Patent Publication Number: US-2022212019-A1

Title: Devices, systems and methods for improving conductive communication between external devices and implantable medical devices

Description:
PRIORITY 
     This application claims priority to U.S. Provisional Patent Application No. 69/291,772, titled DEVICES, SYSTEMS AND METHODS FOR IMPROVING CONDUCTIVE COMMUNICATION BETWEEN EXTERNAL DEVICES AND IMPLANTABLE MEDICAL DEVICES filed Dec. 20, 2021. Additionally, this application is a continuation-in-part (CIP) of U.S. patent application Ser. No. 17/222,242, titled “REMOTE FOLLOW-UP METHODS, SYSTEMS, AND DEVICES FOR LEADLESS PACEMAKER SYSTEMS” filed Apr. 5, 2021 (13881US01_SJUD-01173U52), which claims priority to U.S. Provisional Patent Application No. 63/005,628, filed Apr. 6, 2020, and U.S. Provisional Patent Application No. 63/033,737, filed Jun. 2, 2020. This application is also continuation-in-part (CIP) of U.S. patent application Ser. No. 17/222,279, titled “REMOTE FOLLOW-UP METHODS, SYSTEMS, AND DEVICES FOR LEADLESS PACEMAKER SYSTEMS” filed Apr. 5, 2021 (13881US02_SJUD-01173U53), which claims priority to U.S. Provisional Patent Application No. 63/005,628, filed Apr. 6, 2020, and U.S. Provisional Patent Application No. 63/033,737, filed Jun. 2, 2020. Priority is claimed to each of the above applications, and each of the above applications is incorporated herein by reference. 
    
    
     FIELD OF TECHNOLOGY 
     Embodiments described herein generally relate to devices, systems and methods that enable an external device, such as an external programmer or a remote monitor, to perform conductive communication with one or more implantable medical devices implanted within a patient using external electrodes that are in contact with the patient. 
     BACKGROUND 
     From time to time a non-implanted device needs to communicate with an implantable medical device (IMD), such as a leadless pacemaker (LP), so that the non-implanted device can, for example, program the IMD, interrogate the IMD, and/or obtain notifications and/or other types of diagnostic information from the IMD. Such a non-implanted device, which can also be referred to as an external device, can be, e.g., an external programmer or a remote monitor. 
     Communication between an external device and one or more IMDs (e.g., LPs) may be facilitated by conductive communication via patient tissue, whereby two skin electrodes (that are part of or coupled to the external device) are attached to skin of a patient within which (i.e., in whom) one or more IMDs is/are implanted, and the two skin electrodes are used to transmit information to and/or receive information from the IMD(s) via conduction through body tissue of the patient. In other words, the two skin electrodes can be used by the external device to transmit conductive communication signals via patient tissue to individual IMDs. Additionally, or alternatively, the two skin electrodes can be used by the external device to receive conductive communication signals from individual IMDs. The communication signals transmitted from an external device to an IMD, or vice versa, to achieve conductive communication can be referred to herein as conductive communication signals. The skin electrodes are examples of external electrodes, i.e., non-implanted electrodes. 
     Where conductive communication signals are transmitted from an external programmer to an IMD, the conductive communication signals can also be referred to more specifically as conductive programmer-to-implant (p2i) communication signals, or more succinctly as conductive p2i signals. Where the conductive communication signals are transmitted from an IMD to an external programmer, the conductive communication signals can also be referred to more specifically as conductive implant-to-programmer (i2p) communication signals, or more succinctly as conductive i2p signals. Conductive communication signals are also referred to sometimes as conducted communication signals, and these terms are often used interchangeably. 
     One potential problem with using conductive communication signals is that the orientation of the IMD(s) can cause fading that can adversely affect both conductive p2i and i2p communication. Additionally, the locations of the two skin electrodes, which define a communication vector for the external device, may not provide for good communication signal quality between the external device and an IMD. More generally, the orientation and location of an IMD and the locations of the external electrodes can affect the communication quality between an external device and an IMD. These problems may be exacerbated when there is a need or desire for the external device to communicate with multiple (i.e., two or more) IMDs. For example, it may be the case that a communication vector associated with two skin electrodes attached to a patient&#39;s skin provides for good conductive communication signal quality with only one of multiple IMDs. To overcome this problem, the two skin electrodes attached to the patient&#39;s skin at first locations can first be used to provide for conductive communication between the external device and a first IMD. The two skin electrodes can then be moved to second locations and then used to provide for conductive communication between the external device and a second IMD. If the patient also includes a third IMD, the two skin electrodes can then be moved to third locations and then used to provide for conductive communication between the external device and the third IMD. Even if a patient only includes a single IMD, it still may be necessary to move one or both of the two skin electrodes one or more times before acceptable conductive communication signal quality is achieved between the external device and the single IMD. This repositioning or moving of the two skin electrodes can be time consuming for both the patient and the medical personnel, as well as costly in terms of increasing the patient&#39;s medical bills. 
     SUMMARY 
     Certain embodiments of the present technology are related to methods for use by an external device that is configured to communicate with an IMD implanted within a patient using conductive communication, wherein the external device includes or is communicatively coupled to at least three external electrodes that are in contact with the patient. The external device can be, for example, an external programmer or a remote monitor, but is not limited thereto. The IMD can be, e.g., a leadless cardiac pacemaker, an insertable cardiac monitor (ICM), or a non-vascular implantable cardioverter defibrillator (NV-ICD), but is not limited thereto. Such a method can include determining a respective indicator of conductive communication quality for each communication vector, of a plurality of communication vectors that can be used to communicate with the IMD, wherein each of the plurality of communication vectors comprises a different combination (e.g., pair) of the at least three external electrodes that are in contact with the patient. Unless stated otherwise, when an external electrode is in contact with a patient it is presumed that the electrode is directly in contact with skin of the patient. The method can also include identifying which one of the plurality of communication vectors is a preferred communication vector for communicating with the IMD, based on the respective indicators of conductive communication quality that are determined for the plurality of communication vectors, and communicating with the IMD using the preferred communication vector for communicating with the IMD, after the preferred communication vector is identified. 
     In accordance with certain embodiments, the method can include, while or after communicating with the IMD using the preferred communication vector for communicating with the IMD, determining whether there should be a reassessment of which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD. The method can also include, in response to determining that there should be the reassessment, repeating the determining and the identifying steps, which may result in a new preferred communication vector being identified. The determining that there should be the reassessment can occur in response to the indicator of conductive communication quality associated with the preferred communication vector for communicating with the IMD falling below a corresponding threshold, in response to a loss of conductive communication with the IMD, or in response to a specified period of time elapsing since the preferred communication vector for communicating with the IMD was most recently identified. 
     In accordance with certain embodiments, a plurality of IMDs that are configured to perform conductive communication are implanted within the patient, and the external device is configured to communicate with each of the plurality of IMDs using conductive communication. In certain such embodiments, the determining, the identifying, and the communicating steps are each separately performed for each of the plurality of IMDs, such that a respective preferred communication vector is separately identified for each of the plurality of IMDs that are configured to perform conductive communication. This can result in a different preferred communication vector being identified for each of the IMDs. Alternatively, it is possible that the same preferred communication vector can be identified for two or more of the IMDs. 
     In accordance with certain embodiments, instructions can be provided to a user of the external device to modify at least one of where or how one or more of the at least three external electrodes contact the patient, in response to determining that the indicators of conductive communication quality for communicating with the IMD, which are determined for the plurality of communication vectors, are below a corresponding threshold. Such instructions can be provided via a display of the external device, and/or may be auditory type instructions. Other variations are also possible. 
     In accordance with certain embodiments, determining the respective indicator of conductive communication quality for each communication vector, of the plurality of communication vectors that can be used to communicate with the IMD, includes for each communication vector: determining a plurality of different measures of conductive communication quality and/or surrogates thereof for the communication vector; and combining the plurality of different measures of conductive communication quality and/or surrogates thereof to produce the respective indicator of conductive communication quality for the communication vector. 
     In accordance with certain embodiments, the plurality of different measures of conductive communication signal quality and/or the surrogates thereof that are determined for each communication vector, of the plurality of communication vectors that can be used to communicate with the IMD, are indicative of at least two of the following: a noise floor associated with the communication vector; a measure of amplitude of at least a portion of a conductive communication signal received by the external device from the IMD using the communication vector; a measure of amplitude of at least a portion of a conductive communication signal received by the IMD from the external device; a magnitude of at least a portion of a conductive communication signal received by the external device from the IMD after rectification and integration thereof; a magnitude of at least a portion of a conductive communication signal received by the IMD from the external device after rectification and integration thereof; a signal-to-noise ratio (SNR) of at least a portion of a conductive communication signal received by the external device from the IMD; a SNR of at least a portion of a conductive communication signal received by the IMD from the external device; a total energy of at least a portion of a conductive communication signal received by the external device from the IMD, after rectification and integration thereof; a total energy of at least a portion of a conductive communication signal received by the IMD from the external device, after rectification and integration thereof; a bit-error-rate (BER) associated with at least a portion of a conductive communication signal received by the external device from the IMD; and a BER associated with at least a portion of a conductive communication signal received by the IMD from the external device. The use of additional and/or alternative measures of conductive communication signal quality and/or the surrogates thereof is also possible and within the scope of the embodiments described herein. 
     In accordance with certain embodiments, identifying which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD, comprises ranking the plurality of communication vectors, and identifying a highest ranked one of the plurality of communication vectors as the preferred communication vector for communicating with the IMD. 
     In accordance with certain embodiments, identifying which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD, comprises identifying which one of the plurality of communication vectors has a highest indicator of conductive communication quality. 
     In accordance with certain embodiments, a plurality of IMDs that are configured to perform conductive communication are implanted within the patient, and the external device is configured to communicate with each of the plurality of IMDs using conductive communication. In certain such embodiments, the determining step is separately performed for each of the plurality of IMDs, such that for each of the IMDs a respective indicator of conductive communication quality is determined for each communication vector, of the plurality of communication vectors that can be used to communicate with the IMD, and the identifying step is performed collectively for the plurality of IMDs to thereby identify one preferred communication vector for communicating with the plurality of IMDs. Communicating with the plurality of IMDs is then performed using the one preferred communication vector for communicating with the IMD, after the one preferred communication vector is identified. 
     Certain embodiments of the present technology are related to an external device that is configured to communicate with an IMD implanted within a patient using conductive communication, wherein the external device comprises a conductive communication receiver, switches, and a controller, with the switches being between the conductive communication receiver and at least three external electrodes that are configured to be placed in contact with the patient. In accordance with certain embodiments, the controller of the external device is configured to control the switches to thereby control which communication vector, of a plurality of communication vectors that can be used to communicate with the IMD, is coupled to the conductive communication receiver, wherein each of the plurality of communication vectors comprises a different combination (e.g., pair) of the at least three external electrodes. The controller is also configured to determine a respective indicator of conductive communication quality for each communication vector, of the plurality of communication vectors that can be used to communicate with the IMD. Additionally, the controller is configured to identify which one of the plurality of communication vectors is a preferred communication vector for communicating with the IMD, based on the respective indicators of conductive communication quality that are determined for the plurality of communication vectors, and use the preferred communication vector to communicate with the IMD, after the preferred communication vector is identified. 
     In accordance with certain embodiments, the controller of the external device is also configured to determine when there should be a reassessment of which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD. In certain such embodiments, the controller is configured to determine that there should be the reassessment in response to at least one of the following: the indicator of conductive communication quality associated with the preferred communication vector for communicating with the IMD falling below a corresponding threshold; a loss of conductive communication with the IMD; or a specified period of time elapsing since the preferred communication vector for communicating with the IMD was most recently identified. Other variations are also possible and within the scope of the embodiments described herein. 
     In accordance with certain embodiments, the external device is configured to communicate with each of a plurality of IMDs that are configured to perform conductive communication. In certain such embodiments, for each IMD of the plurality of IMDs that are configured to perform conductive communication, the controller of the external device is configured to: determine a respective indicator of conductive communication quality for each communication vector, of the plurality of communication vectors that can be used to communicate with the IMD; identify which one of the plurality of communication vectors is a preferred communication vector for communicating with the IMD, based on the respective indicators of conductive communication quality that are determined for the plurality of communication vectors; and use the preferred communication vector to communicate with the IMD, after the preferred communication vector is identified, such that different said preferred communication vectors can be identified and used for communicating with different ones of the plurality of IMDs. 
     In accordance with certain embodiments, in order to determine the respective indicator of conductive communication quality for each communication vector, of the plurality of communication vectors that can be used to communicate with the IMD, the controller is configured to determine, for each communication vector, a plurality of different measures of conductive communication quality and/or surrogates thereof for the communication vector, and combine the plurality of different measures of conductive communication quality and/or surrogates thereof to produce the respective indicator of conductive communication quality for the communication vector. Examples of the different measures of conductive communication signal quality and/or the surrogates thereof were provided above, and thus need not be repeated. 
     In accordance with certain embodiments, in order to identify which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD, the controller is configured to rank the plurality of communication vectors, and identify a highest ranked one of the plurality of communication vectors as the preferred communication vector for communicating with the IMD. In accordance with certain embodiments, in order to identify which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD, the controller is configured to identify which one of the plurality of communication vectors has a highest indicator of conductive communication quality, without necessarily performing any ranking. 
     In accordance with certain embodiments, the external device is configured to communicate with each of a plurality of IMDs that are configured to perform conductive communication. In certain such embodiments, for each IMD of the plurality of IMDs that are configured to perform conductive communication, the controller is configured to determine a respective indicator of conductive communication quality for each communication vector of the plurality of communication vectors that can be used to communicate with the IMD. Additionally, the controller is configured to collectively identify one preferred communication vector for communicating with the plurality of IMDs, based on the respective indicators of conductive communication quality that have been determined. The controller is further configured to use the one preferred communication vector to communicate with the plurality of IMDs, after the one preferred communication vector is identified. 
     Certain embodiments of the present technology are directed to a method for use by an external device that is configured to communicate with each IMD, of a plurality of IMDs implanted within a patient, using conductive communication, wherein the external device includes or is communicatively coupled to at least three external electrodes that are in contact with the patient. Certain such embodiments comprise, for each IMD, of the plurality of IMDs, determining a respective indicator of conductive communication quality for each communication vector, of a plurality of communication vectors that can be used to communicate with the IMD, wherein each of the plurality of communication vectors comprises a different combination (e.g., pair) of the at least three external electrodes that are in contact with the patient. Additionally, for each IMD, of the plurality of IMDs, the method comprises identifying which one of the plurality of communication vectors is a preferred communication vector for communicating with the IMD, based on the respective indicators of conductive communication quality that are determined for the plurality of communication vectors. The method also comprises for each IMD, of the plurality of IMDs, communicating with the IMD using the preferred communication vector for communicating with the IMD, after the preferred communication vector for communicating with the IMD is identified. Further, the method comprises, for a said IMD, of the plurality of IMDs, determining that there should be a reassessment of which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD, and in response thereto, repeating the determining and the identifying steps for the IMD to thereby identify an updated preferred communication vector for communicating with the IMD. In accordance with certain such embodiments, the determining that there should be a reassessment of which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD occurs in response to at least one of the following: the indicator of conductive communication quality associated with the preferred communication vector for communicating with the IMD falling below a corresponding threshold; or a loss of conductive communication with the IMD. 
     This summary is not intended to be a complete description of the embodiments of the present technology. Other features and advantages of the embodiments of the present technology will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present technology relating to both structure and method of operation may best be understood by referring to the following description and accompanying drawings, in which similar reference characters denote similar elements throughout the several views: 
         FIG. 1  illustrates a system that includes a plurality of implantable medical devices that are implanted in a patent and an external programmer that can be used to program and/otherwise communicate with the implantable devices. The external programmer in  FIG. 1  is an example of an external device. 
         FIG. 2  is a high level block diagram of an example LP. 
         FIG. 3  illustrates an example form factor for an LP. 
         FIG. 4  depicts a sample configuration involving an external programmer and two endocardially implanted LPs. 
         FIG. 5  depicts a sample configuration involving an external programmer and two LPs implanted epicardially (on the external heart surface). 
         FIG. 6  depicts an example of an external device communicatively coupled to three electrodes that are in contact with the chest of a patient. 
         FIG. 7  is a high level block diagram illustrating example details of an external device that is configured to communicate with one or more IMDs implanted within a patient using conductive communication, wherein the external device includes or is communicatively coupled to at least three external electrodes that are in contact with the patient. 
         FIG. 8  is a high level flow diagram used to summarize certain methods for use by an external device that is configured to communicate with an IMD implanted within a patient using conductive communication, wherein the external device includes or is communicatively coupled to at least three external electrodes that are in contact with the patient. 
         FIG. 9  is a high level flow diagram used to describe how an external device can test communication vectors as part of an interrogation of one or more IMDs. 
         FIG. 10  is a high level flow diagram used to describe how an external device may from time to time reassess which communication vector should be used to conductively communicate with each of one or more IMDs. 
         FIG. 11  includes a timing diagram that shows how an external device can conductively communicate with a first IMD during a first period of time using a preferred communication vector identified by the external device for communicating with the first IMD, and can conductively communicate with a second IMD during a second period of time using a preferred communication vector identified by the external device for communicating with the second IMD. 
         FIG. 12  includes a timing diagram that shows that during a first period of time an external device communicates with a first IMD using a first communication vector, and during a second period of time communicates with a second IMD using a second communication vector. 
         FIG. 13  is a high level flow diagram that is used to summarize a method in which an external device can select a preferred communication vector, from among three or more external electrodes, for receiving conductive communication signals from an implanted IMD. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present technology can be used to improve conductive communication between an external device and one or more implantable medical devices (IMDs) in time and cost efficient manners. Certain such embodiments improve and preferably optimize the conductive communication signal quality between an external device and each IMD, of a plurality of IMDs, by dynamically switching between multiple electrode combinations (e.g., pairs) to find a preferred conductive communication vector. Certain embodiments described herein provide details of how the communication signal quality is assessed for a given communication vector, and the criteria used to determine a preferred communication vector for each IMD. However, before providing addition details of the specific embodiments of the present technology, an example environment in which embodiments of the present technology can be useful will first be described with reference to  FIGS. 1-3 . More specifically,  FIGS. 1-3  will be used to describe an example cardiac pacing system, wherein pacing and sensing operations can be performed by multiple IMDs. Such IMDs may include one or more leadless cardiac pacemakers, an implantable cardioverter defibrillator (ICD), such as a non-vascular ICD (NV-ICD), an insertable cardiac monitor (ICM), and an external programmer to reliably and safely coordinate pacing and/or sensing operations. A leadless cardiac pacemaker can also be referred to more succinctly herein as a leadless pacemaker (LP). Where the system includes an ICD, the system is also capable of performing defibrillation. Where the only IMD is an ICM, the system may only be capable of performing monitoring without performing any therapy. 
       FIG. 1  illustrates a system  100  that is configured to be at least partially implanted in a heart  101 . The system  100  includes LPs  102   a  and  102   b  located in different chambers of the heart  101 . The LP  102   a  is located in a right atrium, while LP  102   b  is located in a right ventricle. The LPs  102   a  and  102   b  can communicate with one another to inform one another of various local physiologic activities, such as local intrinsic events, local paced events, and/or the like. The LPs  102   a  and  102   b  may be constructed in a similar manner, but operate differently based upon which chamber LP  102   a  or  102   b  is located. The LPs  102   a  and  102   b  may sometimes be referred to collectively herein as the LPs  102 , or individually as an LP  102 . 
     In certain embodiments, the LPs  102   a  and  102   b  communicate with one another, and/or with an ICM  104 , and/or with an ICD  106 , by conductive communication through the same electrodes that are used for sensing and/or delivery of pacing therapy. The LPs  102   a  and  102   b  also use conductive communication to communicate with a non-implanted device, e.g., an external programmer  109 , having two electrodes  115   a  and  115   b  placed on the skin of a patient within which the LPs  102   a  and  102   b  are implanted. While not shown (and not preferred, since it would increase the size and power consumption of the LPs  102   a  and  102   b ), the LPs  102   a  and  102   b  can potentially include an antenna and/or telemetry coil that would enable them to communicate with one another, the ICD  106  and/or a non-implanted device using RF and/or inductive communication. While only two LPs  102  are shown in  FIG. 1 , it is possible that more than two LPs can be implanted in a patient. For example, to provide for bi-ventricular pacing and/or cardiac resynchronization therapy (CRT), in addition to having LPs implanted in or on the right atrial (RA) chamber and the right ventricular (RV) chamber, a further LP can be implanted in or on the left ventricular (LV) chamber. It is also possible that a single LP be implanted within a patient, e.g., in or on the RV chamber, the RA chamber, or the LV chamber, but not limited thereto. It would also be possible for more than one LP to be implanted in or on a same cardiac chamber. 
     In some embodiments, one or more of the LPs  102   a ,  102   b  can be co-implanted with the ICM  104  and/or the ICD  106 . In such embodiments, the ICM  104  and/or the ICD  106  are examples of other types of IMDs that may need to communicate with an external device, such as an external programmer, from time to time. The ICM  104  and/or the ICD  106  may utilize conductive communication to communicate with the LPs  102 , as well as to communicate with an external device. It may alternatively or additionally be possible for the ICM  104  and/or the ICD  106  to utilize radio frequency (RF) communication and/or inductive communication to communicate with an external device, depending upon the specific implementation, and depending upon the capabilities of the external device. 
     Each LP  102   a ,  102   b  uses two or more electrodes located within, on, or within a few centimeters of the housing of the pacemaker, for pacing and sensing at the cardiac chamber, for bidirectional conductive communication with one another, with the programmer  109 , the ICD  106 , and/or the ICM  104 . Such an ICM  104  can be intended for subcutaneous implantation at a site near the heart  101 . The ICM  104  can include, for example, a pair of spaced-apart sense electrodes positioned with respect to a housing, wherein the sense electrodes provide for detection of far-field EGM signals, and can also be used for conductive communication with one or more other implanted devices, such as the LP(s)  102   a  and/or  102   b  and/or the ICD  106 . Such an ICM can also include an antenna that is configured to wirelessly communicate with an external device, such as an external programmer  109 , in accordance with one or more wireless communication protocols (e.g., Bluetooth, Bluetooth low energy, Wi-Fi, etc.). The housing of the ICM  104  can include various other components such as: sense electronics for receiving signals from the electrodes, a microprocessor for processing the signals in accordance with algorithms, a loop memory for temporary storage of cardiac activity (CA) data, a device memory for long-term storage of CA data upon certain triggering events, sensors for detecting patient activity and a battery for powering components. 
     Each LP  102   a ,  102   b  and/or other type of IMD can transmit an advertisement sequence of pulses using at least two electrodes of the IMD (e.g., LP) from time to time so that an external device (e.g., an external programmer, or a remote monitor) that has or is communicatively coupled to external electrodes that are in contact with the patient (within which the LP(s) and/or other IMD(s) is/are implanted) can detect the presence of the IMD(s) and optionally establish a communication session with one or more IMD(s). For a more specific example, an LP (or other type of IMD) can transmit an advertisement sequence of pulses every specified number of cardiac cycles (e.g., every eight cardiac cycles), or every specified period of time (e.g., every 5 seconds), but not limited thereto. In accordance with certain embodiments, the advertisement sequence of pulses is a predetermined sequence of pulses that indicates to an external device (e.g., an external programmer, or a remote monitor) that an LP (or other type of IMD) is implanted within a patient. The advertisement sequence of pulses can also be referred to as a sniff sequence of pulses, or more succinctly as a sniff. In accordance with certain embodiments of the present technology, which are described below, an external device can use the sniff pulses to identify which one of a plurality of communication vectors is a preferred communication vector for communicating with the IMD that transmitted the sniff pulses. For example, where the external device has or is communicatively coupled to three external electrodes, i.e., first, second, and third external electrodes, the external device can test and select among first, second, and third subsets of the external electrodes, wherein the first subset includes the first and second external electrodes, the second subset includes the first and third external electrodes, and the third subset includes the second and third external electrodes. 
     Referring to  FIG. 2 , a block diagram shows an example embodiment for portions of the electronics within the LPs  102   a ,  102   b  configured to provide conductive communication through the same electrodes that are used for cardiac pacing and/or sensing. Each of the LPs  102   a ,  102   b  includes at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and uni-directional and/or bi-directional communication. In  FIG. 2  (and  FIG. 3 ) the two electrodes shown therein are labeled  108   a  and  108   b . Such electrodes can be referred to collectively as the electrodes  108 , or individually as an electrode  108 . An LP  102 , or other type of IMD, can include more than two electrodes, depending upon implementation. 
     In  FIG. 2 , each of the LPs  102   a ,  102   b  is shown as including a conductive communication receiver  120  that is coupled to the electrodes  108  and configured to receive conductive communication signals from the other LP  102 , the ICM  104  and/or the ICD  106 , but not limited thereto. The conductive communication receiver  120  and the electrodes  108  can also be used to receive conductive communication signals from the external programmer  109 , and/or another type of external device. Although one receiver  120  is depicted in  FIG. 2 , in other embodiments, each LP  102   a ,  102   b  may also include one or more additional receivers. As will be described in additional detail below, the pulse generator  116  can function as a transmitter that transmits conductive communication signals using the electrodes  108 , under the control of the controller  112 . In certain embodiments, the LPs  102   a ,  102   b  may communicate over more than just first and second communication channels  105  and  107 . In certain embodiments, the LPs  102   a ,  102   b  may communicate over one common communication channel  105 . More specifically, the LPs  102   a  and  102   b  can communicate conductively over a common physical channel via the same electrodes  108  that are also used to deliver pacing pulses. Usage of the electrodes  108  for conductive communication enables the one or more LPs  102   a ,  102   b  to perform antenna-less and inductive coil-less communication. Where multiple implantable devices (such as the LPs  102   a  and  102   b ) communicate with one another using conductive communication, such conductive communication can be referred to as implant-to-implant (i2i) conductive communication, or more succinctly, as i2i conductive communication. 
     Optionally, an LP (or other IMD) that receives any conductive communication signal from another LP (or other IMD) or from a non-implanted device (e.g., a programmer  109 ) may transmit a receive acknowledgement indicating that the receiving LP (or other IMD, or external device) received the conductive communication signal. In certain embodiments, where an IMD expects to receive a conductive communication signal within a window, and fails to receive the conductive communication signal within the window, the IMD may transmit a failure-to-receive acknowledgement indicating that the receiving IMD failed to receive the conductive communication signal. Other variations are also possible and within the scope of the embodiments described herein. Each conductive communication signal can include one or more sequences of conductive communication pulses. In accordance with certain embodiments, conductive communication pulses are delivered during cardiac refractory periods that are identified or detected by the LP(s) and/or other IMD(s). In accordance with certain embodiments, conductive communication pulses are sub-threshold, i.e., they are below the capture threshold for the patient. 
     The LPs  102   a ,  102   b  can exchange event messages within i2i conductive communication signals to enable synchronized therapy and additional supportive features (e.g., measurements, etc.). To maintain synchronous therapy, each of the LPs  102   a ,  102   b  is made aware (through the event messages) when an event occurs in the chamber containing the other LP  102   a ,  102   b . Example additional details of i2i event messages that are sent between LPs  102  are provided in U.S. patent application Ser. No. 17/222,242, filed Apr. 5, 2021, titled REMOTE FOLLOW-UP METHODS, SYSTEMS, AND DEVICES FOR LEADLESS PACEMAKER SYSTEMS, which is incorporated herein by reference above. 
     For synchronous event signaling, LPs  102   a  and  102   b  may maintain synchronization and regularly communicate at a specific interval. Synchronous event signaling allows the transmitter and receiver in each LP  102   a ,  102   b  to use limited (or minimal) power as each LP  102   a ,  102   b  is only powered for a small fraction of the time in connection with transmission and reception. For example, LP  102   a ,  102   b  may transmit/receive (Tx/Rx) communication messages in time slots having duration of 10-20 μs, where the Tx/Rx time slots occur periodically (e.g., every 10-20 ms). Such time slots can also be referred to as windows. 
     Still referring to  FIG. 2 , each LP  102   a ,  102   b  is shown as including a controller  112  and a pulse generator  116 . The controller  112  can include, e.g., a microprocessor (or equivalent control circuitry), RAM and/or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry, but is not limited thereto. The controller  112  can further include, e.g., timing control circuitry to control the timing of the stimulation pulses (e.g., pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A) delay, or ventricular interconduction (V-V) delay, etc.). Such timing control circuitry may also be used for the timing of refractory periods, blanking intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, and so on. The controller  112  can further include other dedicated circuitry and/or firmware/software components that assist in monitoring various conditions of the patient&#39;s heart and managing pacing therapies. The controller  112  and the pulse generator  116  may be configured to transmit event messages, via the electrodes  108 , in a manner that does not inadvertently capture the heart in the chamber where LP  102   a ,  102   b  is located, such as when the associated chamber is not in a refractory state. In addition, a LP  102   a ,  102   b  that receives an event message may enter an “event refractory” state (or event blanking state) following receipt of the event message. The event refractory/blanking state may be set to extend for a determined period of time after receipt of an event message in order to avoid the receiving LP  102   a ,  102   b  from inadvertently sensing another signal as an event message that might otherwise cause retriggering. For example, the receiving LP  102   a ,  102   b  may detect a measurement pulse from another LP  102   a ,  102   b  or programmer  109 . 
     In accordance with certain embodiments herein, the external programmer  109  may communicate over a programmer-to-LP channel, with LPs  102   a ,  102   b  utilizing the same communication scheme. The external programmer  109  may listen to the event message transmitted between LPs  102   a ,  102   b  and synchronize programmer to implant communication such that the external programmer  109  does not transmit communication signals  113  until after an implant to implant messaging sequence is completed. 
     In some embodiments, an individual LP  102  can comprise a hermetic housing  110  configured for placement on or attachment to the inside or outside of a cardiac chamber and at least two leadless electrodes  108  proximal to the housing  110  and configured for conductive communication with at least one other device within or outside the body. Depending upon the specific implementation, and/or the other device with which an LP is communicating, the conductive communication may be unidirectional or bidirectional. 
       FIG. 2  depicts a single LP  102   a  (or  102   b ) and shows the LP&#39;s functional elements substantially enclosed in a hermetic housing  110 . The LP  102   a  (or  102   b ) has at least two electrodes  108  located within, on, or near the housing  110 , for delivering pacing pulses to and sensing electrical activity from the muscle of the cardiac chamber, and for conductive communication with at least one other device within or outside the body. Hermetic feedthroughs  130 ,  131  conduct electrode signals through the housing  110 . The housing  110  contains a primary battery  114  to supply power for pacing, sensing, and communication. The housing  110  also contains circuits  132  for sensing cardiac activity from the electrodes  108 , receiver  120  for receiving information from at least one other device via the electrodes  108 , and the pulse generator  116  for generating pacing pulses for delivery via the electrodes  108  and also for transmitting information to at least one other device via the electrodes  108 . The housing  110  can further contain circuits for monitoring device health, for example a battery current monitor  136  and a battery voltage monitor  138 , and can contain circuits for controlling operations in a predetermined manner. 
     The electrodes  108  can be configured to communicate bidirectionally among the multiple leadless cardiac pacemakers, the implanted ICD  106  and/or the implanted ICM  104  to coordinate pacing pulse delivery and optionally other therapeutic or diagnostic features using messages that identify an event at an individual pacemaker originating the message and a pacemaker receiving the message react as directed by the message depending on the origin of the message. An LP  102   a ,  102   b  that receives the event message reacts as directed by the event message depending on the message origin or location. In some embodiments or conditions, the two or more leadless electrodes  108  can be configured to communicate bidirectionally among the one or more LPs, the ICD  106 , and/or the ICM  104  and transmit data including designated codes for events detected or created by an individual pacemaker. Individual pacemakers can be configured to issue a unique code corresponding to an event type and a location of the sending pacemaker. The electrodes can also be used to transmit and/or receive conductive communication signals from an external device. 
     As shown in  FIG. 2 , each LP  102   a ,  102   b  can comprise two (or more) leadless electrodes  108  configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with one another and/or the co-implanted ICD  106 . As shown in  FIG. 2 , the primary battery  114  has positive terminal  140  and negative terminal  142 . Current from the positive terminal  140  of primary battery  114  flows through a shunt  144  to a regulator circuit  146  to create a positive voltage supply  148  suitable for powering the remaining circuitry of the pacemaker  102 . The shunt  144  enables the battery current monitor  136  to provide the controller  112  with an indication of battery current drain and indirectly of device health. The illustrative power supply can be a primary battery  114 . The LP is also shown as including a temperature sensor  152  and an accelerometer  154 . 
     In various embodiments, each LP  102   a ,  102   b  can manage power consumption to draw limited power from the battery, thereby reducing device volume. Each circuit in the system can be designed to avoid large peak currents. For example, cardiac pacing can be achieved by discharging a tank capacitor (not shown) across the pacing electrodes. Recharging of the tank capacitor is typically controlled by a charge pump circuit. In a particular embodiment, the charge pump circuit is throttled to recharge the tank capacitor at constant power from the battery. 
     In some embodiments, the controller  112  in one LP  102  can access signals on the electrodes  108  and can examine output pulse duration from another pacemaker for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activating delivery of a pacing pulse following a predetermined delay of zero or more milliseconds. The predetermined delay can be preset at manufacture, programmed via an external programmer, or determined by adaptive monitoring to facilitate recognition of the triggering signal and discriminating the triggering signal from noise. In some embodiments or in some conditions, the controller  112  can examine output pulse waveform from another leadless cardiac pacemaker for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activating delivery of a pacing pulse following a predetermined delay of zero or more milliseconds. 
       FIG. 3  shows an example form factor of an LP  102   a ,  102   b . The LP can include a hermetic housing  202  ( 110 ) with electrodes  108   a  and  108   b  disposed thereon. As shown, electrode  108   a  can be separated from but surrounded partially by a fixation mechanism  205 , and the electrode  108   b  can be disposed on the housing  202 . The fixation mechanism  205  can be a fixation helix, a plurality of hooks, barbs, or other attaching features configured to attach the pacemaker to tissue, such as heart tissue. The electrodes  108   a  and  108   b  are examples of the electrodes  108  shown in and discussed above with reference to  FIG. 2 . The housing can also include an electronics compartment  210  within the housing that contains the electronic components necessary for operation of the pacemaker, including, e.g., a pulse generator, receiver, a battery, and a processor for operation. The hermetic housing  202  can be adapted to be implanted on or in a human heart, and can be cylindrically shaped, rectangular, spherical, or any other appropriate shapes, for example. The housing can comprise a conductive, biocompatible, inert, and anodically safe material such as titanium, 316L stainless steel, or other similar materials. The housing  202  can further comprise an insulator disposed on the conductive material to separate electrodes  108   a  and  108   b . The insulator can be an insulative coating on a portion of the housing between the electrodes, and can comprise materials such as silicone, polyurethane, parylene, or another biocompatible electrical insulator commonly used for implantable medical devices. In the embodiment of  FIG. 3 , a single insulator  208  is disposed along the portion of the housing between electrodes  108   a  and  108   b . In some embodiments, the housing itself can comprise an insulator instead of a conductor, such as an alumina ceramic or other similar materials, and the electrodes can be disposed upon the housing. 
     As shown in  FIG. 3 , the pacemaker can further include a header assembly  212  to isolate electrodes  108   a  and  108   b . The header assembly  212  can be made from PEEK, tecothane or another biocompatible plastic, and can contain a ceramic to metal feedthrough, a glass to metal feedthrough, or other appropriate feedthrough insulator as known in the art. The term metal, as used herein, also encompasses alloys that are electrically conductive. The electrodes  108   a  and  108   b  can comprise pace/sense electrodes, or return electrodes. A low-polarization coating can be applied to the electrodes, such as sintered platinum, platinum-iridium, iridium, iridium-oxide, titanium-nitride, carbon, or other materials commonly used to reduce polarization effects, for example. In  FIG. 3 , electrode  108   a  can be a pace/sense electrode and electrode  108   b  can be a return electrode. The electrode  108   b  can be a portion of the conductive housing  202  that does not include an insulator  208 . 
     Several techniques and structures can be used for attaching the housing  202  to the interior or exterior wall of the heart. A helical fixation mechanism  205 , can enable insertion of the device endocardially or epicardially through a guiding catheter. A torqueable catheter can be used to rotate the housing and force the fixation device into heart tissue, thus affixing the fixation device (and also the electrode  108   a  in  FIG. 2 ) into contact with stimulable tissue. Electrode  108   b  can serve as an indifferent electrode for sensing and pacing. The fixation mechanism may be coated partially or in full for electrical insulation, and a steroid-eluting matrix may be included on or near the device to minimize fibrotic reaction, as is known in conventional pacing electrode-leads. 
       FIGS. 4 and 5  are schematic pictorial views depicting how an external programmer  109  coupled to two skin electrodes  115   a ,  115   b  can communicate with the LP  102   a  and/or the LP  102   b  via conductive communication, which is also referred to interchangeably herein as conducted communication. Such communication may take place via bidirectional communication pathways comprising a receiving pathway that decodes information encoded on pulses generated by one or more of the LPs  102   a  or  102   b  and conductive through body tissue to the external programmer  109 . According to the illustrative arrangement, the bidirectional communication pathways can be configured for communication with multiple LPs  102   a  and  102   b  via two or more electrodes and conduction through body tissue. 
     The external programmer  109  is connected by a communication transmission channel and has transmitting and receiving functional elements for a bidirectional exchange of information with one or more IMDs, such as LP  102   a  and/or LP  102   b . The communication channel includes two external electrodes  115   a  and  115   b  which can be affixed or secured to the surface of the skin. From the point of the skin, the communication transmission channel is wireless, includes the ion medium of the intra- and extra-cellular body liquids, and enables electrolytic-galvanic coupling between the external electrodes, which can also be referred to as surface electrodes, and the LPs, or more generally, IMDs. The bidirectional communication pathways can further comprise a transmitting pathway that passes information from the external programmer  109  to one or more of the LPs  102   a  and/or  102   b  by direct conduction through the body tissue by modulation that avoids skeletal muscle stimulation using modulated signals at a frequency in a range from approximately 10 kHz to 100 kHz, or at higher frequencies. For example, p2i communication signals may be transmitted at a center frequency (fc) of 500 kHz. 
     Information transmitted from the external programmer  109  to the implanted LPs is conveyed by modulated signals at the approximate range of 10 kHz to 100 kHz which is a medium-high frequency, or at higher frequencies. The signals are passed through the communication transmission channel by direct conduction. A modulated signal in the frequency range has a sufficiently high frequency to avoid any depolarization within the living body which would lead to activation of the skeletal muscles and discomfort to the patient. The frequency is also low enough to avoid causing problems with radiation, crosstalk, and excessive attenuation by body tissue. Thus, information may be communicated at any time, without regard to the heart cycle or other bodily processes. The use of other frequency ranges is also possible and within the scope of the embodiments described herein. 
       FIG. 4  depicts a sample configuration involving the external programmer  109  and two endocardially implanted LPs  102   a  and  102   b . The external programmer  109  is physically connected to the skin surface via two external electrodes  115   a  and  115   b  (also referred to as surface electrodes), which can serve three functions. The external electrodes  115   a  and  115   b  can be referred to individually as an external electrode  115  (or a surface electrode  115 ), or collectively as external electrodes  115  (or surface electrodes  115 ). First, the electrodes  115  can be used transmit encoded information from the programmer  109  to the LPs or other IMD(s) using a modulated signal, e.g., at a medium frequency 10 kHz to 100 kHz. Second, the external electrodes  115  can be used to receive information from individual LPs or other IMD(s) by detecting encoded information in the pacing pulses of the LP(s). Third, the external electrodes  115  can receive or sense a surface electrocardiogram for display and analysis by the programmer  109 . 
     In  FIG. 4 , the two LPs  102   a  and  102   b  are implanted within the heart  101  endocardially. Alternatively, as shown in  FIG. 5 , the two LPs  102   a  and  102   b  can be implanted by affixing to the exterior surface of the heart  101 . The external electrodes  115  and the external programmer  109  function similarly in arrangements shown in  FIGS. 4 and 5  whether the LPs  102   a  and  102   b  are implanted endocardially or epicardially (on the external heart surface). No restriction is imposed that the LPs are all implanted inside or all implanted outside the heart. One or more may be implanted endocardially along with others implanted on the outer surface of the heart. The functioning of the programmer  109  is substantially the same. 
     As explained above in the Background, a potential problem with using conductive communication signals to provide for communication between an external device and one or more IMDs, is that the orientation of the IMD(s) can cause fading that can adversely affect conductive communication. Additionally, the locations of the two external electrodes, which define a communication vector for the external device, may not provide for good communication signal quality between the external device and an IMD. These problems may be exacerbated when there is a need or desire for the external device to communicate with multiple (i.e., two or more) IMDs. For example, it may be the case that a communication vector associated with two external electrodes attached to a patient&#39;s skin provides for good conductive communication signal quality with only one of multiple IMDs. To overcome this problem, the two externals electrode attached to the patient&#39;s skin at first locations can first be used to provide for conductive communication between the external device and a first IMD. The two external electrodes can then be moved to second locations and then used to provide for conductive communication between the external device and a second IMD. If the patient also includes a third IMD, the two external electrodes can then be moved to third locations and then used to provide for conductive communication between the external device and the third IMD. Even if a patient only includes a single IMD, it still may be necessary to move one or both of the two external electrodes one or more times before an acceptable conductive communication signal quality is achieved between an external device and the signal IMD. This repositioning or moving of the two external electrodes can be time consuming for both the patient and the medical personnel, as well as costly to the patient in terms of increasing their medical bills. 
     Use of Multiple Communication Vectors 
     Certain embodiments of the present technology described herein can be used improve (and preferably optimize) conductive communication signal quality between an external device (e.g., an external programmer or a remote monitor) and each of one or more IMDs by dynamically switching between multiple electrode combinations (e.g., pairs) to find a preferred communication vector for use with each of the IMDs. Certain embodiments described herein assess the communication signal quality for a given communication vector, and criteria are used to determine a preferred communication vector for each IMD. In order for an external device to be able to select among different communication vectors for performing conductive communication with one or more IMDs, the external device should include or be communicatively coupled to at least three external electrodes that are in contact with a patient, within which the one or more IMDs is/are implanted. An example of this is shown in  FIG. 6 . 
     Referring to  FIG. 6 , an external device  602  is shown as being communicatively coupled to three external electrodes  615   a ,  615   b , and  615   c , which can be referred to collectively as the external electrodes  615 , or individually as an external electrode  615 . In  FIG. 6  the external electrodes  615  are shown as being in contact with the chest of a patient, however, that need not be the case. Alternatively, one or more external electrodes can be in contact with the back of the patient, and/or with limbs or digits of the patient, but are not limited thereto. The external electrodes  615  can be physically separated from one another to thereby enable each of the electrodes to be independently placed in contact with the patient&#39;s skin at any desired location. Alternatively, the external electrodes  615 , while electrically isolated from one another, can be physically attached to a same patch or substrate, e.g., a triangular or Y-shaped patch, or the like, that can be configured to be placed on a patient&#39;s chest or back, but not limited thereto. The external device  602  can optionally be communicatively coupled to a remote patient care network, e.g., via one or more wired and/or wireless communication network(s). Example details of the external device  602  are described below with reference to  FIG. 7 . 
     Referring to  FIG. 7 , shown therein is an example block diagram of the external device  602  (e.g., the external programmer  109 , or a remote monitor), which is configured to communicate with one or more IMDs implanted within a patient using conductive communication, wherein the external device  602  includes or is communicatively coupled to at least three external electrodes  615  that are in contact with the patient. 
     Where the external device  602  is an external programmer (e.g.,  109 ), the external device is capable of programming one or more IMDs, such as one or more LPs, an ICM and/or an ICD. The external device  602  can also be used to obtain diagnostic information from one or more IMDs. Where the external device  602  is a remote monitor, it may not be capable of programming any IMDs. The external device  602  is shown as including a controller  712 , a display  716 , a user interface  718 , a network interface  720 , and a battery/supply regulator  726 . The battery and/or supply regulator  726  provides one or more constant voltages to the various components of the external device  602  during normal operation. The external device  602  is also shown as including an ECG amplifier and/or filter  714 , a conductive communication receiver (RX)  742 , and a conductive communication transmitter (TX)  732 . The receiver  742 , in this example embodiment, is shown as including a message amplifier and/or filter  740 , and a message decoder  738 , and is configured to receive conductive communication signals from one or more LPs (e.g.,  102   a  and/or  102   b ). The controller  712 , which is used to control the operation of the external device  702 , can include, e.g., one or more processors (or equivalent control circuitry), RAM and/or ROM memory, logic and timing circuitry, state machine circuitry, and/or I/O circuitry, but is not limited thereto. The controller  712  can also include a clock circuit, or a separate clock circuit (not shown) can provide a clock signal to the controller  712 . 
     In the embodiment shown in  FIG. 7 , the external device  602  is shown as being connected to three external electrodes  615   a ,  615   b , and  615   c , which can be referred to collectively as the electrodes  615 , or individually as an electrode  615 . The electrodes  615  are shown as being connected to switches  713 , which are shown as including first, second, and third sets of switches  713   a ,  713   b , and  713   c . The external electrodes  615  can be located on a housing of the external device  602 , or can be separate from such a housing. Where the electrodes  615  are separate from a housing of the external device  602 , each of the electrodes  615  can be attached to a separate respective wire, or the electrodes  615  can be attached to a further housing that is communicatively coupled to the external device  602  via one or more wires, or via a wireless connection, e.g., using Bluetooth or WiFi, but not limited thereto. Other variations are also possible and within the scope of the embodiments described herein. The external electrodes  615 , as will be described in more detail below, can be used to transmit and receive conductive communication signals to/from one or more LPs, and/or one or more other types of IMDs, and optionally can also be used to sense an electrocardiogram (ECG). 
     The external electrodes  615  are intended to come into contact with the skin of a patient. For example, the external electrodes can be skin electrodes that are configured to be attached to a patient&#39;s torso (e.g., chest and/or back) via an adhesive and/or gel. For another example, the external electrodes  615  can be configured to be touched by one or more digits on each hand of a patient, or to come into contact with a patient&#39;s wrist, a patient&#39;s limb, or a patient&#39;s chest, but are not limited thereto. A set of switches  713   a  is connected between the electrodes  615  and the ECG amplifier and/or filter  714 , a set of switches  714   b  is connected between the electrodes  615  and the receiver  742 , and a further set of switches  713   c  is connected between the electrodes  615  and the transmitter  732 . The various sets of switches are controlled by the controller  712 . In certain embodiments, the amplifiers and/or filters  714 ,  740 , and  736  are each differential circuits that are intended to be connected to a pair of the electrodes  615  by the switches  713  under the control of the controller  712 . For an example, the switches  713   b  can be controlled to connect any pair of the electrodes  615   a ,  615   b ,  615   c  to the message amplifier and/or filter  740 . For an example, the switches  713   b  can connected the electrode  615   a  to a first input of the message amplifier/filter  740 , and connect the electrode  615   b  to a second input of the message amplifier/filter  740 , and not connect electrode  615   c  to any input of the message amplifier/filter  740 . It is also possible that the switches can connect two electrodes  615  directly to one another. For an example, the switches  713   b  can connect the electrode  615   a  to a first input of the message amplifier/filter  740 , and connect the electrodes  615   b  and  615   c  to one another and to a second input of the message amplifier/filter  740 . Beneficially, connecting together two or more electrodes (e.g.,  615   b  and  615   c ) to a same node (e.g., to the same input node of the message amplifier/filter  740 ) can effectively average or create a virtual vector which is between the two or more electrode locations, which enables sensing of a signal that is effectively an average of the signals detected at the two separate electrodes. This is an example of where a combination of the three electrodes  615   a ,  615   b , and  615   c  includes all three of the electrodes, with the electrode  615   a  being separate from the other electrodes, and the electrodes  615   b  and  615   c  being electrically coupled to one another. The inclusion of three external electrodes  615  enables an ECG to be sensed at multiple vectors and/or enables selection from among the multiple vectors for conductive communication with one or more implanted LPs so that conductive communication quality can be improved or maximized. 
     As noted above, the conductive communication receiver  742 , which is shown as including the message amplifier and/or filter  740 , and the message decoder  738 , is configured to receive conductive communication signals from one or more LPs (e.g.,  102   a  and/or  102   b ). The message amplifier and/or filter  740  is configured to amplify and/or filter conductive communication signals received from an LP (e.g.,  102   a  and/or  102   b ). The amplifier portion can be used to increase the relatively small amplitudes of such conductive communication signals. The filter portion can be a high-pass filter or a bandpass filter adapted to separate an ECG signal from conductive communication signals. The message decoder  738  can be configured to decode conductive communication signals received from an LP into a format that the controller  712  can understand. The specific type of decoding performed by the message decoder  738  depends upon the specific coding of the conductive communication signals received from an LP, e.g., on-off keying, frequency-shift keying, frequency modulation, or amplitude shift keying, but not limited thereto. 
     The conductive communication transmitter  732  is configured to transmit (under the controller of the controller  712 ) conductive communication signals to one or more IMDs implanted within a patient. One example of a conductive communication signal that may be transmitted by the external device  602 , such as an external programmer or a remote monitor, is an acknowledgement (ACK) sequence of conductive communication pulses, which informs one or more LPs that the external device  602  is in proximity to the LP(s) and/or other types of IMD(s) and capable of receiving data (encoded into conducted communicate pulses) from the LP(s) and/or other types of IMD(s). The conductive communication signals can also be used to program, interrogate, and/or obtain notifications and/or other types of diagnostic information from one or more IMD(s). 
     The transmitter  732 , in this example embodiment, is shown as including a message encoder and/or modulator  730  and an amplifier  736 . The message encoder and/or modulator  730  can be configured to encode and/or modulate signals that are output from the controller  712  into a format that IMD(s) can understand. The specific type of encoding performed by the message encoder depends upon the specific type of encoding the IMD(s) can understand, e.g., on-off keying, frequency-shift keying, frequency modulation, or amplitude shift keying, but not limited thereto. The amplifier  736  is coupled to the encoder/modulator  730  to increase amplitudes of pulses included in a conductive communication signals to a level sufficient to enable one or more IMD(s) to receive conductive communication signals from the external device  602 . 
     The controller  712  may receive ECG data and optionally displays an ECG using the display  716  and can also display information included in other data acquired from the implanted IMD(s) acquired through the encoded pulses included in conductive communication signals, such as battery voltage, sensed cardiac signal amplitude, or other system status information. The controller  712  also can accept input from a user via a user interface  718 , which can include, e.g., a keyboard and/or touch-screen, but is not limited thereto. The controller  712  can also communicate over a network interface  720  to other data entry or display units, such as a handheld computer or laptop/desktop unit. The network interface  720  can be cabled or wireless and can also enable communication to a local area network or the Internet for greater connectivity. More specifically, the network interface  720  can be used to send ECG data, diagnostic data, and other types of data collected from one or more IMD(s) to a patient care network associated with a medical group and/or facility. For more specific examples, the network interface can include a Bluetooth antenna, a WiFi antenna, and/or an Ethernet connection, but is not limited thereto. 
     The controller  712 , which can include one or more processors, and/or the like, can execute operations based on firmware stored in non-volatile memory (Flash). The non-volatile memory can also be used to store parameters or values that are to be maintained when power is removed. The controller  712  can use volatile memory or random access memory (RAM) as general storage for information such as ECG data, status information, swap memory, and other data. 
     The external device  602  can include or be coupled to more than three external electrodes  615 . For example, the external device  602  can included or be coupled to four, five, or six external electrodes  615 , but not limited thereto, wherein the greater the number of external electrodes the greater the number of potential communication vectors to test and select from. 
     The external electrodes (e.g.,  615 ) of an external device (e.g.,  602 ) described herein can be used to sense ECG signals, as well as sense conductive communication signals output by one or more IMDs. It is also possible for the external electrodes of an external device to be used to receive electrogram (EGM) signal data included in conductive communication signals output by one or more IMDs, which EGM signal data can be received by the external device (using the external electrodes) and used to reproduce one or more electrogram signals that were sensed by one or more IMDs, wherein an EGM signal can also be referred to as an intracardiac electrogram (IEGM) signal. In addition to being able to communicate with one or more IMDs via conductive communication, the external device  602  can optionally have an antenna and RF communication capabilities that enable the external device  602  to wirelessly communicate with an implantable device, such as the ICM  104 , via a wireless communication protocol, examples of which were discussed above. It would also be possible for the external device  602  to also include an inductive coil that enables the external device to perform inductive communication with an IMD that has such a capability. 
     The external device  602  can take many physical forms, but fundamentally it should be able to establish a conductive communication vector with the patient so that it can detect one or more IMDs&#39; conductively communicated transmissions, decipher the communication protocol utilized by the IMD(s), and upload any acquired follow-up information to a patient care network, such as the Merlin.net™ patient care network operated by Abbott Laboratories (headquartered in the Abbott Park Business Center in Lake Bluff, Ill.). 
     For example, where the external device  602  has or is communicatively coupled to three external electrodes  615   a ,  615   b  and  615   c , which can be referred to respectively as first, second, and third external electrodes, the external device can test and select among first, second, and third subsets of the external electrodes, wherein the first subset includes the first and second external electrodes (i.e.,  615   a  and  615   b ), the second subset includes the first and third external electrodes (i.e.,  615   a  and  615   c ), and the third subset includes the second and third external electrodes (i.e.,  615   b  and  615   c ). In accordance with certain embodiments of the present technology, which are described below, the external device  602  can identify which one of a plurality of the subsets, or more generally, which one of the plurality of possible communication vectors, is a preferred communication vector for communicating with an IMD. Further, as will be described in additional details below, where multiple IMDs are implanted within a patient, the external device can determine that different communication vectors are preferred for different IMDs. However, it is also possible that the external device may determine that a same communication vector is preferred for communicating with two or more different IMDs. 
     The high level flow diagram of  FIG. 8  will now be used to summarize certain methods for use by an external device that is configured to communicate with an IMD implanted within a patient using conductive communication, wherein the external device includes or is communicatively coupled to at least three external electrodes (e.g., external electrodes  615   a ,  615   b  and  615   c ) that are in contact with the patient. Referring to  FIG. 8 , step  802  involves determining a respective indicator of conductive communication quality for each communication vector, of a plurality of communication vectors that can be used to communicate with the IMD, wherein each of the plurality of communication vectors comprises a different combination (e.g., pair) of the at least three external electrodes that are in contact with the patient. Step  804  involves identifying which one of the plurality of communication vectors is a preferred communication vector for communicating with the IMD, based on the respective indicators of conductive communication quality that are determined for the plurality of communication vectors. Step  806  involves communicating with the IMD using the preferred communication vector for communicating with the IMD, after the preferred communication vector is identified, at step  804 . 
     In accordance with certain embodiments, step  802  involves, for each communication vector, determining a plurality of different measures of conductive communication quality and/or surrogates thereof for the communication vector, and combining the plurality of different measures to produce the respective indicator of conductive communication quality for the communication vector. When combining the various different measures and/or surrogates thereof, the different measures and/or surrogates thereof can be equally or non-equally weighted, depending upon the specific implementation. Example measures of conductive communication quality and/or surrogates thereof that can be determined for a communication vector include, but are not limited to: a noise floor associated with the communication vector, a measure of amplitude of at least a portion of a conductive communication signal received by the external device from the IMD using the communication vector, a measure of amplitude of at least a portion of a conductive communication signal received by the IMD from the external device, a magnitude of at least a portion of a conductive communication signal received by the external device from the IMD after rectification and integration thereof, a magnitude of at least a portion of a conductive communication signal received by the IMD from the external device after rectification and integration thereof, a signal-to-noise ratio (SNR) of at least a portion of a conductive communication signal received by the external device from the IMD, a SNR of at least a portion of a conductive communication signal received by the IMD from the external device, a total energy of at least a portion of a conductive communication signal received by the external device from the IMD after rectification and integration thereof, a total energy of at least a portion of a conductive communication signal received by the IMD from the external device after rectification and integration thereof, a bit-error-rate (BER) associated with at least a portion of a conductive communication signal received by the external device from the IMD, and a BER associated with at least a portion of a conductive communication signal received by the IMD from the external device. Other variations are also possible and within the scope of the embodiments described herein. 
     In order for one or more measures of the quality of conductive communication signals received by an IMD from the external device to be used by the external device to identify a preferred communication vector for the external device to use for communicating with the IMD, the IMD should provide such measure(s) to the external device so that external device has such measure(s) available for use in identifying the preferred communication vector. Alternatively, in certain embodiments, the external device only considers measures of the quality of conductive communication signals received by the external device from an IMD when identifying the preferred communication vector that the external device should use for communicating with the IMD. 
     In accordance with certain embodiments, step  804  involves ranking the plurality of communication vectors, and identifying a highest ranked one of the plurality of communication vectors as the preferred communication vector for communicating with the IMD. Alternatively, step  804  can more simply involve identifying which one of the plurality of communication vectors has the highest indicator of conductive communication quality. 
     Still referring to  FIG. 8 , step  808  involves determining whether there should be a reassessment of which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD. Step  808  can be performed while or after communicating with the IMD using the preferred communication vector for communicating with the IMD. If the answer to the determination at step  806  is No, then flow returns to step  806 . If the answer to the determination at step  806  is Yes, then flow returns to step  802 , and steps  802  and  804  are performed again. 
     In accordance with certain embodiments, step  808  can involve determining whether an indicator of conductive communication quality associated with the preferred communication vector for communicating with the IMD has fallen below a corresponding threshold. If the answer is No then flow returns to step  806 , and if the answer is Yes then flow returns to step  802 . Alternatively, or additionally, step  808  can involve determining whether the external device has lost conductive communication with the IMD. Alternatively, or additionally, step  808  can involve determining whether a specified period of time has elapsed since the preferred communication vector for communicating with the IMD was most recently identified. If the answer is No then flow returns to step  806 , and if the answer is Yes then flow returns to step  802 . 
     In accordance with certain embodiments, instructions can be provided to a user of the external device to modify at least one of where or how one or more of the at least three external electrodes contact the patient, in response to determining that all of the indicators of conductive communication quality for communicating with the IMD, which are determined for the plurality of communication vectors, are below a corresponding threshold. Such instructions can be provided via a display of the external device, and/or may be auditory type instructions. Other variations are also possible. 
     If a plurality of IMDs that are configured to perform conductive communication are implanted within the patient, the external device can perform the steps described above with reference to  FIG. 8  for each of the plurality of IMDs, such that a respective preferred communication vector is separately identified for each of the plurality of IMDs that are configured to perform conductive communication. For example, steps  802 ,  804 , and  806  can initially be performed by the external device for a first IMD, then steps  802 ,  804 , and  806  can be performed by the external device for a second IMD, etc. Alternatively, step  802  can be performed by the external device for each of a plurality of IMDs (e.g., a first IMD, a second IMD, etc.), then step  804  can be performed by the external device for each of a plurality of IMDs (e.g., the first IMD, the second IMD, etc.), and then step  806  can be performed by the external device for each of a plurality of IMDs (e.g., the first IMD, the second IMD, etc.). Other variations are also possible and within the scope of the embodiments described herein. 
     In alternative embodiments, where a plurality of IMDs that are configured to perform conductive communication are implanted within a patient, rather than an external device identifying a separate preferred communication vector for each of the plurality of IMDs, the external device can instead identify a universally preferred communication vector. In certain such embodiments, the external device can perform step  802  in  FIG. 8  for each of the plurality of IMDs, such that for each of the IMDs a respective indicator of conductive communication quality is determined for each communication vector, of the plurality of communication vectors that can be used to communicate with the IMD. Step  804  can be performed collectively for the plurality of IMDs to thereby identify one preferred communication vector for communicating with the plurality of IMDs. Then, after the one preferred communication vector is identified at step  804 , step  806  can involve communicating with the plurality of IMDs using the one preferred communication vector for communicating with the IMDs. The one preferred communication vector can also be referred to as a universally preferred communication vector. An advantage of identifying and using a universally preferred communication vector is that switching between different communication vectors need not be performed each time the external device wants or needs to communicate with a different one of the IMDs. 
     Where multiple IMDs are synchronized with one another or to a common reference, it may be viable for the external device to switch between different communication vectors each time the external device attempts to communicate with a different one of the IMDs. However, where multiple IMDs operate asynchronously, it may be difficult or impossible for an external device to attempt to communicate with different ones of the IMDs using different preferred communication vectors. Accordingly, where multiple IMDs operate asynchronously, it would likely be more viable for an external device to communicate with the multiple IMDs using a universally preferred communication vector. Such a universally preferred communication vector may not (and will likely not) provide for the highest available level of communication quality for each of the multiple IMDs, but should provide at least a minimally acceptable level of communication quality for each of the multiple IMDs. For example, there can be a minimal acceptable level of communication quality that the external device can use to successfully communicate with an IMD using conductive communication quality. The universally preferred communication vector should provide at least this minimally acceptable level of communication quality for each of the IMDs with which the external device is to conductively communicate. 
     For a specific example, assume an external device is to communicate with a first IMD (i.e., IMD1) and a second IMD (i.e., IMD2). Also assume that there are three different communication vectors that the external device can select, which can be referred to as Vector A, Vector B, and Vector C. Also assume for simplicity that indicators of conductive communication quality are specified as values between 0 and 10, with 0 being the worst and 10 being the best. An example of this is shown in Table 1 below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Vector A 
                 Vector B 
                 Vector C 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 IMD 1 
                 10 
                 4 
                 0 
               
               
                   
                 IMD 2 
                 6 
                 8 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     Assuming that the minimally acceptable level of communication quality corresponds to a value of 5, it can be appreciated from Table 1 that the external device would select Vector A as the universally preferred communication vector, even though vector B provide for better communication quality for the IMD2. Where there are multiple different vectors that can provide the minimally acceptable level of communication quality for all the IMDs that the external device is to communicate with, the external device may identify as the universally preferred vector the specific one of the vectors that provides for the highest level of communication quality among the multiple viable vectors. In a specific embodiment, where there are multiple different vectors that can provide the minimally acceptable level of communication quality for all the IMDs that the external device is to communicate with, the external device may identify as the universally preferred vector the specific one of the vectors that provides for the highest sum or average of the values of communication quality. 
     The high level flow diagram of  FIG. 9  will now be used to describe how communication vectors can be tested as part of, and more specifically at the beginning of, an interrogation of one or more IMDs by an external device. Such an interrogation can be performed, for example, by an external programmer when a patient visits a clinic for the purpose of uploading data that has been stored within the IMD(s) and/or reprogramming the IMD(s), but not limited thereto. It would also be possible that the interrogation is performed by a remote monitor that is not capable of programming the IMD(s), for the purpose of uploading data and/or notification that have been stored within the IMD(s). Other variations are also possible and within the scope of the embodiments described herein. 
     Referring to  FIG. 9 , at step  902  an interrogation of one or more IMDs by an external device is started. At step  904  the external device configures a communication vector to use for performing conductive communication with an IMD, which can involve coupling two of three or more external electrodes (e.g.,  615   a ,  615   b , and  615   c ) to a conductive communication receiver (e.g.,  120  or  742 ). Step  904  can be performed by controlling switches, such as the switches  713  in  FIG. 7 , to control which electrodes are coupled to the conductive communication receiver of the external device. 
     At step  906  an IMD search is started, which can involve monitoring for sniff pulses from an IMD. At step  908  there is a determination of whether an IMD was found, which can involve determining whether sniff pulses were detected from an IMD. If the answer to the determination at step  908  is Yes (i.e., if an IMD was found), then one or more measures of communication quality can be determined and stored (e.g., in memory of the external device) at step  910 . In certain embodiments, the measures of communication quality are from the perspective of the external device, meaning the measures are indicative of the quality of signals (e.g., sniff signals, but not limited thereto) received by the external device from an IMD. Alternatively, or additionally, the measures of communication quality can be from the perspective of the IMD, meaning the measures can be indicative of the quality of signals received by an IMD from the external device. Examples of measures of communication quality, or surrogates thereof, that can be measured and stored at instances of step  910  were discussed above with reference to step  802  of  FIG. 8 , and thus, need not be described again. 
     After an instance of step  910 , or when the answer to the determination at step  908  is No, flow goes to step  912 . At step  912  there is a determination as to whether a search for an IMD has timed out. Step  912  can be performed, for example, using a countdown or count-up timer, but is not limited thereto. If the answer to the determination at step  912  is No, then flow returns to step  908 . If the answer to the determination at step  912  is Yes, then flow goes to step  914 . 
     At step  914  there is a determination of whether all possible communication vectors have been tested. For example, if there are three total external electrodes that include first, second, and third electrodes, then there are three communication vectors that should be tested, including a first communication vector made up of the first and second electrodes, a second communication vector made up of the first and third electrodes, and a third communication vector made of the second and third electrodes. For another example, if there are four external electrodes, then there are six different communication vectors that can be tested, assuming each communication vector includes a different combination of two of the four external electrodes. For still another example, if there are five external electrodes, then there are ten different communication vectors that can be tested, assuming each communication vector includes a different combination of two of the five external electrodes. More generally, the total number (T) of different communication vectors to test can be determined using the equation T=n! /(2! (n−2)!), where n is the total number of external electrode, and the exclamation mark ! represents a factorial. If the answer to the determination at step  914  is No, then flow returns to step  904  and a different one of the total number T of different communication vectors is configured at  904 . When the answer to the determination at step  914  is Yes, then flow goes to step  916 . 
     At step  916  the plurality of communication vectors that were tested are ranked from best to worst. At step  918  there is a determination of whether at least one of the communication vectors tested for an IMD was successful at conductively communicating with the IMD. In certain embodiments, step  918  can involve determining whether a sniff pulse was successfully detected from the IMD. Additionally, or alternatively, step  918  can involve determining whether at least some threshold level of communication quality was achieved. If the answer to the determination at step  918  is Yes, then flow goes to step  920 . If the answer to the determination at step  918  is No, then flow goes to step  922 . 
     At step  920  a preferred communication vector for the IMD is identified. This can involve identifying the highest ranking communication vector for the IMD, following the ranking performed at step  916 . It is also possible to eliminate step  916 , and at step  920  identify the communication vector that provided for the highest communication quality. 
     At step  922  that is a determination that conductive communication was unable to be established using any of the possible communication vectors. In certain embodiments, at or following step  922 , instructions can be provided to a user of the external device to modify at least one of where or how one or more of the external electrodes contact the patient. 
     At step  924  there is a determination of whether communication vectors for all of the IMDs have been ranked. If the answer to the determination at step  924  is No, then flow returns to step  916  and step  916  and the following steps are repeated for another IMD. If the answer to the determination at step  924  is Yes, then flow goes to step  926 . At step  926  the conductive communication status for each of the IMDs is displayed to the user of the external device. As indicated at step  928 , the interrogation is continued. This can include, for example, using the preferred conductive communication vector identified at step  920  for each IMD (of one or more IMDs) to upload information from the IMD and/or to program or reprogram the IMD. 
     As was explained above with reference to step  808  in  FIG. 8 , in accordance with certain embodiments, after a preferred conductive communication vector has been identified and used for communicating with an IMD, there may be a reassessment of which one of the plurality of communication vectors is the preferred communication vector for communicating with the IMD. As was also explained above, this can involve determining whether an indicator of conductive communication quality associated with the preferred communication vector for communicating with the IMD falls below a corresponding threshold, or whether conductive communication between the external device and the IMD was lost. Additional details of when and how such a reassessment may occur, in accordance with certain embodiments of the present technology, will now be described below with reference to  FIG. 10 . 
     Referring to  FIG. 10 , following a start at step  1002  there is a determination at step  1004  of whether the conductive communication quality associated with the preferred communication vector for communicating with an IMD has fallen below a corresponding threshold. If the answer to the determination at step  1004  is No, then there is no change to what is considered the preferred communication vector for communicating with the IMD, as indicated at step  1006 , and then flow returns to step  1004 . If the answer to the determination at step  1004  is Yes, then flow goes to step  1008 . It is noted that if there is a loss of conductive communication between the external device and an IMD, the answer to the determination at step  1004  will be Yes. 
     At step  1008 , a list of possible communication vectors to test for the IMD is set or reset. At step  1010  (which is similar to step  904  described above with reference to  FIG. 9 ), the external device configures a communication vector to use for performing conductive communication with an IMD, which can involve coupling two of three or more external electrodes (e.g.,  615   a ,  615   b , and  615   c ) to a conductive communication receiver (e.g.,  120  or  742 ). Step  1010  can be performed by controlling switches, such as the switches  713  in  FIG. 7 , to control which electrodes are coupled to the conductive communication receiver of the external device. 
     At step  1012  there is a determination of whether the IMD with which the external device is attempting to communicate (which can be referred to as the target IMD) was detected. This step can involve determining whether any communication pulses, such as sniff pulses, were detected by the external device from the target IMD. If the answer to the determination at step  1012  is No then flow goes to step  1014 . At step  1014  (which is similar to step  912  described above with reference to  FIG. 9 ) there is a determination as to whether a search for the IMD has timed out. Step  1014  can be performed, for example, using a countdown or count-up timer, but is not limited thereto. If the answer to the determination at step  1014  is No, then flow returns to step  1012 . If the answer to the determination at step  1014  is Yes, then flow goes to step  1022 . 
     Returning to step  1012 , if the answer to the determination at step  1012  is Yes, then flow goes to step  1016 . At step  1016  there is a determination of whether a valid message is received from the IMD. The external device can use cyclic redundancy check (CRC) or some other type of error detection and correction scheme to determine whether a message the external device receives from the IMD is a valid message or an invalid message. If there answer to the determination at step  1016  is No, then it is determined that the external device was unsuccessful at communicating with the target IMD as indicated at step  1018 , and then flow goes to step  1014 , which was discussed above. If the answer to step  1014  is Yes, then flow goes to step  1022 . 
     Returning to step  1016 , if the answer to the determination at step  1016  is Yes, then flow goes to step  1020 . At step  1020  (which is similar to step  910  described above with reference to  FIG. 9 ), one or more measures of communication quality are determined and stored (e.g., in memory of the external device). In certain embodiments, the measures of communication quality are from the perspective of the external device, meaning the measures are indicative of the quality of signals received by the external device from an IMD. Alternatively, or additionally, the measures of communication quality can be from the perspective of the IMD, meaning the measures can be indicative of the quality of signals received by an IMD from the external device. Examples of measures of communication quality, or surrogates thereof, that can be measured and stored at instances of step  1020  were discussed above with reference to step  802  of  FIG. 8 , and thus, need not be described again. 
     At step  1022  (which is similar to step  914  described above with reference to  FIG. 9 ), there is a determination of whether all possible communication vectors have been tested. For example, if there are three total external electrodes that include first, second, and third electrodes, then there are three communication vectors that should be tested, including a first communication vector made up of the first and second electrodes, a second communication vector made up of the first and third electrodes, and a third communication vector made of the second and third electrodes. More generally, the total number (T) of different communication vectors to test can be determined using the equation T=n! /(2! (n−2)!), where n is the total number of external electrode, and the exclamation mark ! represents a factorial. If the answer to the determination at step  1022  is No, then flow returns to step  1010  and a different one of the total number T of different communication vectors is configured at  1010 . When the answer to the determination at step  1022  is Yes, then flow goes to step  1024 . 
     At step  1024  (which is similar to step  916  described above with reference to  FIG. 9 ) the plurality of communication vectors that were tested are ranked from best to worst. At step  1026  there is a determination of whether at least one of the communication vectors tested for an IMD was successful at conductively communicating with the IMD. In certain embodiments, step  1026  can involve determining whether a sniff pulse was successfully detected from the IMD. Additionally, or alternatively, step  1026  can involve determining whether at least some threshold level of communication quality was achieved. If the answer to the determination at step  1026  is Yes, then flow goes to step  1028 . If the answer to the determination at step  1026  is No, then flow goes to step  1030 . 
     At step  1028  a preferred communication vector for the IMD is identified. This can involve identifying the highest ranking communication vector for the IMD, following the ranking performed at step  1024 . It is also possible to eliminate step  1024 , and at step  1028  identify the communication vector that provided for the best communication quality. At step  1030  there is a determination that conductive communication was unable to be reestablished using any of the possible communication vectors, and thus, the conductive communication with the target IMD was lost (or continues to be lost). In certain embodiments, at or following step  1030 , instructions can be provided to a user of the external device to modify at least one of where or how one or more of the external electrodes contact the patient. 
       FIG. 11  includes a timing diagram that shows how an external device can conductively communicate with a first IMD (IMD 1 ) during a first period of time using a preferred communication vector identified by the external device for communicating with the first IMD.  FIG. 11  also shows that the external device can thereafter conductively communicate with a second IMD (IMD 2 ) during a second period of time using a preferred communication vector identified by the external device for communicating with the second IMD. Such preferred communication vectors can be identified using one of the embodiments described above, e.g., with reference to  FIG. 8  and  FIG. 9 . Depending upon the positions of the IMDs relative to the external electrodes, the preferred conductive communication vector for communicating with the second IMD can be different (or potentially the same) as the preferred conductive communication vector for communicating with the first IMD. 
       FIG. 12  includes a timing diagram that shows that during a first period of time, during which the target IMD is a first IMD (IMD 1 ), the external device communicates with the first IMD using a communication vector that includes a third electrode (Electrode 3 ) and a second electrode (Electrode 2 ).  FIG. 12  also shows that during a second period of time, during which the target IMD is a second IMD (IMD 2 ), the external device communicates with the second IMD using a communication vector that includes the second electrode (Electrode 2 ) and a first electrode (Electrodes). For this example, it is assumed that the conductive communication transceiver, which includes a conductive communication receiver, has differential inputs, including a positive (+) input and a negative (−) input.  FIG. 12  also shows that communication with each target device (e.g., IMD 1  or IMD 2 ) can include one or more communication frames/packets/bursts. 
     In accordance with certain embodiments of the present technology, an external device can perform the steps described below with reference to  FIG. 13  to identify a preferred communication vector, and receive a notification sequence, or more generally a conductive communication signal, from an implanted IMD (e.g., an LP) using the identified preferred communication vector. For the embodiment described below with reference to  FIG. 13 , it is assumed that three electrodes are in contact with the patient, wherein the three electrodes can be referred to as first, second, and third electrodes. Where a communication vector is being used to receive a conductive communication signal, the communication vector can be referred to as a sensing vector. 
       FIG. 13  is a high level flow diagram that is used to summarize a specific method in which an external device can select a preferred communication vector (e.g., sensing vector), from among three or more external electrodes, for receiving conductive communication signals from an implanted IMD. Step  1302  involves the external device monitoring for the advertisement sequence of pulses using first, second, and third subsets of the external electrodes, the first subset including the first and second external electrodes, the second subset including the first and third external electrodes, and the third subset including the second and third external electrodes. For example, at step  1302  the external device monitors for conductive communication pulses (output by an implanted LP, or other type of IMD) using a plurality of different communication vectors. The advertisement sequence of pulses can also be referred to as a sniff sequence of pulses, or more succinctly as a sniff, as was noted above. 
     At step  1304 , the external device measures for each subset of the external electrodes, of the first, second, and third subsets, a respective metric indicative of power and/or quality of a communication signal received from the LP using the subset of electrodes. More generally, at step  1302  the external device determines a metric of power and/or quality for each of the plurality of different communication vectors. 
     At step  1306 , the external device identifies, based on the results of step  1304 , a preferred one of the first, second, and third subsets of the external electrodes. More generally, at step  1306  the external device selects a preferred communication vector, based on the results of step  1304 . 
     At step  1308 , the external device uses the preferred one of the first, second, and third subsets of the external electrodes, which was identified at step  1306 , to receive the notification sequence of pulses from the LP. More generally, at step  1308  the external device uses the identified preferred communication vector to receive one or more conductive communication signals from an LP. 
     At step  1310 , the external device stores within memory of the external device and/or transmits to a patient care network, raw data associated with the notification sequence of pulses and/or information decoded from the notification sequence of pulses received from the LP using the preferred one of the first, second, and third subsets of the external electrodes. More generally, at step  1310  the external device stores and/or forwards data it obtained from one or more conductive communication signals received from an implanted LP using the identified preferred communication vector. 
     In accordance with certain alternative embodiments, rather than an external device identifying a preferred communication vector for communicating with each of one or more IMDs, and then using the preferred communication vector(s) for communicating with the IMD(s), the external device can from time to time change what communication vector is uses for communicating with IMD(s), e.g., in a round robin manner, using even or odd skipping, etc. This may result in the external device from time to time being unable to communicate with one or more IMDs. However, over time, as the communication vectors that are used for performing conductive communication are changed, the external device should be able to successfully communicate with the IMD(s). 
     It is to be understood that the subject matter described herein is not limited in its application to the details of construction and the arrangement of components set forth in the description herein or illustrated in the drawings hereof. The subject matter described herein is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Further, it is noted that the term “based on” as used herein, unless stated otherwise, should be interpreted as meaning based at least in part on, meaning there can be one or more additional factors upon which a decision or the like is made. For example, if a decision is based on the results of a comparison, that decision can also be based on one or more other factors in addition to being based on results of the comparison. 
     Embodiments have been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have often been defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. For example, it would be possible to combine or separate some of the steps shown in the various flow diagrams. It would also be possible to just perform a subset of the steps shown in the various flow diagrams. For another example, it is possible to change the boundaries of some of the block diagrams. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments without departing from its scope. While the dimensions, types of materials and coatings described herein are intended to define the parameters of the embodiments of the present technology, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the embodiments of the present technology should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means—plus-function format and are not intended to be interpreted based on 35 U.S.C. § 112(f), unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.