Method and apparatus for selecting a sensing vector configuration in a medical device

A method and medical device for determining sensing vectors that includes sensing cardiac signals from a plurality of electrodes, the plurality of electrodes forming a plurality of sensing vectors, determining a sensing vector metric in response to the sensed cardiac signals, determining a morphology metric associated with a morphology of the sensed cardiac signals, determining vector selection metrics in response to the determined sensing vector metric and the determined morphology setting, and selecting a sensing vector of the plurality of sensing vectors in response to the determined vector selection metrics.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, in particular, to an apparatus and method for selecting a sensing vector in a medical device.

BACKGROUND

Implantable medical devices are available for preventing or treating cardiac arrhythmias by delivering anti-tachycardia pacing therapies and electrical shock therapies for cardioverting or defibrillating the heart. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to a number of rate zones in order to detect episodes of tachycardia or fibrillation.

Upon detecting an abnormal rhythm, the ICD delivers an appropriate therapy. Pathologic forms of ventricular tachycardia can often be terminated by anti-tachycardia pacing therapies. Anti-tachycardia pacing therapies are followed by high-energy shock therapy when necessary. Termination of a tachycardia by a shock therapy is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a form of tachycardia that is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is commonly referred to as “defibrillation.” Accurate arrhythmia detection and discrimination are important in selecting the appropriate therapy for effectively treating an arrhythmia and avoiding the delivery of unnecessary cardioversion/defibrillation (CV/DF) shocks, which are painful to the patient.

In past practice, ICD systems have employed intra-cardiac electrodes carried by transvenous leads for sensing cardiac electrical signals and delivering electrical therapies. Emerging ICD systems are adapted for subcutaneous or submuscular implantation and employ electrodes incorporated on the ICD housing and/or carried by subcutaneous or submuscular leads. These systems, referred to generally herein as “subcutaneous ICD” or “SubQ ICD” systems, do not rely on electrodes implanted in direct contact with the heart. SubQ ICD systems are less invasive and are therefore implanted more easily and quickly than ICD systems that employ intra-cardiac electrodes. However, greater challenges exist in reliably detecting cardiac arrhythmias using a subcutaneous system. The R-wave amplitude on a SubQ ECG signal may be on the order of one-tenth to one-one hundredth of the amplitude of intra-ventricular sensed R-waves. Furthermore, the signal quality of subcutaneously sensed ECG signals are likely to be more affected by myopotential noise, environmental noise, patient posture and patient activity than intra-cardiac myocardial electrogram (EGM) signals.

The ability of a subcutaneous ICD to detect tachyarrhythmias and reject noise depends on its ECG signal characteristics. ECG vectors with higher amplitude R-wave waves, higher frequency (high slew rate) R-waves, higher R/T wave ratios, lower frequency signal (e.g., P and T waves) around R-waves, lower susceptibility to skeletal myopotentials, and greater R-wave consistency from cycle to cycle are preferred to ECG vectors without these attributes. A subcutaneous ICD with a minimum of 2 ECG leads or vectors (using a minimum of 3 electrodes) in a plane may use these physical vectors to generate virtual ECG vectors using a linear combination of the physical vector ECGs. However, choosing the optimal vector may sometimes be a challenge given the changing environment of a subcutaneous system. As such, systems and methods that promote reliable and accurate sensing detection of arrhythmias using optimal available sensing vectors to sense ECG signals via subcutaneous electrodes are needed.

DETAILED DESCRIPTION

FIG. 1is a conceptual diagram of a patient12implanted with an example extravascular cardiac defibrillation system10. In the example illustrated inFIG. 1, extravascular cardiac defibrillation system10is an implanted subcutaneous ICD system. However, the techniques of this disclosure may also be utilized with other extravascular implanted cardiac defibrillation systems, such as a cardiac defibrillation system having a lead implanted at least partially in a substernal or submuscular location. Additionally, the techniques of this disclosure may also be utilized with other implantable systems, such as implantable pacing systems, implantable neurostimulation systems, drug delivery systems or other systems in which leads, catheters or other components are implanted at extravascular locations within patient12. This disclosure, however, is described in the context of an implantable extravascular cardiac defibrillation system for purposes of illustration.

Extravascular cardiac defibrillation system10includes an implantable cardioverter defibrillator (ICD)14connected to at least one implantable cardiac defibrillation lead16. ICD14ofFIG. 1is implanted subcutaneously on the left side of patient12. Defibrillation lead16, which is connected to ICD14, extends medially from ICD14toward sternum28and xiphoid process24of patient12. At a location near xiphoid process24, defibrillation lead16bends or turns and extends subcutaneously superior, substantially parallel to sternum28. In the example illustrated inFIG. 1, defibrillation lead16is implanted such that lead16is offset laterally to the left side of the body of sternum28(i.e., towards the left side of patient12).

Defibrillation lead16is placed along sternum28such that a therapy vector between defibrillation electrode18and a second electrode (such as a housing or can25of ICD14or an electrode placed on a second lead) is substantially across the ventricle of heart26. The therapy vector may, in one example, be viewed as a line that extends from a point on the defibrillation electrode18to a point on the housing or can25of ICD14. In another example, defibrillation lead16may be placed along sternum28such that a therapy vector between defibrillation electrode18and the housing or can25of ICD14(or other electrode) is substantially across an atrium of heart26. In this case, extravascular ICD system10may be used to provide atrial therapies, such as therapies to treat atrial fibrillation.

The embodiment illustrated inFIG. 1is an example configuration of an extravascular ICD system10and should not be considered limiting of the techniques described herein. For example, although illustrated as being offset laterally from the midline of sternum28in the example ofFIG. 1, defibrillation lead16may be implanted such that lead16is offset to the right of sternum28or more centrally located over sternum28. Additionally, defibrillation lead16may be implanted such that it is not substantially parallel to sternum28, but instead offset from sternum28at an angle (e.g., angled lateral from sternum28at either the proximal or distal end). As another example, the distal end of defibrillation lead16may be positioned near the second or third rib of patient12. However, the distal end of defibrillation lead16may be positioned further superior or inferior depending on the location of ICD14, location of electrodes18,20, and22, or other factors.

Although ICD14is illustrated as being implanted near a midaxillary line of patient12, ICD14may also be implanted at other subcutaneous locations on patient12, such as further posterior on the torso toward the posterior axillary line, further anterior on the torso toward the anterior axillary line, in a pectoral region, or at other locations of patient12. In instances in which ICD14is implanted pectorally, lead16would follow a different path, e.g., across the upper chest area and inferior along sternum28. When the ICD14is implanted in the pectoral region, the extravascular ICD system may include a second lead including a defibrillation electrode that extends along the left side of the patient such that the defibrillation electrode of the second lead is located along the left side of the patient to function as an anode or cathode of the therapy vector of such an ICD system.

ICD14includes a housing or can25that forms a hermetic seal that protects components within ICD14. The housing25of ICD14may be formed of a conductive material, such as titanium or other biocompatible conductive material or a combination of conductive and non-conductive materials. In some instances, the housing25of ICD14functions as an electrode (referred to as a housing electrode or can electrode) that is used in combination with one of electrodes18,20, or22to deliver a therapy to heart26or to sense electrical activity of heart26. ICD14may also include a connector assembly (sometimes referred to as a connector block or header) that includes electrical feedthroughs through which electrical connections are made between conductors within defibrillation lead16and electronic components included within the housing. Housing may enclose one or more components, including processors, memories, transmitters, receivers, sensors, sensing circuitry, therapy circuitry and other appropriate components (often referred to herein as modules).

Defibrillation lead16includes a lead body having a proximal end that includes a connector configured to connect to ICD14and a distal end that includes one or more electrodes18,20, and22. The lead body of defibrillation lead16may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. However, the techniques are not limited to such constructions. Although defibrillation lead16is illustrated as including three electrodes18,20and22, defibrillation lead16may include more or fewer electrodes.

Defibrillation lead16includes one or more elongated electrical conductors (not illustrated) that extend within the lead body from the connector on the proximal end of defibrillation lead16to electrodes18,20and22. In other words, each of the one or more elongated electrical conductors contained within the lead body of defibrillation lead16may engage with respective ones of electrodes18,20and22. When the connector at the proximal end of defibrillation lead16is connected to ICD14, the respective conductors may electrically couple to circuitry, such as a therapy module or a sensing module, of ICD14via connections in connector assembly, including associated feedthroughs. The electrical conductors transmit therapy from a therapy module within ICD14to one or more of electrodes18,20and22and transmit sensed electrical signals from one or more of electrodes18,20and22to the sensing module within ICD14.

ICD14may sense electrical activity of heart26via one or more sensing vectors that include combinations of electrodes20and22and the housing or can25of ICD14. For example, ICD14may obtain electrical signals sensed using a sensing vector between electrodes20and22, obtain electrical signals sensed using a sensing vector between electrode20and the conductive housing or can25of ICD14, obtain electrical signals sensed using a sensing vector between electrode22and the conductive housing or can25of ICD14, or a combination thereof. In some instances, ICD14may sense cardiac electrical signals using a sensing vector that includes defibrillation electrode18, such as a sensing vector between defibrillation electrode18and one of electrodes20or22, or a sensing vector between defibrillation electrode18and the housing or can25of ICD14.

ICD may analyze the sensed electrical signals to detect tachycardia, such as ventricular tachycardia or ventricular fibrillation, and in response to detecting tachycardia may generate and deliver an electrical therapy to heart26. For example, ICD14may deliver one or more defibrillation shocks via a therapy vector that includes defibrillation electrode18of defibrillation lead16and the housing or can25. Defibrillation electrode18may, for example, be an elongated coil electrode or other type of electrode. In some instances, ICD14may deliver one or more pacing therapies prior to or after delivery of the defibrillation shock, such as anti-tachycardia pacing (ATP) or post shock pacing. In these instances, ICD14may generate and deliver pacing pulses via therapy vectors that include one or both of electrodes20and22and/or the housing or can25. Electrodes20and22may comprise ring electrodes, hemispherical electrodes, coil electrodes, helix electrodes, segmented electrodes, directional electrodes, or other types of electrodes, or combination thereof. Electrodes20and22may be the same type of electrodes or different types of electrodes, although in the example ofFIG. 1both electrodes20and22are illustrated as ring electrodes.

Defibrillation lead16may also include an attachment feature29at or toward the distal end of lead16. The attachment feature29may be a loop, link, or other attachment feature. For example, attachment feature29may be a loop formed by a suture. As another example, attachment feature29may be a loop, link, ring of metal, coated metal or a polymer. The attachment feature29may be formed into any of a number of shapes with uniform or varying thickness and varying dimensions. Attachment feature29may be integral to the lead or may be added by the user prior to implantation. Attachment feature29may be useful to aid in implantation of lead16and/or for securing lead16to a desired implant location. In some instances, defibrillation lead16may include a fixation mechanism in addition to or instead of the attachment feature. Although defibrillation lead16is illustrated with an attachment feature29, in other examples lead16may not include an attachment feature29.

Lead16may also include a connector at the proximal end of lead16, such as a DF4 connector, bifurcated connector (e.g., DF-1/IS-1 connector), or other type of connector. The connector at the proximal end of lead16may include a terminal pin that couples to a port within the connector assembly of ICD14. In some instances, lead16may include an attachment feature at the proximal end of lead16that may be coupled to an implant tool to aid in implantation of lead16. The attachment feature at the proximal end of the lead may separate from the connector and may be either integral to the lead or added by the user prior to implantation.

Defibrillation lead16may also include a suture sleeve or other fixation mechanism (not shown) located proximal to electrode22that is configured to fixate lead16near the xiphoid process or lower sternum location. The fixation mechanism (e.g., suture sleeve or other mechanism) may be integral to the lead or may be added by the user prior to implantation.

The example illustrated inFIG. 1is exemplary in nature and should not be considered limiting of the techniques described in this disclosure. For instance, extravascular cardiac defibrillation system10may include more than one lead. In one example, extravascular cardiac defibrillation system10may include a pacing lead in addition to defibrillation lead16.

In the example illustrated inFIG. 1, defibrillation lead16is implanted subcutaneously, e.g., between the skin and the ribs or sternum. In other instances, defibrillation lead16(and/or the optional pacing lead) may be implanted at other extravascular locations. In one example, defibrillation lead16may be implanted at least partially in a substernal location. In such a configuration, at least a portion of defibrillation lead16may be placed under or below the sternum in the mediastinum and, more particularly, in the anterior mediastinum. The anterior mediastinum is bounded laterally by pleurae, posteriorly by pericardium, and anteriorly by sternum28. Defibrillation lead16may be at least partially implanted in other extra-pericardial locations, i.e., locations in the region around, but not in direct contact with, the outer surface of heart26. These other extra-pericardial locations may include in the mediastinum but offset from sternum28, in the superior mediastinum, in the middle mediastinum, in the posterior mediastinum, in the sub-xiphoid or inferior xiphoid area, near the apex of the heart, or other location not in direct contact with heart26and not subcutaneous. In still further instances, the lead may be implanted at a pericardial or epicardial location outside of the heart26.

FIG. 2is an exemplary schematic diagram of electronic circuitry within a hermetically sealed housing of a subcutaneous device according to an embodiment of the present invention. As illustrated inFIG. 2, subcutaneous device14includes a low voltage battery153coupled to a power supply (not shown) that supplies power to the circuitry of the subcutaneous device14and the pacing output capacitors to supply pacing energy in a manner well known in the art. The low voltage battery153may be formed of one or two conventional LiCFx, LiMnO2or LiI2cells, for example. The subcutaneous device14also includes a high voltage battery112that may be formed of one or two conventional LiSVO or LiMnO2cells. Although two both low voltage battery and a high voltage battery are shown inFIG. 2, according to an embodiment of the present invention, the device14could utilize a single battery for both high and low voltage uses.

Further referring toFIG. 2, subcutaneous device14functions are controlled by means of software, firmware and hardware that cooperatively monitor the ECG signal, determine when a cardioversion-defibrillation shock or pacing is necessary, and deliver prescribed cardioversion-defibrillation and pacing therapies. The subcutaneous device14may incorporate circuitry set forth in commonly assigned U.S. Pat. No. 5,163,427 “Apparatus for Delivering Single and Multiple Cardioversion and Defibrillation Pulses” to Keimel and U.S. Pat. No. 5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” to Keimel for selectively delivering single phase, simultaneous biphasic and sequential biphasic cardioversion-defibrillation shocks typically employing ICD IPG housing electrodes28coupled to the COMMON output123of high voltage output circuit140and cardioversion-defibrillation electrode24disposed posterially and subcutaneously and coupled to the HVI output113of the high voltage output circuit140.

The cardioversion-defibrillation shock energy and capacitor charge voltages can be intermediate to those supplied by ICDs having at least one cardioversion-defibrillation electrode in contact with the heart and most AEDs having cardioversion-defibrillation electrodes in contact with the skin. The typical maximum voltage necessary for ICDs using most biphasic waveforms is approximately 750 Volts with an associated maximum energy of approximately 40 Joules. The typical maximum voltage necessary for AEDs is approximately 2000-5000 Volts with an associated maximum energy of approximately 200-360 Joules depending upon the model and waveform used. The subcutaneous device14of the present invention uses maximum voltages in the range of about 300 to approximately 1500 Volts and is associated with energies of approximately 25 to 150 joules or more. The total high voltage capacitance could range from about 50 to about 300 microfarads. Such cardioversion-defibrillation shocks are only delivered when a malignant tachyarrhythmia, e.g., ventricular fibrillation is detected through processing of the far field cardiac ECG employing the detection algorithms as described herein below.

InFIG. 2, sense amp190in conjunction with pacer/device timing circuit178processes the far field ECG sense signal that is developed across a particular ECG sense vector defined by a selected pair of the subcutaneous electrodes18,20,22and the can or housing25of the device14, or, optionally, a virtual signal (i.e., a mathematical combination of two vectors) if selected. For example, the device may generate a virtual vector signal as described in U.S. Pat. No. 6,505,067 “System and Method for Deriving Virtual ECG or EGM Signal” to Lee, et al; both patents incorporated herein by reference in their entireties. In addition, vector selection may be selected by the patient's physician and programmed via a telemetry link from a programmer.

The selection of the sensing electrode pair is made through the switch matrix/MUX191in a manner to provide the most reliable sensing of the ECG signal of interest, which would be the R wave for patients who are believed to be at risk of ventricular fibrillation leading to sudden death. The far field ECG signals are passed through the switch matrix/MUX191to the input of the sense amplifier190that, in conjunction with pacer/device timing circuit178, evaluates the sensed EGM. Bradycardia, or asystole, is typically determined by an escape interval timer within the pacer timing circuit178and/or the control circuit144. Pace Trigger signals are applied to the pacing pulse generator192generating pacing stimulation when the interval between successive R-waves exceeds the escape interval. Bradycardia pacing is often temporarily provided to maintain cardiac output after delivery of a cardioversion-defibrillation shock that may cause the heart to slowly beat as it recovers back to normal function. Sensing subcutaneous far field signals in the presence of noise may be aided by the use of appropriate denial and extensible accommodation periods as described in U.S. Pat. No. 6,236,882 “Noise Rejection for Monitoring ECGs” to Lee, et al and incorporated herein by reference in its' entirety.

Detection of a malignant tachyarrhythmia is determined in the Control circuit144as a function of the intervals between R-wave sense event signals that are output from the pacer/device timing178and sense amplifier circuit190to the timing and control circuit144. It should be noted that the present invention utilizes not only interval based signal analysis method but also supplemental sensors and morphology processing method and apparatus as described herein below.

Supplemental sensors such as tissue color, tissue oxygenation, respiration, patient activity and the like may be used to contribute to the decision to apply or withhold a defibrillation therapy as described generally in U.S. Pat. No. 5,464,434 “Medical Interventional Device Responsive to Sudden Hemodynamic Change” to Alt and incorporated herein by reference in its entirety. Sensor processing block194provides sensor data to microprocessor142via data bus146. Specifically, patient activity and/or posture may be determined by the apparatus and method as described in U.S. Pat. No. 5,593,431 “Medical Service Employing Multiple DC Accelerometers for Patient Activity and Posture Sensing and Method” to Sheldon and incorporated herein by reference in its entirety. Patient respiration may be determined by the apparatus and method as described in U.S. Pat. No. 4,567,892 “Implantable Cardiac Pacemaker” to Plicchi, et al and incorporated herein by reference in its entirety. Patient tissue oxygenation or tissue color may be determined by the sensor apparatus and method as described in U.S. Pat. No. 5,176,137 to Erickson, et al and incorporated herein by reference in its entirety. The oxygen sensor of the '137 patent may be located in the subcutaneous device pocket or, alternatively, located on the lead18to enable the sensing of contacting or near-contacting tissue oxygenation or color.

Certain steps in the performance of the detection algorithm criteria are cooperatively performed in microcomputer142, including microprocessor, RAM and ROM, associated circuitry, and stored detection criteria that may be programmed into RAM via a telemetry interface (not shown) conventional in the art. Data and commands are exchanged between microcomputer142and timing and control circuit144, pacer timing/amplifier circuit178, and high voltage output circuit140via a bi-directional data/control bus146. The pacer timing/amplifier circuit178and the control circuit144are clocked at a slow clock rate. The microcomputer142is normally asleep, but is awakened and operated by a fast clock by interrupts developed by each R-wave sense event, on receipt of a downlink telemetry programming instruction or upon delivery of cardiac pacing pulses to perform any necessary mathematical calculations, to perform tachycardia and fibrillation detection procedures, and to update the time intervals monitored and controlled by the timers in pacer/device timing circuitry178.

When a malignant tachycardia is detected, high voltage capacitors156,158,160, and162are charged to a pre-programmed voltage level by a high-voltage charging circuit164. It is generally considered inefficient to maintain a constant charge on the high voltage output capacitors156,158,160,162. Instead, charging is initiated when control circuit144issues a high voltage charge command HVCHG delivered on line145to high voltage charge circuit164and charging is controlled by means of bi-directional control/data bus166and a feedback signal VCAP from the HV output circuit140. High voltage output capacitors156,158,160and162may be of film, aluminum electrolytic or wet tantalum construction.

The negative terminal of high voltage battery112is directly coupled to system ground. Switch circuit114is normally open so that the positive terminal of high voltage battery112is disconnected from the positive power input of the high voltage charge circuit164. The high voltage charge command HVCHG is also conducted via conductor149to the control input of switch circuit114, and switch circuit114closes in response to connect positive high voltage battery voltage EXT B+ to the positive power input of high voltage charge circuit164. Switch circuit114may be, for example, a field effect transistor (FET) with its source-to-drain path interrupting the EXT B+ conductor118and its gate receiving the HVCHG signal on conductor145. High voltage charge circuit164is thereby rendered ready to begin charging the high voltage output capacitors156,158,160, and162with charging current from high voltage battery112.

High voltage output capacitors156,158,160, and162may be charged to very high voltages, e.g., 300-1500V, to be discharged through the body and heart between the electrode pair of subcutaneous cardioversion-defibrillation electrodes113and123. The details of the voltage charging circuitry are also not deemed to be critical with regard to practicing the present invention; one high voltage charging circuit believed to be suitable for the purposes of the present invention is disclosed. High voltage capacitors156,158,160and162may be charged, for example, by high voltage charge circuit164and a high frequency, high-voltage transformer168as described in detail in commonly assigned U.S. Pat. No. 4,548,209 “Energy Converter for Implantable Cardioverter” to Wielders, et al. Proper charging polarities are maintained by diodes170,172,174and176interconnecting the output windings of high-voltage transformer168and the capacitors156,158,160, and162. As noted above, the state of capacitor charge is monitored by circuitry within the high voltage output circuit140that provides a VCAP, feedback signal indicative of the voltage to the timing and control circuit144. Timing and control circuit144terminates the high voltage charge command HVCHG when the VCAP signal matches the programmed capacitor output voltage, i.e., the cardioversion-defibrillation peak shock voltage.

Control circuit144then develops first and second control signals NPULSE 1 and NPULSE 2, respectively, that are applied to the high voltage output circuit140for triggering the delivery of cardioverting or defibrillating shocks. In particular, the NPULSE 1 signal triggers discharge of the first capacitor bank, comprising capacitors156and158. The NPULSE 2 signal triggers discharge of the first capacitor bank and a second capacitor bank, comprising capacitors160and162. It is possible to select between a plurality of output pulse regimes simply by modifying the number and time order of assertion of the NPULSE 1 and NPULSE 2 signals. The NPULSE 1 signals and NPULSE 2 signals may be provided sequentially, simultaneously or individually. In this way, control circuitry144serves to control operation of the high voltage output stage140, which delivers high energy cardioversion-defibrillation shocks between the pair of the cardioversion-defibrillation electrodes18and25coupled to the HV-1 and COMMON output as shown inFIG. 2.

Thus, subcutaneous device14monitors the patient's cardiac status and initiates the delivery of a cardioversion-defibrillation shock through the cardioversion-defibrillation electrodes18and25in response to detection of a tachyarrhythmia requiring cardioversion-defibrillation. The high HVCHG signal causes the high voltage battery112to be connected through the switch circuit114with the high voltage charge circuit164and the charging of output capacitors156,158,160, and162to commence. Charging continues until the programmed charge voltage is reflected by the VCAP signal, at which point control and timing circuit144sets the HVCHG signal low terminating charging and opening switch circuit114. The subcutaneous device14can be programmed to attempt to deliver cardioversion shocks to the heart in the manners described above in timed synchrony with a detected R-wave or can be programmed or fabricated to deliver defibrillation shocks to the heart in the manners described above without attempting to synchronize the delivery to a detected R-wave. Episode data related to the detection of the tachyarrhythmia and delivery of the cardioversion-defibrillation shock can be stored in RAM for uplink telemetry transmission to an external programmer as is well known in the art to facilitate in diagnosis of the patient's cardiac state. A patient receiving the device14on a prophylactic basis would be instructed to report each such episode to the attending physician for further evaluation of the patient's condition and assessment for the need for implantation of a more sophisticated ICD.

Subcutaneous device14desirably includes telemetry circuit (not shown inFIG. 2), so that it is capable of being programmed by means of external programmer20via a 2-way telemetry link (not shown). Uplink telemetry allows device status and diagnostic/event data to be sent to external programmer20for review by the patient's physician. Downlink telemetry allows the external programmer via physician control to allow the programming of device function and the optimization of the detection and therapy for a specific patient. Programmers and telemetry systems suitable for use in the practice of the present invention have been well known for many years. Known programmers typically communicate with an implanted device via a bi-directional radio-frequency telemetry link, so that the programmer can transmit control commands and operational parameter values to be received by the implanted device, so that the implanted device can communicate diagnostic and operational data to the programmer. Programmers believed to be suitable for the purposes of practicing the present invention include the Models 9790 and CareLink® programmers, commercially available from Medtronic, Inc., Minneapolis, Minn.

Various telemetry systems for providing the necessary communications channels between an external programming unit and an implanted device have been developed and are well known in the art. Telemetry systems believed to be suitable for the purposes of practicing the present invention are disclosed, for example, in the following U.S. patents: U.S. Pat. No. 5,127,404 to Wyborny et al. entitled “Telemetry Format for Implanted Medical Device”; U.S. Pat. No. 4,374,382 to Markowitz entitled “Marker Channel Telemetry System for a Medical Device”; and U.S. Pat. No. 4,556,063 to Thompson et al. entitled “Telemetry System for a Medical Device”. The Wyborny et al. '404, Markowitz '382, and Thompson et al. '063 patents are commonly assigned to the assignee of the present invention, and are each hereby incorporated by reference herein in their respective entireties.

According to an embodiment of the present invention, in order to automatically select the preferred ECG vector set, it is necessary to have an index of merit upon which to rate the quality of the signal. “Quality” is defined as the signal's ability to provide accurate heart rate estimation and accurate morphological waveform separation between the patient's usual sinus rhythm and the patient's ventricular tachyarrhythmia.

Appropriate indices may include R-wave amplitude, R-wave peak amplitude to waveform amplitude between R-waves (i.e., signal to noise ratio), low slope content, relative high versus low frequency power, mean frequency estimation, probability density function, or some combination of these metrics.

Automatic vector selection might be done at implantation or periodically (daily, weekly, monthly) or both. At implant, automatic vector selection may be initiated as part of an automatic device turn-on procedure that performs such activities as measure lead impedances and battery voltages. The device turn-on procedure may be initiated by the implanting physician (e.g., by pressing a programmer button) or, alternatively, may be initiated automatically upon automatic detection of device/lead implantation. The turn-on procedure may also use the automatic vector selection criteria to determine if ECG vector quality is adequate for the current patient and for the device and lead position, prior to suturing the subcutaneous device14device in place and closing the incision. Such an ECG quality indicator would allow the implanting physician to maneuver the device to a new location or orientation to improve the quality of the ECG signals as required. The preferred ECG vector or vectors may also be selected at implant as part of the device turn-on procedure. The preferred vectors might be those vectors with the indices that maximize rate estimation and detection accuracy. There may also be an a priori set of vectors that are preferred by the physician, and as long as those vectors exceed some minimum threshold, or are only slightly worse than some other more desirable vectors, the a priori preferred vectors are chosen. Certain vectors may be considered nearly identical such that they are not tested unless the a priori selected vector index falls below some predetermined threshold.

Depending upon metric power consumption and power requirements of the device, the ECG signal quality metric may be measured on the range of vectors (or alternatively, a subset) as often as desired. Data may be gathered, for example, on a minute, hourly, daily, weekly or monthly basis. More frequent measurements (e.g., every minute) may be averaged over time and used to select vectors based upon susceptibility of vectors to occasional noise, motion noise, or EMI, for example.

Alternatively, the subcutaneous device14may have an indicator/sensor of patient activity (piezo-resistive, accelerometer, impedance, or the like) and delay automatic vector measurement during periods of moderate or high patient activity to periods of minimal to no activity. One representative scenario may include testing/evaluating ECG vectors once daily or weekly while the patient has been determined to be asleep (using an internal clock (e.g., 2:00 am) or, alternatively, infer sleep by determining the patient's position (via a 2- or 3-axis accelerometer) and a lack of activity). In another possible scenario, the testing/evaluating ECG vectors may be performed once daily or weekly while the patient is known to be exercising.

If infrequent automatic, periodic measurements are made, it may also be desirable to measure noise (e.g., muscle, motion, EMI, etc.) in the signal and postpone the vector selection measurement until a period of time when the noise has subsided.

Subcutaneous device14may optionally have an indicator of the patient's posture (via a 2- or 3-axis accelerometer). This sensor may be used to ensure that the differences in ECG quality are not simply a result of changing posture/position. The sensor may be used to gather data in a number of postures so that ECG quality may be averaged over these postures, or otherwise combined, or, alternatively, selected for a preferred posture.

In one embodiment, vector quality metric calculations may be performed by the clinician using a programmer either at the time of implant, during a subsequent visit in a clinic setting, or remotely via a remote link with the device and the programmer. According to another embodiment, the vector quality metric calculations may be performed automatically for each available sensing vector by the device a predetermined number of times, such multiple times daily, once per day, weekly or on a monthly basis. In addition, the values could be averaged for each vector over the course of one week, for example. Averaging may consist of a moving average or recursive average depending on time weighting and memory considerations.

FIG. 3is a flowchart of a method for selecting a sensing vector in a medical device, according to one embodiment. As illustrated inFIG. 3, according to an embodiment of the disclosure, the device senses a cardiac signal for each available sensing vector102-106, using sensing techniques known in the art, such as described, for example, in U.S. patent application Ser. No. 14/250,040 (U.S. Patent Publication No. 2015/0290468), incorporated herein by reference in it's entirety. The device obtains a sensed R-wave of the cardiac signal for each available sensing vector102-106, Block124, and determines both a vector quality metric, Block126, for determining the quality of a sensing for the vector, and a morphology quality metric, Block128, for determining the quality of a morphology analysis, associated with the sensed R-wave for that sensing vector102-106, as described below. Once both a vector quality metric, Block126and a morphology metric, Block128, associated with the sensed R-wave has been determined for each sensing vector102-106, the device determines whether the vector quality metric and the morphology metric has been determined for a predetermined threshold number of cardiac cycles for each of the sensing vectors102-106, Block130. If the vector quality metric and the morphology metric has not been determined for the predetermined threshold number of cardiac cycles for each sensing vector102-106, No in Block130, the device gets the next R-wave124for each sensing vector102-106, and the process is repeated for a next sensed cardiac cycle for each of the sensing vectors102-106. According to one embodiment, the vector quality metric and the morphology metric is determined for 15 cardiac cycles, for example.

Once the vector metric and the morphology metric have been determined for the predetermined threshold number of cardiac cycles for each sensing vector102-106, Yes in Block130, the device determines selection metrics using the determined vector quality metrics and morphology metrics, Block132, and selects one or more vectors, Block134, to be utilized during subsequent sensing and arrhythmia detection by the device based on the determined selection metrics, as described below. Depending on the amount of time programmed to occur between updating of the sensing vectors102-106, i.e., an hour, day, week or month, for example, the device waits until the next scheduled vector selection determination, Block136, at which time the vector selection process is repeated.

FIG. 4is a graphical representation of cardiac signals sensed along multiple sensing vectors during selection of a sensing vector in a medical device according to one embodiment. As illustrated inFIG. 4, during the vector selection process, the device senses a cardiac signal100for each available sensing vector102-106, using sensing techniques known in the art, such as described, for example, in U.S. patent application Ser. No. 14/250,040 (U.S. Patent Publication No. 2015/0290468), incorporated herein by reference in it's entirety. For example, as illustrated inFIG. 4, according to one embodiment, the device senses an ECG signal100from each of the available sensing vectors, including a horizontal sensing vector102extending between the housing or can25and electrode22, a diagonal sensing vector104extending between the housing or can25and electrode20, and a vertical sensing vector106extending between electrodes20and22. The device determines a sensed R-wave108for each sensing vector102-106as occurring when the sensed signal exceeds a time-dependent self-adjusting sensing threshold110.

Once the R-wave108is sensed, the device determines a vector quality metric and a morphology metric for the sensed R-wave, Blocks126and128ofFIG. 3. As illustrated inFIG. 4, in order to determine the vector quality metric, Block126ofFIG. 3, for example, the device sets a vector quality metric detection window112, based on the sensed R-wave108for each of the sensing vectors102-106, for determining a vector quality metric associated with the sensing vectors102-106. According to an embodiment, the device sets a quality metric detection window112to start at a start point114located a predetermined distance116from the R-wave108, and having a detection window width118so as to allow an analysis of the signal100to be performed in an expected range of the signal100where a T-wave of the QRS signal associated with the sensed R-wave108is likely to occur. For example, the device sets the quality metric detection window112as having a width118of approximately 200 ms, with a start point114of the quality metric detection window112located between approximately 150-180 milliseconds from the sensed R-wave108, and the width118extending 200 ms from the detection window start point114to a detection window end point120, i.e., at a distance of approximately 350-380 ms from the detected R-wave108. Once the quality metric detection window112is set, the device determines a minimum signal difference122between the sensed signal100and the sensing threshold110within the quality metric detection window112, i.e., the minimum distance extending between the sensed signal100and the sensing threshold110. This determined minimum signal difference122for each of the three sensing vectors102-106is then set as the vector quality metric for the simultaneously sensed R-waves108in the sensing vectors, Block126.

FIG. 5is a flowchart of a method for determining a morphology metric for selecting a sensing vector, according to one embodiment. In order to determine the morphology metric, Block126ofFIG. 3, the device determines a narrow pulse count, i.e., pulse number, for the R-wave108. For example, in order to determine the narrow pulse count for each R-wave108associated with the sensing vectors102-106, the device determines individual pulses associated with the R-wave using known techniques, such as described in commonly assigned U.S. patent application Ser. No. 13/826,097 (U.S. Pat. No. 8,983,586) and Ser. No. 14/255,158 (U.S. Patent Publication No. US 2015/0297907), for example, incorporated herein by reference in their entireties. For each identified pulse, the device determines whether the width of the pulse is less than a predetermined threshold. In particular, as illustrated inFIG. 5, the device gets a single pulse of the identified pulses associated with the R-wave, Block200, determines a pulse width associated with the pulse, Block202, and determines whether the pulse width is less than or equal to a pulse width threshold, Block204.

In addition to determining whether the pulse width of the individual pulse is less than or equal to the pulse width threshold, Yes in Block204, the device may also determine whether the absolute amplitude of the pulse is greater than an amplitude threshold, Block206. According to an embodiment, the pulse width threshold may be set as 23 milliseconds, for example, and the amplitude threshold is set as a fraction, such as one eighth, for example, of a maximum slope used in the determination of whether the slope threshold was met during the aligning of the beat with the template, described in commonly assigned U.S. patent application Ser. No. 13/826,097 (U.S. Pat. No. 8,983,586) and Ser. No. 14/255,158 (U.S. Patent Publication No. US 2015/0297907), incorporated herein by reference in their entireties.

While the pulse width determination, Block204, is illustrated as occurring prior to the amplitude threshold determination, Block206, it is understood that the determinations of Blocks204and206may be performed in any order. Therefore, if either the pulse width of the individual pulse is not less than or equal to the pulse width threshold, No in Block204, or the absolute amplitude of the pulse is not greater than the amplitude threshold, No in Block206, the pulse is determined not to be included in the narrow pulse count. The device continues by determining whether the determination of whether the number of pulses satisfying the narrow pulse count parameters has been made for all of the identified pulses for the R-wave beat, Block210. If the determination has not been made for all of the identified pulses, No in Block210, the device identifies the next pulse associated with the R-wave, Block200, and the process of determining a narrow pulse count for that beat, Blocks202-208, is repeated for the next pulse.

If both the pulse width of the individual pulse is less than or equal to the pulse width threshold, Yes in Block204, and the absolute amplitude of the pulse is greater than the amplitude threshold, Yes in Block206, the number of pulses satisfying the width and amplitude thresholds for the individual R-wave, i.e., the narrow pulse count, is increased by one, Block208.

Once the determination has been made for all of the identified pulses associated with the R-wave, Yes in Block210, the device sets the narrow pulse count for the R-wave, Block212, equal to the resulting updated narrow pulse count, Block208. In this way, the narrow pulse count for the R-wave is the total number of pulses of the identified pulses for the R-wave that satisfy both the width threshold, i.e., the number of pulses that have a pulse width less than 23 milliseconds, and the amplitude threshold, i.e., the number of pulses that have an absolute amplitude greater than one eighth of the maximum slope used during the aligning of the beat with the template, for example. The final narrow pulse count from Block212is then stored as the morphology metric for each R-wave.

In this way, after the process is repeated for multiple R-waves sensed along each of the sensing vectors102-106so that both the vector quality metric and the morphology metric has been determined for the predetermined threshold number of cardiac cycles for each of the sensing vectors102-106, such as 15, for example, Block130, the device determines the selection metrics, Block132ofFIG. 3, i.e., a vector selection metric and a morphology selection metric. As illustrated inFIGS. 3 and 4, once the minimum signal difference122has been determined for all of the predetermined threshold number of cardiac cycles, Yes in Block130, the device determines a vector selection metric for each vector102-106based on the 15 minimum signal differences122determined for that sensing vector. For example, according to an embodiment, the device determines the median of the 15 minimum signal differences122for each sensing vector and sets the vector selection metric for that sensing vector equal to the determined median of the associated minimum signal differences122. Once a single vector selection metric is determined for each of the sensing vectors102-106, the device ranks the vector selection metrics for the sensing vectors102-106. For example, the device ranks the determined vector selection metrics from highest to lowest, so that in the example ofFIG. 4, the diagonal sensing vector104would be ranked first since the median minimum signal difference for that vector was 0.84 millivolts, the horizontal sensing vector102would be ranked second, since the median minimum signal difference for that vector is 0.82 millivolts, and the vertical sensing vector106would be ranked last, since the median minimum signal difference for that sensing vector is 0.55 millivolts.

Similarly, in order to determine morphology selection metrics in Block132ofFIG. 3, the device may determine an average, a mean or a maximum pulse count of the 15 determined narrow pulse counts for each of the sensing vectors102-106. Based on the determined average, median or maximum narrow pulse count for R-waves simultaneously sensed along the sensing vectors102-106, the device ranks the vectors based on the determined morphology selection metrics as being one of a low pulse count, a medium pulse count and a high pulse count. For example, according to one embodiment, if the average, mean or maximum pulse count associated with a sensing vector is greater than 5, the final pulse count for that vector, i.e., morphology selection metric, is determined to be “high”. If the average, mean or maximum pulse count associated with a sensing vector is less than or equal to 5, but greater than or equal to 2, the final pulse count for that vector, i.e., morphology selection metric, is determined to be “medium”. Otherwise, if the average, mean or maximum pulse count associated with a sensing vector is less than or equal to 1, the final pulse count for that vector, I.e., morphology selection metric, is determined to be “low”.

According to another embodiment, the sensing vectors102-106may be relatively ranked based on the morphology selection metric, so that the sensing vector having the greatest pulse count would be identified as being “high”, the sensing vector having the second greatest pulse count would be identified as being “medium”, and the sensing vector having the lowest pulse count would be identified as being “low”,

FIG. 6is a chart illustrating a method of utilizing determined selection metrics for selecting a sensing vector, according to an exemplary embodiment. As illustrated inFIG. 6, assuming that the result of the determination of the vector selection metric, described above, is that sensing vector102is ranked first, sensing vector104is ranked second and sensing vector106is ranked third, and if the sensing vectors102-106are relatively ranked based on the morphology selection metric, the six possible scenarios are shown, so that the result of the morphology selection metric may be illustrated by any one of six possible scenarios. In a first morphology selection scenario,300, sensing vector102is determined to have a low relative narrow pulse count (i.e., relative to sensing vectors104and106) over the 15 cardiac cycles, sensing vector104is determined to have a medium relative narrow pulse count (i.e., relative to sensing vectors102and106), and sensing vector106is determined to have a high relative narrow pulse count (i.e., relative to sensing vectors102and104). In a second morphology selection scenario,302, sensing vector102is determined to have a low relative narrow pulse count over the 15 cardiac cycles, sensing vector104is determined to have a high relative narrow pulse count, and sensing vector106is determined to have a medium relative narrow pulse count.

In a third morphology selection scenario,304, sensing vector102is determined to have a medium relative narrow pulse count over the 15 cardiac cycles, sensing vector104is determined to have a high relative narrow pulse count, and sensing vector106is determined to have a low relative narrow pulse count. In a fourth morphology selection scenario,306, sensing vector102is determined to have a medium relative narrow pulse count over the 15 cardiac cycles, sensing vector104is determined to have a low relative narrow pulse count, and sensing vector106is determined to have a high relative narrow pulse count. In a fifth morphology selection scenario,308, sensing vector102is determined to have a high relative narrow pulse count over the 15 cardiac cycles, sensing vector104is determined to have a low relative narrow pulse count, and sensing vector106is determined to have a medium relative narrow pulse count. Finally, in a sixth morphology selection scenario310, sensing vector102is determined to have a high relative pulse count over the 15 cardiac cycles, sensing vector104is determined to have a medium relative narrow pulse count, and sensing vector106is determined to have a low relative narrow pulse count.

FIG. 7is a flowchart of a method for selecting sensing vectors using determined vector selection metrics and morphology selection metrics according to an embodiment. As illustrated inFIGS. 6 and 7, once the vector selection metrics and the morphology selection metrics have been determined for the sensing vectors102-106, the device identifies the resulting first and second ranked vectors, Block320, which in the example ofFIG. 6are sensing vectors102and104, and determines whether the morphology selection metric of one of the corresponding determined morphology selection metrics has a “High” pulse count, Block322. In the example ofFIG. 6, this occurs, Yes in Block322, in morphology selection scenarios302,304,308and310, and does not occur, No in Block322, in morphology selection scenarios300and306. If neither one of the determined morphology selection metrics associated with the first and second ranked vectors is a “High” morphology selection metric, No in Block322, the first and second ranked vectors are selected as the sensing vectors, Block324.

If the morphology selection metric of either the first ranked vector or the second ranked vector is a “High” morphology selection metric, Yes in Block322, the device sets the other vector as the first ranked vector, Block326. For example, in morphology selection metric scenarios308and310, the second ranked vector, i.e.; sensing vector104, is set as the first ranked vector and sensing vector102is set as the updated second ranked vector, and in morphology selection metric scenarios302and304, the first ranked sensing vector, i.e., sensing vector102is set (remains) as the first ranked vector.

In order to determine which one of the remaining two sensing vectors is chosen as the second ranked vector, the device then determines whether a difference between the morphology metrics of the updated second and third vectors is less than a morphology metric difference threshold, Block328and whether a difference between the vector metrics of the updated second and third vectors is greater than a vector metric difference threshold, Block330. For example, according to one embodiment, the device may determine in Block328whether the difference between the narrow pulse count determined, as described above, for the vector identified as having the “HIGH” morphology selection metric and the third ranked vector is greater than or equal to three.

By way of illustration, in morphology selection metric scenarios308and310, the device determines whether the difference between sensing vector102and sensing vector106is greater than the morphology metric difference threshold by subtracting the morphology metric, i.e., narrow pulse count, determined above, for the third ranked sensing vector from the morphology metric determined for the vector identified as having the “HIGH” morphology selection metric, i.e., sensing vector102. Similarly, in morphology selection metric scenarios302and304, the device determines whether the difference between sensing vector104and sensing vector106is greater than the morphology metric difference threshold by subtracting the morphology metric, i.e., narrow pulse count, determined above, for the third ranked sensing vector from the morphology metric determined for the vector identified as having the “HIGH” morphology selection metric, i.e., sensing vector104.

If the difference between the morphology metrics of the updated second and third vectors is not greater than a morphology metric difference threshold, No in Block328, the first and second ranked vectors are selected as the sensing vectors, Block324.

Similarly, for example, according to one embodiment, in order to determine whether a difference between the vector metrics of the updated second and third vectors is less than a vector metric difference threshold, Block330, the device may determine whether the difference between the minimum signal difference determined, as described above, for the vector identified as having the “HIGH” morphology selection metric and the third ranked vector is less than a nominal minimum threshold, such as 0.10 millivolts, for example.

By way of illustration, in morphology selection metric scenarios308and310, the device determines whether the difference between sensing vector102and sensing vector106is greater than the vector metric difference threshold by subtracting the vector metric, i.e., minimum signal difference, determined above, for the third ranked sensing vector from the vector metric determined for the vector identified as having the “HIGH” morphology selection metric, i.e., sensing vector102. Similarly, in morphology selection metric scenarios302and304, the device determines whether the difference between sensing vector104and sensing vector106is less than the vector metric difference threshold by subtracting the vector metric, i.e., minimum signal difference, determined above, for the third ranked sensing vector from the vector metric determined for the vector identified as having the “HIGH” morphology selection metric, i.e., sensing vector104.

If the difference between the vector metrics of the updated second and third vectors is not less than the vector metric difference threshold, No in Block330, the first and second ranked vectors are selected as the sensing vectors, Block324. If both the difference between the morphology metrics of the updated second and third vectors is greater than the morphology metric difference threshold, Yes in Block328, and the difference between the vector metrics of the updated second and third vectors is less than the vector metric difference threshold, Yes in Block330, the updated first and the third vectors are selected as the sensing vectors, Block332. For example, assuming both the morphology metric difference threshold and the vector metric difference threshold are satisfied, Yes in Blocks328and330, in morphology selection metric scenarios308and310, vectors104and106are selected as the sensing vectors, and in morphology selection metric scenarios302and304, vectors102and106are selected as sensing vectors.

In some instances, the morphology selection metric for two or more of the sensing vectors102-106may have the same ranking. Therefore, according to one embodiment, if two sensing vectors have the same morphology selection metric, the device may select the first and second ranked vectors from the vector selection metric, i.e., vectors102and104in the example shown inFIG. 7, as the sensing vectors to be utilized. Or according to another embodiment, if the morphology selection metric for two or more of the sensing vectors102-106is “High”, then the device may select the first and second ranked vectors from the vector selection metric, i.e., vectors102and104in the example shown inFIG. 7, as the sensing vectors to be utilized. In both situations, the sensing vectors102and104were chosen based only on the determined minimum signal differences for the sensing vectors102-106, and therefore no updating of the first and second raked sensing vectors would occur.

It is understood that in addition to the three sensing vectors102-106described above, optionally, a virtual signal (i.e., a mathematical combination of two vectors) may also be utilized in addition to, thus utilizing more than three sensing vectors, or in place of the sensing vectors described. For example, the device may generate a virtual vector signal as described in U.S. Pat. No. 6,505,067 “System and Method for Deriving Virtual ECG or EGM Signal” to Lee, et al; both patents incorporated herein by reference in their entireties. In addition, vector selection may be selected by the patient's physician and programmed via a telemetry link from a programmer.

In addition, while the use of a minimum signal difference is described, the device may utilize other selection criteria for ranking vectors. For example, according one embodiment, the device may determine, for each vector, a maximum signal amplitude within the detection window for each R-wave, determine the difference between the maximum amplitude and the sensing threshold for each of the maximum amplitudes, and determine a median maximum amplitude difference for each sensing vector over 15 cardiac cycles. The device would then select the vector(s) having the greatest median maximum amplitude difference as the sensing vector(s) to be utilized during subsequent sensing and arrhythmia detection by the device.

Thus, a method and apparatus for selecting a sensing vector configuration in a medical device have been presented in the foregoing description with reference to specific embodiments. It is appreciated that various modifications to the referenced embodiments may be made without departing from the scope of the disclosure as set forth in the following claims.