Patent Publication Number: US-2017368362-A1

Title: Electrocardiogram monitoring

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. patent application Ser. No. 14/924,363, entitled “Electrocardiogram Monitoring,” filed Oct. 27, 2015, which is set to issue on Sep. 12, 2017 as U.S. Pat. No. 9,757,578, which is a continuation of U.S. patent application Ser. No. 13/650,570, entitled “Electrocardiogram Monitoring,” filed Oct. 12, 2012, issued on Dec. 1, 2015 as U.S. Pat. No. 9,198,593, which is a division of U.S. patent application Ser. No. 11/679,154, entitled “Electrocardiogram Monitoring,” filed Feb. 26, 2007, issued on Nov. 20, 2012 as U.S. Pat. No. 8,315,693, which claims the benefit of U.S. provisional application no. 60/777,308, entitled “Electrocardiogram Monitoring,” filed on Feb. 28, 2006, now expired, all of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to medical devices for treating cardiac conditions, and more particularly, to medical devices using an electrocardiogram. 
     BACKGROUND 
     In a typical implementation, the electrocardiogram (ECG) represents a difference in potential between two or more electrodes placed upon the body of the patient. Leads connect the electrodes to the inputs of a differential amplifier. The differential amplifier detects the ECG signals associated with the contraction of the heart and amplifies the ECG signals so that the ECG signals can be analyzed and/or displayed for analysis. 
     An external defibrillator, such as an automated external defibrillator (AED), is an example of a device that may analyze and/or display ECG signals. For example, based upon the ECG signals, an AED may assess whether a defibrillation shock is indicated, and charge an energy storage element in preparation for giving the shock. When a shock is indicated, the AED may cue the operator to administer the shock, or the AED may administer the shock automatically. The patient receives the shock through the same electrodes. 
     It is possible to reduce outside electrical noise associated with ECG signals by applying a third electrode to the patient and connecting the third electrode to a driven reference input of the common mode of the other two electrodes via a driven reference lead circuit. When noise is reduced, the ECG signals are generally easier to analyze. Consequently, as an example, an AED may be more capable of determining whether shock therapy is appropriate and what degree of therapy is appropriate. Further, noise reduction is particularly important in an ECG that is displayed for human analysis. For example, it is generally desirable that a displayed ECG be of “diagnostic quality,” e.g., conform to the standards for diagnostic ECG devices relating to noise, artifacts, and the like, promulgated by the Association for the Advancement of Medical Instrumentation (AAMI). 
     Another factor that may influence the clarity of the ECG signal is the quality of the connection of the electrodes to the patient. In the case of a patient with a hairy chest, for example, an electrode placed on the chest may lose contact with the patient&#39;s skin, resulting in a poor electrical connection. An inadequate electrical connection for one or more electrodes may, for example, result in an inability to detect the ECG signal, or an ECG signal that is not of adequate quality for analysis. 
     SUMMARY 
     In general, the invention is directed to techniques for detecting whether the leads coupled to an ECG monitoring device, e.g., the three leads coupled to a three wire electrocardiogram monitoring device, are adequately connected to a patient. More particularly, an ECG monitoring device according to the invention injects an integrated signal via one of the leads, and determines whether one or more of the leads are not adequately connected based on the response at the other leads. The integrated signal includes a test signal and a common mode signal from the other, e.g., non-driven, leads. In some embodiments, the ECG monitoring device may advantageously be able to identify more specifically which one or more of the leads are not adequately connected. 
     If the ECG monitoring device determines that one or more leads are not adequately connected to the patient, the device may provide an indication to that effect to a user. The indication may be a general indication that one or more of the leads are not adequately connected. In other embodiments, the indication may more specifically direct the user&#39;s attention to a particular one or more leads. In either case, the indication may allow the user to address the inadequate connection such that an ECG of adequate quality may be detected. 
     The ECG monitoring device may include an integrator that integrates a test signal and a common mode signal from the other leads to generate an integrated signal. The ECG monitoring device may determine whether one or more leads are not adequately connected based on the integrated signal. For example, the ECG monitoring device may compare one or both of an AC amplitude and a DC offset of the integrated signal to respective thresholds. If the AC amplitude or DC offset exceeds the threshold, the device may indicate that either the lead driven with the integrated signal, all other leads, or all leads are not adequately connected. 
     The ECG monitoring device may also include a difference unit, which may be difference amplifier, that generates a difference signal as a function of signals detected via the non-driven leads. The device may determine whether one or more leads are not adequately connected based on the difference signal. For example, the device may compare the AC difference signal to a threshold, and determine that one of the non-driven leads is not adequately connected to the patient if the signal exceeds a threshold. In some embodiments, the device may determine which of the leads is off based on the phase of the difference signal. 
     An ECG monitoring device may take the form of an external defibrillator, such as an AED. The defibrillator may provide ECG monitoring and therapy delivery via a common set of three or more electrodes. In such embodiments, the defibrillator may monitor a driven, e.g., three-wire, ECG for the purpose of determining whether a defibrillation pulse should be delivered to a patient. Two or more of the leads may include electrodes with a larger surface area for delivery of defibrillation pulses or other electrical therapy. 
     In other embodiments, the defibrillator may be coupled to different cables, which provide either ECG electrodes or defibrillation electrodes, via a common receptacle of the defibrillator. Such embodiments may allow a user to use less expensive ECG monitoring electrodes if ECG monitoring of the patient is desired, reserving use of a defibrillation electrode and cable set for situations in which therapy will be delivered to the patient. The defibrillator may detect which type of cable is received by the receptacle, and select an operational mode based on the cable. 
     For example, the defibrillator may detect an ECG monitoring cable and enter an ECG monitoring mode in which delivery of therapy via the ECG electrodes is avoided. In the ECG monitoring mode, the defibrillator may begin driving one of the leads to provide a diagnostic quality ECG signal. The ECG monitoring mode may also include monitoring whether leads are adequately connected to a patient, as discussed above. If the defibrillator detects a therapy, i.e.; defibrillation cable, the defibrillator may enter a therapy mode in which the defibrillator operates as an AED or manual defibrillator, i.e., is able to deliver therapy via the leads. 
     The defibrillator may include a cable-type identification circuit, and at least one cable able to be used with the defibrillator may include a cable-type identification conductor separate from the patient therapy and monitoring leads. The cable-type identification conductor may short at least a portion of cable-type identification circuit, the defibrillator may detect the short to identify the type of the cable. The defibrillator may detect the presence or absence of the conductor, e.g., the presence or absence of a short, to identify which one of two cable-types is coupled to the defibrillator. In some embodiments, different configurations of the cable-type identification circuits may short different portions of the cable-type identification circuit, and the defibrillator may be able to identify more than two cable types based on the which one or more portions of the circuit are shorted. 
     For example, in some embodiments, the defibrillator may additionally detect a combined three-wire monitoring and therapy cable, i.e., with at least two defibrillation leads and an additional lead, which was discussed above. In response to detecting the combined cable, the defibrillator may enter an integrated three-wire monitoring and therapy mode that includes monitoring a 3-wire ECG via the leads, with one of the leads provided by the cable driven, and delivery of defibrillation therapy via the leads that include defibrillation electrodes. Further, in some embodiments, a defibrillator may additionally or alternatively detect cables intended for use with particular types of patients, such as cables intended for use with pediatric patients. In response to detecting such cables, the defibrillator may enter a different mode, or modify some aspect of therapy or monitoring in a way that is particularized for the patient. For example, in response to detecting a pediatric cable, the defibrillator may deliver, or recommend delivery of, defibrillation pulses with energy levels that are reduced relative to those for adult patients. 
     In one embodiment the invention is directed to a method comprising receiving a common mode signal via a first lead and a second lead coupled to an electrocardiogram monitoring device, integrating a test signal and the common mode signal to generate an integrated signal, and injecting the integrated signal into a third lead coupled to the electrocardiogram monitoring device. The method further comprising generating a difference signal based on a difference between a first signal obtained via the first lead and a second signal obtained via the second lead, and indicating whether any of the first lead, the second lead, and the third lead are not adequately connected to a patient based on the integrated signal and the difference signal. 
     In another embodiment, the invention is directed to an electrocardiogram monitoring device comprising an integrator that receives a common mode signal via a first lead and a second lead coupled to an electrocardiogram monitoring device, and integrates a test signal and the common mode signal to generate an integrated signal. The device further comprises a drive circuit that injects the integrated signal into a third lead coupled to the electrocardiogram monitoring device, and a difference unit that generates a difference signal based on a difference between a first signal obtained via the first lead and a second signal obtained via the second lead. The device further comprises a processor that indicates whether any of the first lead, the second lead, and the third lead are not adequately connected to a patient based on the integrated signal and the difference signal. 
     In another embodiment, the invention is directed to an electrocardiogram monitoring device comprising means for receiving a common mode signal via a first lead and a second lead coupled to an electrocardiogram monitoring device, means for integrating a test signal and the common mode signal to generate an integrated signal, means for injecting the integrated signal into the third lead coupled to the electrocardiogram monitoring device, means for generating a difference signal based on a difference between a first signal obtained via the first lead and a second signal obtained via the second lead, and means for indicating whether any of the first lead, the second lead, and the third lead are not adequately connected to a patient based on the integrated signal and the difference signal. 
     In another embodiment, the invention is directed to a system comprising an elongated external cable having a proximal end and a distal end, and an external defibrillator. The cable comprises a plurality of leads that extend from the proximal end to the distal end and convey electrical signals between a patient at the distal end and the proximal end, and a connector located at the proximal end that includes a cable-type identification conductor separate from the leads located on an external surface of the connector. The external defibrillator comprises circuitry that at least one of monitors or delivers therapy to the patient, a cable-type identification circuit, and a receptacle that receives the connector, couples the leads to the circuitry that at least one of monitors or delivers therapy to the patient, and couples the cable-type identification conductor to the cable type detection circuit. The cable-type identification conductor creates a short circuit in the cable-type identification circuit, and the external defibrillator detects the short circuit and identifies a type of the external cable based on the detection. 
     The invention may provide one or more advantages. For example, the invention may allow detection of whether one or more leads are not adequately connected to a patient for ECG monitoring. In some embodiments, the invention may advantageously allow identification of which one or more leads are not adequately connected. 
     Further, by identifying the type of cable coupled to a defibrillator, the defibrillator may be able to provide separate therapy and monitoring modes. Separate therapy and monitoring modes may allow lower cost ECG monitoring electrodes to be used monitoring, and avoid delivery of therapy via such electrodes. Further, the separate modes may allow the device to determine whether to perform therapy functions, e.g., automated evaluation of the ECG and recommendation of therapy, or monitoring functions, e.g., driving a monitoring lead and determining whether the leads are adequately connected to a patient. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an example system including defibrillator coupled to a patient by a three-wire ECG monitoring cable. 
         FIG. 2  is a block diagram illustrating an ECG monitoring circuit used to detect whether one or more leads of an ECG monitoring cable are not adequately connected to a patient. 
         FIG. 3  is a bode plot illustrating a response of the ECG monitoring circuit to one or more leads not being adequately connected to the patient. 
         FIG. 4  is a flow diagram illustrating an example technique for determining whether one or more leads are not adequately connected to a patient. 
         FIG. 5A  is a view illustrating a three-wire ECG monitoring cable. 
         FIG. 5B  is a view illustrating a portion of an example external defibrillator. 
         FIG. 6  is a perspective view illustrating a plurality of connectors of a plurality of cables. 
         FIG. 7  is a block diagram illustrating example cable-type identification conductors and an example cable-type identification circuit which may be included in an external defibrillator. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an example system  10  including a defibrillator  28  coupled to a patient  12  by an elongated three-wire ECG monitoring cable  13 . Cable  13  includes three leads  20 ,  22  and  24  that convey electrical signals between defibrillator  28  and patient  12 . Cable  13  also includes or is coupled to three ECG monitoring electrodes  14 ,  16  and  18  at its distal end with respect to the defibrillator  28 , i.e., leads  20 ,  22  and  24  are coupled to the electrodes at their distal ends. Electrodes  14 ,  16  and  18  may be adhesive electrodes pads, which may include a snap or other connection to the end of leads  20 ,  22  and  24 , as is known in the art. At its proximal end with respect to the defibrillator  28 , cable  13  includes a connector  26  that is received by a receptacle  30  of defibrillator  28  to physically and electrically couple the cable  13  to the defibrillator  28 . 
     As illustrated by  FIG. 1 , system  10  may also include an elongated defibrillator cable  47 , which may be coupled to defibrillator  28  instead of ECG monitoring cable  13 . Defibrillator cable  47  includes leads  52  and  54  that convey electrical signals between patient  12  and defibrillator  28 , as well as electrode pads  48  and  50  at the distal end of leads  52  and  54 . Electrode pads  48  and  50  may be adhesive defibrillation electrode pads known in the art. At its proximal end with respect to the defibrillator  28 , cable  47  may include a connector  56  that is received by receptacle  30  to physically and electrically couple the cable to the defibrillator. 
     In addition to cables  13  and  47 , system  10  may also include a combined three-wire monitoring and therapy cable  51 , which includes leads  57  and  59  attached to electrode pads  53  and  55 , respectively, to allow defibrillator  28  to convey electrical signals between patient  12  and the defibrillator  28 . Electrode pads  53  and  55  may be substantially similar to electrode pads  48  and  50 , and may be sized to facilitate delivery of high-energy defibrillation pulses to patient  12 . In addition to delivery of defibrillation pulses, electrode pads  53  and  55  may facilitate detection of electrical signals within patient  12 , e.g., ECG monitoring. 
     Combined cable  51  also includes a third lead  63  attached to a pad  65  so that defibrillator  28  may monitor a three-wire ECG with the therapy-monitoring cable. Pad  65  may have a smaller surface area suitable for ECG monitoring, but which may not be suitable for delivery of high-energy defibrillation pulses. Third lead  63  may, but does not necessarily, act as the driven lead during three-wire ECG monitoring. Combined cable  51  includes connector  61  at the proximal end of the cable. Combined cable  51  may allow defibrillator  28  to provide a third mode, where the defibrillator is capable of providing a defibrillation shock and a higher quality three-wire ECG without exchanging cables In such embodiments, defibrillator  28  may analyze or allow a user to analyze the three-wire ECG to make therapy delivery decisions, and also deliver the therapy, via the combined cable  51 . 
     Leads  20 ,  22  and  24  of cable  13 , leads  52  and  54  of cable  47 , and leads  57 ,  59  and  63  of cable  51 , may be attached or otherwise bundled along a portion of the length of the cable, as is known in the art. Although illustrated in the context of a three-wire ECG monitoring cable  13  and a combined therapy-monitoring cable  51  with three leads, the invention may be used with cables including more than three leads. 
     In the illustrated embodiment, defibrillator  28  includes a cable identification circuit  38 , through which a processor  36  identifies which type of cable, e.g., which of cables  13 ,  47 , and  51  is coupled to defibrillator  28 . Based on the identified type of cable, processor  36  may cause defibrillator  28  to operate in a selected one of a plurality of operational modes. For example, if processor  36  identifies ECG monitoring cable  13 , processor  36  may cause defibrillator  28  to operate in an ECG monitoring mode. When defibrillator  28  is operating in the ECG monitoring mode, processor  36  may control an ECG monitoring module  34  to drive one of leads  20 ,  22  and  24 , and receive the ECG signal via the other two leads. 
     When defibrillator  28  is operating in the ECG monitoring mode, processor  36  may also control ECG monitoring module  34  to detect whether one or more of the leads are not adequately connected to patient  12 , as will be described in greater detail below. Further, when defibrillator  28  is operating in a therapy mode, processor  36  may control monitoring the ECG via the two leads of cable  47 , and delivery of therapy via the leads of cable  47 , as will be described in greater detail below. By identifying the type of cable and entering an appropriate mode, defibrillator  28  and processor  36  may, for example, avoid delivery of therapy to patient  12  via ECG monitoring cable  13 , which may be harmful to the patient. 
     In addition, when defibrillator  28  is operating in a combined three-wire ECG and therapy mode with combined cable  51 , processor  36  may control monitoring of a three-wire ECG via the leads  57 ,  59  and  63 , and delivery defibrillation pulses via leads  57  and  59 , without changing cable  51 . In this manner, combined cable  51  may be more suitable for patients in critical care or having imminent cardiac problems. Processor  36  may control ECG monitoring module  34  to drive one of leads  57 ,  59  and  63 , and receive the EGG signal via the other two leads. Processor  36  may use the three-wire ECG to decide whether defibrillation therapy should be delivered to the patient via leads  57  and  59  and electrodes  53  and  55 . 
     Further, in some embodiments, when defibrillation therapy is delivered, processor  36  may disable or otherwise protect lead  63  e.g., by modifying switches within interface  32 , to prevent delivery of defibrillation therapy via electrode  65 . As discussed above, electrode  65  may have a smaller surface area than electrode pads  53  and  55  that may be unsuitable for delivery of high-energy defibrillation pulses. In the therapy-monitoring mode, defibrillator  28  may provide leads-off monitoring in the manner described below with reference to cable  13  and the ECG monitoring mode. However, the thresholds for such identification may be different in therapy-monitoring mode as compared to a dedicated ECG mode due to, for example, differences between electrodes  14 ,  16  and  18 , and electrodes  53 ,  55  and  65 . 
     Processor  36  may include any one or more of a microprocessor, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate array (FPGA), or other digital logic circuitry. A memory  37  may store instructions that, when executed by processor  36 , cause processor  36  to provide the functionality ascribed to the processor and defibrillator  28  herein. Memory  37  may also store patient data gathered during treatment or monitoring of patient  12 , as well as treatment or monitoring protocols, including defibrillation pulse energy level protocols or progressions, and thresholds and algorithms used to detect cardiac fibrillation. Memory  37  may also store the thresholds used to detect whether one or more leads of an ECG monitoring cable are not adequately detected to a patient, which will be described in greater detail below. Memory  37  may include, for example, any one or more of a random access memory (RAM), read only memory (ROM), non-volatile RAM (NVRAM), electronically erasable programmable ROM (EEPROM), or flash memory. 
     In addition to receptacle  30 , cables  13 ,  47 , and  51  are coupled to defibrillator by an interface  32 . When defibrillator  28  is coupled to cable  47  and operating in a therapy mode, or coupled to cable  51  and operating in the combined mode, defibrillator  28  may sense electrical activity of the heart of patient  12  and deliver defibrillation pulses to patient  12  via electrodes  48  and  50  or  53  and  55 . Interface  32  may include a switch (not shown in  FIG. 2 ) that, when activated, couples an energy storage circuit  46  to the electrodes. Energy storage circuit  46  stores the energy to be delivered to patient  12  in the form of a defibrillation pulse. The switch may be of conventional design and may be formed, for example, of electrically operated relays. Alternatively, the switch may comprise an arrangement of solid-state devices such as silicon-controlled rectifiers or insulated gate bipolar transistors. 
     Energy storage circuit  46  includes components, such as one or more capacitors, that store the energy to be delivered to patient  12  via the electrodes. Before a defibrillation pulse may be delivered to patient  12 , energy storage circuit  46  must be charged. Processor  36  directs a charging circuit  44  to charge energy storage circuit  46  to a high voltage level. Charging circuit  44  comprises, for example, a flyback charger that transfers energy from a power source  42  to energy storage circuit  46 . 
     Defibrillator  28  may be a manual defibrillator or an AED. Where defibrillator  28  is a manual defibrillator, a caregiver using defibrillator  28  may select an energy level for each defibrillation pulse delivered to patient  12 . Processor  36  may receive the selection made by the caregiver via a user interface  40 , which may include input devices, such as a keypad and various buttons or dials, and output devices, such as various indicator lights, a cathode ray tube (CRT), light emitting diode (LED), or liquid crystal display (LCD) screen, and a speaker. In some embodiments, user interface  40  may include a touch sensitive display that acts both displays information to a user and receives user input. Where defibrillator  28  is an AED, processor  36  may select an energy level from a preprogrammed progression of energy levels stored in memory  37  based on the number of defibrillation pulses already delivered to patient  12 . 
     When the energy stored in energy storage circuit  46  reaches the desired energy level, processor  36  controls user interface  40  to provide an indication to a user that defibrillator  28  is ready to deliver a defibrillation pulse to patient  12 , such as an indicator light, displayed message, or a voice prompt. The defibrillation pulse may be delivered manually or automatically. Where the defibrillation pulse is delivered manually, the user may direct processor  36  to deliver the defibrillation pulse via user interface  40  by, for example pressing a button. In either case, processor  36  activates the switches of interface  32  to electrically connect energy storage circuit  46  to leads  52  and  54  and electrodes  48  and  50 , or leads  57  and  59  and electrodes  53  and  55 , and thereby deliver the defibrillation pulse to patient  12 . 
     Processor  36  may modulate the defibrillation pulse delivered to patient  12 . Processor  36  may, for example, control the switches of interface  32  to regulate the shape and width of the pulse. Processor  36  may control the switches to modulate the pulse to, for example, provide a multiphasic pulse, such as a biphasic truncated exponential pulse, as is known in the art. 
     Processor  36  may perform other functions as well, such as monitoring electrical activity of the heart of patient  12  sensed via electrodes. Processor  36  may determine whether the heart of patient  12  is fibrillating based upon the sensed electrical activity in order to determine whether a defibrillation pulse should be delivered to patient  12 . Where a defibrillation pulse has already been delivered, processor  36  may evaluate the efficacy of the delivered defibrillation pulse by determining if the heart is still fibrillating in order to determine whether an additional defibrillation pulse is warranted. Processor  36  may automatically deliver defibrillation pulses based on these determinations, or may advise the caregiver of these determinations via user interface  40 . 
     When defibrillator  28  is operating in the therapy mode, processor  36  may display an electrocardiogram (ECG) that reflects the electrical activity sensed via electrodes  48  and  50  via user interface  40 . Because defibrillator  28  is operating in the therapy mode rather than the ECG monitoring mode when coupled to cable  47  or therapy-monitoring mode when coupled to cable  51 , and therefore does not provide a driven lead, the displayed ECG may not be of as high a quality as a three-wire ECG. As mentioned above and discussed in greater detail below, processor may identify a type of cable, e.g., whether cable  13 ,  47 , or  51  is attached, via a cable identification circuit  38 , and enter either the therapy mode, an ECG monitoring mode, or a combined mode based on the identification. 
       FIG. 2  is a block diagram illustrating an ECG monitoring module  34 . ECG monitoring module  34  may provide a driven lead to allow detection a higher-quality, three-wire ECG signal via the other leads of an ECG monitoring cable  13  or combined cable  51  ( FIG. 1 ). ECG monitoring module  34  may also allow processor  36  ( FIG. 1 ) to determine whether one or more of the leads are not adequately connected to patient  12 . Processor  36  may control ECG monitoring module  34  to perform these functions when, for example, defibrillator  28  ( FIG. 1 ) is operating in an ECG monitoring mode or a combined three-wire ECG and therapy mode. 
     As illustrated in  FIG. 2 , both ECG monitoring module  34  and a defibrillation module  70  are coupled to the leads of whichever of cables  13 ,  47 , or  51  is coupled to defibrillator  28  by common channels  67 - 69  of interface  32 . Interface  32  may include switches (not shown), and processor  36  may control the switches to control which of defibrillation module  70  and ECG monitoring module is coupled to channels  67 - 69  via the switches based on whether defibrillator  28  is in an ECG monitoring mode, a therapy mode, or a therapy-monitoring mode. In this manner, channels  68  and  69  are shared between ECG monitoring module  34  and a defibrillation module  70 . Defibrillation module  70  may include charging circuit  44  and energy storage circuit  46  illustrated in  FIG. 1 . 
     When ECG monitoring module  34  is coupled to leads  20 ,  22  and  24  of cable  13  or coupled to leads  57 ,  59  and  63  of cable  51  by channels  67 - 69 , processor  36  may control monitoring module  34  to drive one of the leads with a signal, and detect an ECG signal via the other two non-driven leads. More particularly, processor  36  may control an integrator  64  to generate an integrated signal  76  that drives one of the leads, and may receive the 
     ECG based on a difference signal  72  generated by a difference unit, such as a difference amplifier  62 , which is the difference between the signals detected by the other two leads. Although not shown in  FIG. 2 , ECG monitoring module  34  may include a variety of circuitry to condition difference signal  72  prior to delivery to processor  36  as an ECG, such analog or digital filters, amplifiers, and analog to digital conversion circuitry. For example, a band pass filter may filter difference signal  72  before the difference signal is sent to a processor. Processor  36  may display the ECG via a display of user interface  42  ( FIG. 1 ). 
     The integrator  64  generates the integrated signal based on a common mode signal  74  of the non-driven leads. Common mode signal  74  provides negative feedback from the patient  12 . In general, during ECG monitoring, integrated signal  76  acts to cancel at least lower frequency noise present in the common mode signal. 
     In the embodiment illustrated by  FIG. 2 , ECG monitoring module  34  includes a multiplexer  60 . Processor  36  may control which of a plurality of leads is coupled to the output of integrator  64 , e.g., which of the leads is the driven lead, and which of the leads are coupled to the inputs of differential amplifier  62  via multiplexer  60 . In this manner, defibrillator  28  may display any of a Lead I, II or III ECG signal, as is known in the art. Processor  36  may receive selection of the Lead I, II or III ECG signal from a user via user interface  40 , and configure the leads via multiplexer  60  according to the selection. 
     In other embodiments, ECG monitoring module  34  need not include multiplexer  60 . In such embodiments, the connections between leads and the output of integrator  64  and inputs of differential amplifier  62 , may be fixed. As an example, if lead  22  of cable  13  is connected to integrator  64  via channel  67 , lead  24  is connected to the non-inverting (+) input of differential amplifier  62  via channel  69 , and lead  20  is connected to the inverting (−) input of the differential amplifier via channel  68 , the ECG derived from the signal output by the differential amplifier will be a Lead II ECG. 
     Processor  36  may also determine whether one or more of the leads are not adequately connected to the patient via ECG monitoring module  34 . More particularly, processor  36  may control injection of a test signal  66  into integrator  64 , which outputs an integrated signal to one of the leads, and determine whether one or more of the leads are not adequately connected to patient  12  based on the response detected at the non-driven leads and the driven lead  67 . 
     As an example, the test signal  66  used to drive one of the leads may be a sinusoidal frequency at approximately 275 Hz. Test signal  66  may have a minor de bias. Although test signal  66  need not be sinusoidal in some embodiments, a sinusoidal test signal may advantageously reduce the electromagnetic interference from outside sources, and reduce the likelihood of charge buildup on the electrodes coupled to the patient, which may itself affect ECG signal quality. Further, test signal  66  may be at a frequency other than approximately 275 Hz. For example, test signal  66  may have any frequency between 150 Hz and 2 kHz. Test signal frequencies within this range may advantageously avoid interfering with the ECG signal, which primarily occurs below 150 Hz, and detection of implanted cardiac pacer activity, which general occurs above 2 kHz. 
     In general, if all of the leads are adequately connected to patient  12 , test signal  66  will be small in amplitude at integrated signal  76  due to the cancellation from the integrated negative feedback. Test signal  66  is substantially absent from difference signal  72  because the test signal is a common mode signal simultaneously applied to both inputs of differential amplifier  62 . If either the driven lead, or both of the non-driven leads, are not adequately connected to the patient, test signal  66 , which is the 275 Hz sine wave in the above example, will be more significantly present in integrated signal  76 . The elevated 275 Hz test signal  66  will exceed a pre-set threshold that indicates that the leads are not adequately connected to patient  12 , i.e. the “leads-off” indication. If common-mode signal  74  does not include test signal  66 , and therefore does not provide negative feedback from patient  12  to integrator  64  that includes test signal  66 , the peak-to-peak amplitude and absolute value of the DC offset of the 275 Hz test signal within integrated signal  76  output by integrator  64  may increase relative to the input test signal  66 . Accordingly, processor  36  may compare an amplitude, which may be the peak-to-peak amplitude, and/or the DC offset, i.e., the absolute value of the DC offset, of test signal  66  within integrated signal  76  to a threshold. Processor  36  may indicate that one or more leads are not adequately connected to patient  12  if the threshold is exceeded. Processor  36  may provide a general “leads-off” indication, or may more particularly indicate that either the driven lead, both of the non-driven leads, or all of the leads  20 - 24  are not adequately connected to patient  12 , if the threshold is exceeded. Detecting the presence, absence, or strength of test signal  66  in common-mode signal  74  by monitoring integrated signal  76  is one way in which processor  36  may monitor the response at the non-driven leads to injection of test signal  66  at the driven lead. 
     Another way in which the processor may monitor the response to the test signal to detect whether one or more leads are not adequately connected is to monitor difference signal  72 , which is generated by differential amplifier  62  based on the signals received from the non-driven leads. In general, if both of the non-driven leads connected to the inputs of differential amplifier  62  are adequately connected to patient  12 , test signal  66  will be substantially absent from difference signal  72 . However, if one of the non-driven leads is not adequately connected to the patient, the test signal will be present in the differential signal. Accordingly, processor  36  may compare the amplitude of difference signal to a threshold, and indicate that one or more leads are not adequately connected to patient  12  if the threshold is exceeded. Processor  36  may provide a general “leads-off” indication, or may more particularly indicate which of the non-driven leads is not adequately connected to patient  12 , if the threshold is exceeded. 
     Further, in some embodiments, processor  36  may determine which of the non-driven leads is not adequately connected based on the phase of difference signal  72 . For example, because integrator  64  includes an inverter, if processor  36  determines that difference signal  72  is substantially in phase with test signal  66 , processor  36  may determine that the lead coupled to the non-inverting (+) input of difference amplifier  62  is not adequately connected to patient  12 . If processor  36  determines that difference signal  72  is substantially out of phase with test signal  66 , processor  36  may determine that the lead coupled to the inverting (−) input of difference amplifier  62  is not adequately connected to patient  12 . In such embodiments, processor  36  may advantageous identify to a user which of the non-driven leads is not adequately connected to patient  12 . Whether a general or more specific in the manner described above, processor  36  may indicate whether one or more leads are not adequately connected to patient  12  via user interface  42  of defibrillator  28 , e.g., via a display, speaker, or one or more indicator lights. 
       FIG. 3  is a bode plot  86  illustrating a response of ECG monitoring circuit  34  to one or more of leads  20 ,  22  and  24  not being adequately connected to patient  12 . More particularly, bode plot  86  illustrates a closed-loop gain  78  of integrated signal  76  relative to test signal  66 , corresponding to a condition all of the leads are adequately connected, and an open-loop gain  80  of the integrated signal relative to the test signal, corresponding to a condition either the driving lead or both non- driven leads are completely disconnected, as a function of frequency. As illustrated by bode plot  86 , open-loop gain  80  is greater than closed-loop gain  78  across a relatively large range of frequencies, and particularly so around the frequency of the injected test signal  66  and at DC  84 . Accordingly, processor  36  may monitor the amplitude of integrated signal  76  at approximately the test frequency signal and/or DC, e.g., the DC gain to detect whether one or more leads are not adequately connected to patient  12 . When the leads are not adequately connected, the loop gain will generally fall between the boundaries of  78  and  80 . A leads-off threshold can be specified, based on clinic preference, within the range defined by these two boundaries. 
       FIG. 4  is a flow diagram illustrating an example technique for determining whether one or more of leads are not adequately connected to a patient. The illustrated technique may be performed by a defibrillator as controlled by a processor thereof. The illustrated technique may be performed by, for example, defibrillator  28  and processor  36  as described above with reference to  FIGS. 1-3 . 
     According to the example technique, processor  36  controls generation of a test signal  66  ( 90 ). An integrator  64  generates an integrated signal  76  based on a common-mode signal  74  from non-driven leads and the test signal, and injects the integrated signal into a driven one of the leads. Processor  36  detects at least one of an amplitude or DC offset of the integrated signal  76  ( 92 ). Processor  36  compares the amplitude or DC offset, e.g., the peak-to-peak amplitude or absolute value of the DC offset, to a threshold ( 94 ). If the threshold is exceeded, processor  36  provides an indication to a user via user interface  40  that one or more of the leads are not adequately connected to patient  12  ( 96 ). In some embodiments, the processor provides a more specific indication that the driven lead, both non-driven leads, or all leads are not adequately connected if the amplitude or DC offset of the integrated signal exceeds the threshold. 
     If the amplitude or DC offset does not exceed the threshold, processor  36  detects an amplitude of a difference signal  72  generated by a difference unit, e.g., a difference amplifier  62 , based on signals detected by the non-driven leads ( 98 ). Processor  36  compares the amplitude to a threshold ( 100 ). If the amplitude of the difference signal does not exceed the threshold, processor  36  may provide an indication of adequate lead attachment e.g., “leads on,” via user interface  42  ( 102 ). In other embodiments, processor  36  may provide no indication if the leads are adequately attached, i.e., may only provide lead attachment related indications if one or more leads are not adequately attached to patient  12 . 
     If the amplitude of difference signal  72  exceeds the threshold, processor  36  may indicate that a lead is not adequately connected to patient  12 , or may more specifically indicate that one of the non-driven leads is not adequately connected, In other embodiments, as illustrated in  FIG. 4 , processor  36  may detect the phase of the difference signal relative to the test signal if its amplitude exceeds the threshold ( 104 ). If difference signal  72  is substantially out of phase with test signal  66 , processor  36  may indicate that the lead coupled to the inverting input of the difference unit is not adequately connected to patient  12  ( 108 ). If difference signal  72  is substantially in phase with test signal  66 , processor  36  may indicate that the lead coupled to the noninverting input of the difference unit is not adequately connected to patient  12  ( 110 ). 
       FIG. 5A  shows three-wire electrocardiogram. (ECG) cable  13 . ECG cable  13  includes electrodes  14 ,  16  and  18  connected to the distal ends of leads  20 ,  22  and  24 , respectively. Leads  20 ,  22  and  24  are combined into one insulated bundle at some location along cable  13  and couple to connector  26  at the proximal ends. Connector  26  provides an electrical contact interface to defibrillator  28 . 
     Electrode  18  may be attached to the left leg of patient  12 , electrode  16  may be attached to the left arm of the patient and electrode  14  may be attached to the right arm of the patient. The exact positioning of each electrode  14 ,  16  or  18  may slightly alter the detected by defibrillator  28 , but the altered signal may still adequately detect the ECG of patient  12 . However, defibrillator  28  may detect if one of electrodes  14 ,  16  or  18  does not make adequately contact to the skin of patient  12 , as described herein. 
     ECG cable  13  may be electrically coupled to defibrillator  28  through receptacle  30  shown in  FIG. 5B , e.g., by receipt of connector  26  into receptacle  30 . Receptacle  30  may be located at an easily accessed location of defibrillator  28  and provide multiple contacts with connector  26 . In addition, receptacle  30  may accept the connectors of other external cables, such as connector  56  of defibrillator cable  47  or connector  61  of therapy-monitoring cable  51  of  FIG. 1 . Defibrillator  28  may automatically identify which external cable is plugged into receptacle  30  by detecting a cable-type identification conductor on the external surface of the connector via conductor contacts  122  located within receptacle  30 . Three conductor contacts are shown in the example receptacle  30  so that defibrillator  28  can identify up to four different cables. In this manner, defibrillator  28  may be operated based upon the external cable plugged into receptacle  30 . Conductor contacts  122  are three electrically conductive surfaces that are selectively coupled to each other via the electrically conductive cable-type identification conductor. Other embodiments of receptacle  30  may include more than three conductor contacts  122  to identify more than four difference cables. 
     Receptacle  30  is shown as an oval shaped structure within a rectangular recession. In other embodiments, receptacle may include other shapes, such as a circle, triangle, trapezoid, or other polygon. Alternatively, receptacle  30  may have an unsymmetrical shape that only accepts the connector of an external cable in a particular orientation. In addition, receptacle  30  may include a locking mechanism which prevents a connector from being removed from the receptacle accidentally. The locking mechanism may include a latch, a pin, a spring loaded lever, one or more mating indents and detents, or a friction fit. 
       FIG. 6  is a perspective view illustrating example embodiments of connectors for cables described herein, such as connectors  26 ,  56  and  61  of cables  13 ,  47  and  51 . For example, connectors  150  and  158  maybe attached to cables  13  and  47 , respectively, while connector  162  is attached to another cable, such as a pediatric defibrillation cable. Connector  150  includes connector housing  152  and cable-type identification conductor  154  located at a proximal end of the connector along one external surface of the housing. Connector  156  includes connector housing  158  and cable-type identification conductor  160  located at a proximal end of the connector along one external surface of the housing. Connector  162  includes connector housing  164  and cable-type identification conductor  166  located at a proximal end of the connector along one external surface of the housing. Connector housings  152 ,  158  and  164  may be shaped to partially fit into receptacle  30  of  FIG. 5B  and to allow a user to hold each housing such that the user may easily insert the respective connector into the receptacle. 
     Conductors  154 ,  160  and  166  are each configured to allow defibrillator  28  to identify the “type” of their respect cables as such without input from a user. Conductors  154 ,  160  and  166  may be constructed of an electrically conductive metal plate or non-conductive metal or molded plastic coated with a conductive paint or coating. Alternatively, the conductors may be formed on the connectors by any one or more of a variety of processes, including vapor-deposition, sputter coating, lithography, or etching. Conductors  154 ,  160  and  166  may be of a shape or size such that it interacts with a cable-type identification circuit of defibrillator  28 , e.g., shorts at least a portion of the circuit. Processor  36  of defibrillator  28  may identify the coupled cable, i.e., ECG monitoring cable  13 , defibrillation cable  47 , therapy-monitoring cable  51 , or other type of cable, based on the interaction. For example, conductor  160  may touch two of three of electrical contacts, where one of the two electrical contacts provides a path to ground. Other cables may be identified by processor  36  through the use of a different conductor configuration, as shown in  FIG. 6  by conductor  154  that touches all three contacts and conductor  166  that touches two other contacts, or the absence of a conductor. In this manner, processor  36  may cause defibrillator  28  to enter a mode of operation appropriate for the connected cable type, e.g., enter an ECG monitoring mode when coupled to an ECG monitoring cable, such as ECG cable  13 . 
     Conductors  154 ,  160  and  166  are located on an external surface of connector housings  152 ,  158  and  162 , respectively. In some embodiments, the external surface maybe parallel or orthogonal to the direction in which a connector housing is inserted in receptacle  30 . Conductors  154 ,  160  and  166  mate to one or more conductive surfaces within receptacle  30 , however neither the conductive surfaces surround a portion of the conductor, nor does the conductor surround the conductive surface, i.e., a male to female type of connection. For example, conductor  154  is a substantially flat electrically conductive surface normal to the direction of connector housing  152  insertion, where the conductor surface mates to three substantially flat connector contacts  122  within receptacle  30  coupled to the cable-type identification circuit of defibrillator  28 . 
       FIG. 7  is a block diagram illustrating example cable-type identification conductors, and an example configuration of cable-type identification circuit  38 . A digital power supply  128  delivers a logic signal to one or more inputs of processor  36  via one or more respective pull-up resistors  130  and  132 . Conductors  140 ,  142  and  144  are example conductors which are configured, e.g., sized, to interact with one or both of contacts  134  and  138  coupled to the processor inputs and the digital power supply via the pull-up resistors, in addition to contact  136  that provides a path to ground. By interacting with such combinations of contacts  134 ,  136  and  138 , the conductors short a selected one or more of the circuits providing a logic signal to processor  36  to change the logic state presented to one or more inputs of the processor, e.g., from high or one, to low or zero. 
     In the case of no conductor connecting any of the contacts  134 ,  136  and  138 , processor  36  may receive a high voltage at both inputs. Conductor  144  is configured to interact with contacts  134  and  136 , thereby connecting a first input of processor  36  to ground and driving it to a low voltage, while a second input other remains high. Conductor  142  interacts with contacts  138  and  136  to provide the opposite effect, i.e., the second input is connected to ground and driven low while the first input remains high. Conductor  140  interacts with all of the contacts to couple both inputs to ground, and thereby drive both inputs low. 
     Processor  36  may determine which cable-type identification conductor is present based upon one or more detected logic states, thereby identifying which external cable is connected to receptacle  30 . In the configuration of cable-type identification circuit  38  illustrated by  FIG. 7 , as many as four different cable-types may be identified, i.e., one for each conductor configuration, and one for the absence of a conductor. A greater number of inputs to processor  36  may be used to detect many more different cable types by selectively coupling certain inputs. Based on a determination that no conductor is present, processor  36  may identify an associated type of cable, or determine that no cable is attached to defibrillator  28 . 
     As discussed above, processor  36  may cause defibrillator to enter one of a plurality of operational modes based on the detected cable type. For example, processor  36  may cause defibrillator  28  to enter an ECG monitoring mode based on detection of cable  13 , or a therapy mode based on detection of cable  47 . Alternatively, processor  36  may cause defibrillator  28  to enter a combined three-wire ECG monitoring and therapy mode based upon detection of cable  51 . Further, in some embodiments, once processor  36  identifies the cable type, processor  36  may provide an indication of the cable type to a user via user interface  40 , e.g., via one or more indicator LEDs or a display. In this manner the user may verify that the attached cable is actually the cable the user desires to use with defibrillator  28 . 
     Various embodiments have been described. However, one of ordinary skill in the art will understand that various modifications may be made to the described embodiments without departing from the scope of the invention. For example, the “circuits,” “circuitry,” “modules,” “units” and the like described herein may be embodied as hardware, software, or any combination thereof 
     Further, although described herein as allowing identification of either an ECG monitoring cable or a therapy cable for determining whether to enter a ECG monitoring mode, a therapy mode, or a combined mode, in other embodiments cable-type identification circuitry and cable- type identification conductors may be used to distinguish other types of cables used with an external defibrillator for other purposes. For example, processor of a defibrillator may adjust the delivery of the therapy to a patient based upon the identified external cable. The processor may identify, as an example, a pediatric cable attached to the defibrillator, and utilize a pediatric defibrillation mode designed for providing defibrillation to small children, e.g., deliver lower energy defibrillation pulses. 
     In other embodiments, the circuits and other hardware described to carry out certain aspects of the disclosure may be recreated with software. The software may be instructions stored on a computer readable medium that cause a processor to perform various tasks as described herein. For example, the computer readable medium may include instructions that cause a processor to provide a “leads-off” indication when one or more leads are not adequately attached to patient  12 . Therefore, it is contemplated to translate the circuitry described herein into a computer readable medium that a processor uses to perform the functions described above. 
     Additionally, although described in the context of a three-wire ECG monitoring a defibrillator, the techniques for detecting whether leads are adequately connected to a patient may be used in any ECG monitoring device coupled to any number of leads. These and other embodiments are within the scope of the following claims.