WO2015153426A1

WO2015153426A1 - Detecting saturation in an electrocardiogram neutral drive amplifier

Info

Publication number: WO2015153426A1
Application number: PCT/US2015/023285
Authority: WO
Inventors: Daniel K. FREEMAN; Ronald Gatzke
Original assignee: Draeger Medical Systems, Inc.
Priority date: 2014-04-02
Filing date: 2015-03-30
Publication date: 2015-10-08

Abstract

A Wilson point can be monitored in an ECG monitor having a plurality of recording ECG leads each electrically connected to a body of a patient. The ECG monitor can have a feedback ECG lead electrically connected to the body of the patient and to an amplifier to compensate for noise on the plurality of recording ECG leads by injecting current into the body of the patient. Each of the plurality of recording ECG leads can have an associated lead-off circuit. The Wilson point can be compared to a threshold. A characterization of the comparison can be provided when the Wilson point is beyond the threshold. Related apparatus, systems, techniques, and articles are also described.

Description

Detecting Saturation in an Electrocardiogram Neutral Drive

Amplifier

RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. § 119 to U.S.

Provisional Application No. 61/974,022 filed on April 2, 2014, the entire content of which is hereby expressly incorporated by reference herein in its entirety.

TECHNICAL FIELD

[0002] The subject matter described herein relates to electrocardiogram signal monitoring and characterization.

BACKGROUND

[0003] An electrocardiogram (ECG) can be used to measure a heart's electrical conduction system. An ECG monitor can include more than two leads with electrodes, for example, left arm (LA), right arm (RA), and left leg (LL) electrodes.

Electrodes are placed on a patient to record voltages on various points of the body. An

ECG monitor can detect electrical impulses generated by the polarization and depolarization of cardiac tissue using electrodes connected to a patient's skin and can translate the electrical impulses into a waveform. This waveform can be used, for example by a clinician, to measure a rate and regularity of heartbeats, as well as size and position of heart chambers, presence of damage to the heart, and effects of drugs or devices used to regulate the heart, such as a pacemaker. The ECG electrodes should be properly placed on a patient's skin to create a sufficient electrical connection. ECG monitoring is an important tool in modem healthcare facilities, such as hospitals, but its utility can be balanced with the burden of frequent false alarms. For example, it has been estimated that greater than 80% of all alarms from patient ECG monitors are false positive.

[0004] Some ECG monitors include a right-leg drive (RLD) or neutral drive system, which can operate to reduce alternating current (AC) noise that couples to a body. Similar systems were introduced into commercial ECG monitors in the 1960's, and were designed to address pronounced ECG noise sources, such as that of mains power (i.e., 50Hz in Europe, 60Hz in the United States). These types of noise couples into the body through stray capacitance to the main power, producing a large AC voltage on the body that can be common to all ECG electrodes, thus serving as a source of common- mode noise in the ECG monitor.

SUMMARY

[0005] In an aspect, a Wilson point can be monitored in an ECG monitor having a plurality of recording ECG leads each electrically connected to a body of a patient. The ECG monitor can have a feedback ECG lead electrically connected to the body of the patient and to an amplifier to compensate for noise on the plurality of recording ECG leads by injecting current into the body of the patient. Each of the plurality of recording ECG leads can have an associated lead-off circuit. The Wilson point can be compared to a threshold. A characterization of the comparison can be provided when the Wilson point is beyond the threshold.

[0006] One or more of the following features can be included in any feasible combination. For example, the Wilson point can be an average of voltages measured on each of the plurality of recording ECG leads. The feedback amplifier can inject current into the body of the patient to drive the Wilson point towards a reference voltage.

Providing the characterization of the comparison can include one or more of generating an alarm, displaying the characterization of the comparison, storing data comprising the characterization of the comparison, loading data comprising the characterization of the comparison, and transmitting data comprising the characterization of the comparison. Each of the associated lead-off circuits can include a pull-down resistor connected to a negative voltage. Each of the associated lead-off circuits can include a pull-up resistor connected to a positive voltage. The providing the characterization of the comparison can occur when the Wilson point is beyond the threshold for a predefined length of time. The threshold can be predetermined to correspond to the feedback amplifier being saturated. The Wilson point can be continuously monitored. The feedback amplifier can be switched to a different ECG lead. The different ECG lead can be selected from one of the plurality of recording ECG leads. Each of the associated lead-off circuits can provide a positive direct current source. Each of the associated lead-off circuits can provide a negative direct current source. Each of the associated lead-off circuits can provide an alternating current source. At least one of the monitoring, comparing, or providing, can be implemented by at least one data processor.

[0007] Computer program products are also described that comprise non- transitory computer readable media storing instructions, which when executed by at least one data processors of one or more computing systems, causes at least one data processor to perform operations herein. Similarly, computer systems are also described that may include one or more data processors and a memory coupled to the one or more data processors. The memory may temporarily or permanently store instructions that cause at least one processor to perform one or more of the operations described herein. In addition, methods can be implemented by one or more data processors either within a single computing system or distributed among two or more computing systems.

[0008] The subject matter described herein provides many advantages. For example, monitoring and detection of saturation of a neutral drive amplifier can be performed without having to record an output of the neutral drive amplifier or voltage on an electrode. Additionally, ECG measurement noise can be reduced, the number of false- positive monitor alarms can be reduced, and ECG monitor performance can be improved.

[0009] The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Other features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0010] FIG 1 is a process flow diagram illustrating a method of monitoring a

Wilson point in an ECG monitor;

[0011] FIG. 2 is an example implementation of an ECG monitoring system configured to monitor the Wilson point;

[0012] FIG. 3 is a circuit model of the example ECG monitoring system;

[0013] FIG. 4 is an example implementation of a circuit for monitoring the Wilson point;

[0014] FIG. 5 is a circuit model of an example ECG monitoring system in which the lead-off circuits include positive direct current sources; [0015] FIG. 6 is a circuit model of an example ECG monitoring system in which the lead-off circuits include alternating current sources; and

[0016] FIG.7 is another example implementation of a circuit for monitoring the Wilson point.

[0017] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0018] FIG 1 is a process flow diagram 100 illustrating a method of monitoring a Wilson point in an ECG monitor to, for example, detect when a neutral drive amplifier is saturated. Saturation of a neutral drive amplifier can lead to increased noise in ECG measurements and poor ECG monitor performance. Additionally, once saturation of the neutral drive amplifier is detected, steps can be taken to correct the condition or notify a clinician or other healthcare provider of the condition.

[0019] The Wilson point in an ECG monitor can, at 1 10, be monitored. A

Wilson point can be an average of voltages or signals measured on a number of recording

ECG leads. The Wilson point can be an average body voltage. Electrical signals detected by the recording ECG leads can be used to create the ECG waveforms. When the ECG monitor is operating under normal conditions with each lead properly connected to a patient, the Wilson point can be at or near zero volts, although there may be transient voltage values. The monitoring can be performed, for example, using analog and/or digital systems.

[0020] FIG. 2 is an example implementation of an ECG monitor 200 configured to monitor the Wilson point and FIG. 3 is a circuit model of the example ECG monitor 200. The example ECG monitor 200 includes three recording leads (205LL, 205- LA, and 205RA) each having a respective recording electrode (210LL, 21 OLA, and 210RA) electrically connected to a body 215 of a patient. The example ECG monitor 200 can further include one or more precordial leads 205p having a precordial electrode 21 Op and a neutral drive lead 205RL having a neutral drive electrode 210RL. The neutral drive lead 205RL can be a feedback lead, and the neutral drive electrode 210RL can be a feedback electrode. The voltage on each lead (205LL, 205LA, 205RA, and 205p) is denoted by

VLL,VLA, VRA, and Vp, respectively. Each of the precordial leads 205p feeds a buffer 255, the output of which can be further processed. The body 215 voltage VB can be modeled to a first order as ground. Commonly used "12-lead ECG" configurations use 10 total electrodes, with 6 precordial leads.

[0021] Each of the respective electrodes (210LL, 21 OLA, 210RA, 21 Op, and 210RL) can be electrically connected to the body 215 through respective skin-electrode interfaces (220LL, 220LA, 220RA, 220p, and 220RL). AS illustrated in FIG. 3, each skin- electrode interface (220LL, 220LA, 220RA, 220p, and 220RL) can be modeled as a parallel RC circuit having some resistance R and some capacitance C. The resistance R and capacitance C of each skin-electrode interface (220LL, 220LA, 220RA, 220p, and 220RL) can vary over time and based on the quality of the physical connection between the body 215 and the corresponding electrode (210LL, 21 OLA, 210RA, 21 Op, and 210RL). For example, a poor electrical connection resulting from a semi-detached electrode can lead to a large resistance R. For modeling purposes, example nominal skin-electrode impedance can derive from the American Association of the Advancement of Medical Instrumentation (AAMI): 50k Ω for skin resistance R and 47nF for skin capacitance C. But impedance can vary over time through a number of factors, such as electrode gel drying out, skin moisture level, or loss of secure contact between electrode and skin.

[0022] An averager 235 can be included to provide an average of the voltage on the recording leads (205LL, 205LA, and 205RA). The output of the averager 235 can be the Wilson point 240. In the example implementation illustrated in FIG. 3, the averager 235 can include equal-valued resistors, each buffered by a high input impedance buffer stage.

[0023] The ECG monitor 200 can include a neutral drive amplifier 230 (e.g., a feedback amplifier) electrically connected to the neutral drive lead 205RL. The neutral drive amplifier 230 operates to inject current into the body 215 via the neutral drive lead 205RL and the neutral drive electrode 210Μ, ΪΟ compensate for noise on one or more recording ECG leads (205LL, 205LA, and 205RA). The neutral drive amplifier 230 can take the Wilson point 240, which can be the average voltage appearing on all recording electrodes (210LL, 21 OLA, and 210RA), invert and amplify this voltage, and apply the inverted amplified voltage to the neutral drive electrode 210RL to effectively cancel out and compensate for any AC noise source on the body 215. The neutral drive amplifier 230 can operate to drive the Wilson point 240 to zero volts. In some implementations, the Wilson point 240 can be a reference voltage or value, can be a voltage or value other than zero, and/or can be predefined or predetermined.

[0024] As illustrated in FIG. 3, an example implementation of a neutral drive amplifier 230 can include an operational amplifier with the Wilson point 240 (also denoted as Vw) connected to the negative input, with a ground reference (or 0V) connected to the positive input. The output VRL is thus the inverted and amplified Wilson point 240. The body 215 and ECG monitor 200 forms a feedback loop in which the Wilson point 240 is driven to the volts reference level potential (e.g., zero volts). In some example implementations, the neutral drive amplifier 230 can have an open loop gain of 10⁵, and a gain bandwidth product of 700kHz.

[0025] The neutral drive amplifier 230 output can generate significant alternating current (AC) voltage on its output particularly when the neutral drive electrode 210RL impedance (modeled as RRL in FIG. 3) becomes high. But the neutral drive amplifier 230 is susceptible to saturation (or "clipping") on its output. Saturation occurs when the neutral drive amplifier 230 attempts to output a voltage greater in magnitude than its power supplies. For example, if the neutral drive amplifier 230 is powered with +3V (e.g., +VSUPPLY), then any attempt to exceed +3V on its output will cause saturation of the neutral drive amplifier 230. When the neutral drive amplifier 230 saturates, it will cease to function properly, the common-mode noise rejection of the ECG monitor 200 will drop, and noise can appear on the ECG output.

[0026] Another cause or contributor to saturation of the neutral drive amplifier 230 can include lead-off systems. For example, the ECG monitor 200 can include a lead-off circuit (225LL, 225LA, 225RA, 225RL, and 225p) for each recording lead (205LL, 205LA, and 205RA); the one or more precordial leads 205p; and the neutral drive amplifier lead 205RL. Each lead-off circuit (225LL, 225LA, 225RA, 225p, and 225RL) can be used to detect if and when the corresponding electrode (210LL, 21 OLA, 210RA, 21 Op, or 210RL) has a poor electrical connection with the body 215. An example implementation of a lead-off circuit (225LL, 225LA, 225RA, 225p, and 225RL) can include a current source ISENSE (as illustrated in FIG. 3) or a pull-down resistor to a negative DC voltage, which acts to change the DC voltage on the electrodes (210LL, 21 OLA, 210RA, 21 Op, or 210RL) when the skin-electrode interface (220LL, 220LA, 220RA, 220p, and 220RL) resistance R becomes high. In some implementations, the DC voltage becomes negative when an electrode contact is poor. For example, if a the right-arm lead-off circuit 225RA

comprises a luA pull-down current, and the corresponding electrode 210RA, resistance RRA increases substantially, such as above 1 ΜΩ, then the right-arm voltage VRA will be some negative voltage. In some implementations, neutral drive amplifier lead 205RL does not include an associated lead-off circuit 225RL.

[0027] In some example implementations, the lead-off circuit (225LL, 225LA, 225RA, 225p, and 225RL) can include a positive current source ISENSE (as illustrated in FIG. 5) or a pull-up resistor to a positive DC voltage. In yet other example implementations, the lead-off circuit (225LL, 225LA, 225RA, 225p, and 225RL) can include an AC source ISENSE (as illustrated in FIG. 6).

[0028] The lead-off circuits (225LL, 225LA, 225RA, and 225p) can be a cause of neutral drive amplifier 230 saturation, such as when electrode (210LL, 21 OLA, and 210RA 21 Op, and 210RL) contact becomes poor. If, for example, the electrode resistance on the right arm (RRA) increases due to poor electrode-skin contact, this will cause the voltage on the right-arm (VRA) to have negative DC voltage. By causing a negative DC voltage on one or more of the recording leads (205RA, 205LA, and 205LL) and precordial leads 205p, the lead-off circuits (225LL, 225LA, 225RA, and 225p) causes the Wilson point 240, which is the average of the recording lead voltages, to become non-zero. The neutral drive amplifier 230, attempting to bring the Wilson point 240 back to zero voltage, can output a relatively high DC voltage. This is because all of the DC current that is drawn into each of the leads (205LL, 205LA, 205RA, and 205p) is supplied from the neutral drive amplifier 230. For example, if the recording lead electrodes (210LL, 21 OLA, and 210RA) are all drawing luA each for each corresponding DC lead-off circuit (225LL, 225LA, 225RA), the neutral drive amplifier 230 is sourcing at least 3uA. Depending on the resistance of the neutral drive electrode (e.g., RRL), a DC voltage can develop on the neutral drive amplifier 230 output (e.g., VRL), which can cause or contribute to saturation of the neutral drive amplifier 230.

[0029] A similar saturation effect can occur when the resistance RRL on the neutral drive electrode 210RL becomes high. For example, because the neutral drive amplifier 230 is supplying current for the lead-off circuits (225LL, 225LA, 225RA, 225p, and 225RL), if the resistance RRL on the neutral drive electrode 21 ORL substantially increases, a relatively high voltage drop will occur across the neutral drive electrode 21 ORL. AS a result, VRL will become positive and can cause or contribute to the neutral drive amplifier 230 saturating. It should be noted that the lead-off circuits (225LL, 225LA, 225RA, 225p, and/or 225RL) can also be chosen to source current rather than sink current without changing the performance of the system. If the lead-off circuits (225LL, 225LA, 225RA, 225p, and/or 225RL) source current, then the neutral drive amplifier 230 would sink this current, and therefore any increase to the resistance RRL will cause the Wilson point 240 to become positive, which in turn causes VRL to become negative and can contribute to the neutral amplifier saturating on its negative power rail. Thus, in some implementations, the Wilson voltage can be monitored to detect when it goes beyond (e.g., greater than) a threshold. In some implementations, the leads-off circuits (225LL, 225LA, 225RA, 225p, and 225RL) can be chosen to provide an AC source rather than a DC source (e.g., as illustrated in FIG. 6). This causes an AC error signal to appear on the Wilson point 240. The Wilson point 240 can be monitored to detect when an AC signal goes beyond a threshold. Thus, in some implementations, each of the lead-off circuits can provide a positive direct current source, a negative direct current source, or an alternating current source.

[0030] Another cause or contributor to saturation of the neutral drive amplifier 230 can include low voltage power supplies. For example, if the neutral drive amplifier 230 is powered at relatively low voltages (e.g., V_SUppiy = +1.5V) as may commonly occur in battery powered ECG monitors, then the neutral drive amplifier 230 output can saturate even under normal conditions. For example, consider a "12-lead" ECG (i.e. 10 total electrodes) in which a sense current of 1 OOnA is used, then the neutral amplifier will be driving 900nA of DC current (lOOnA per electrode) through the neutral electrode 210_RL. If the power supply is 2 V, then saturation will occur when the neutral electrode impedance becomes 2V/900nA = 2.2ΜΩ, which can commonly occur when using low quality electrodes and/or on dry or thick skin.

[0031] Monitoring of the Wilson point can be performed by a controller 245. The controller 245 can monitor the Wilson point by measuring the voltages (V_RA, V_LA, and V_LL) on each of the recording leads (205_RA, 205_LA, and 205_LL) and computing the average. In some implementations, the controller 245 can measure the Wilson point 240 directly on the output of the averager 235. The Wilson point can be continuously monitored.

[0032] The controller 245 can also perform operations related to processing the ECG monitor measurements (e.g., the ECG signal); computing heart rate and other patient physiological parameters; displaying, transmitting, and storing related information; and generating alarms for clinicians related to the ECG signal.

[0033] Referring again to FIG. 1 , at 120 the Wilson point 240 can be compared to a threshold. The threshold can be predetermined or predefined and can be a voltage value. The threshold can correspond to a value that occurs when the neutral drive amplifier 230 is saturating, is close to saturating, or is intermittently saturating. The comparison can further include a time component. For example, comparing the Wilson point 240 to a threshold and length of time the Wilson point 240 has been beyond (e.g., less than or greater than) the threshold. The comparison can be performed by the controller 245, for example, in software.

[0034] A characterization of the comparison can be provided at 130 when the Wilson point is beyond (e.g., less than or greater than) the threshold. The characterization of the comparison can include, for example, generating an alarm or notification for a clinician or other health care provider. The alarm or notification can indicate that the neutral drive amplifier 230 is saturated, that the ECG monitor 200 is not operating under normal conditions or is experiencing increased noise, that one or more electrodes has a poor electrical connection, and the like. Providing the characterization of the comparison can include displaying the characterization (for example, on a display of the ECG monitor 200), storing the characterization of the comparison (for example, in memory of the ECG monitor 200), and transmitting the characterization of the comparison (for example, to a hospital network or remote monitoring system). The providing the characterization of the comparison can occur when the Wilson point 240 is beyond the threshold for a predefined length of time. [0035] In some example implementations, the neutral drive amplifier 230 can be switched or reconfigured to connect to a different ECG lead. For example, referring again to FIG. 2 and FIG. 3, the ECG monitor 200 can include a switch or multiplexer (MUX) 250 between the neutral drive amplifier 230 and the ECG leads (205LL, 205LA, 205RA, 205p, and 205RL). When the neutral drive amplifier 230 is switched to a recording ECG lead, for example left leg ECG lead 205LL, the left-leg recording ECG lead 205LL may no longer be used for recording the ECG measurement (e.g., signal), but rather for injecting current into the body to cancel out noise. The controller 245 can control the MUX 250 to, for example, switch the ECG lead being driving by the neutral drive amplifier 230 from a first lead to a second lead based on whether the neutral drive amplifier 230 is saturating. The lead to which the neutral drive amplifier 230 is switched can be determined based, for example, on a priority list of leads, which can be static or dynamic based on a quality of electrical contact for each lead (e.g., the magnitude of impedance).

[0036] In some example implementations, steps of the method 100 of FIG. 1 can be performed using analog hardware. For example, FIG. 4 is an example

implementation of a circuit for monitoring the Wilson point 240, comparing the Wilson point 240 to a threshold, and providing a characterization of the threshold. The implementation of FIG. 4 includes a comparator circuit 405, which can control the MUX 250 based on whether the Wilson point 240 is beyond (e.g., less than or greater than) the predetermined threshold (denoted as VThresh). As a second example, FIG. 7 is another example implementation of a circuit for monitoring the Wilson point 240, comparing the Wilson point 240 to a threshold, and providing a characterization of the threshold. The implementation of FIG. 7 includes a comparator circuit 705, which can control the MUX 250 based on whether the Wilson point 240 is beyond (e.g., less than or greater than) the predetermined value (denoted as V_REF).

[0037] In some implementations, the controller 245 can monitor the neutral drive electrode 210_RL voltage V_RL directly, rather than the Wilson point 240.

[0038] Various implementations of the subject matter described herein may be realized in digital electronic circuitry, integrated circuitry, specially designed ASICs (application specific integrated circuits), computer hardware, firmware, software, and/or combinations thereof. These various implementations may include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device.

[0039] These computer programs (also known as programs, software, software applications or code) include machine instructions for a programmable processor, and may be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the term "machine-readable medium" refers to any computer program product, apparatus and/or device (e.g., magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions as a machine- readable signal. The term "machine-readable signal" refers to any signal used to provide machine instructions and/or data to a programmable processor. [0040] To provide for interaction with a user, the subject matter described herein may be implemented on a computer having a display device (e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor) for displaying information to the user and a keyboard and a pointing device (e.g., a mouse or a trackball) by which the user may provide input to the computer. Other kinds of devices may be used to provide for interaction with a user as well; for example, feedback provided to the user may be any form of sensory feedback (e.g., visual feedback, auditory feedback, or tactile feedback); and input from the user may be received in any form, including acoustic, speech, or tactile input.

[0041] The subject matter described herein may be implemented in a computing system that includes a back-end component (e.g., as a data server), or that includes a middleware component (e.g., an application server), or that includes a front- end component (e.g., a client computer having a graphical user interface or a Web browser through which a user may interact with an implementation of the subject matter described herein), or any combination of such back-end, middleware, or front-end components. The components of the system may be interconnected by any form or medium of digital data communication (e.g., a communication network). Examples of communication networks include a local area network ("LAN"), a wide area network ("WAN"), and the Internet.

[0042] The computing system may include clients and servers. A client and server are generally remote from each other and typically interact through a

communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

[0043] Although a few variations have been described in detail above, other modifications are possible. For example, the implementations described above can be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flows depicted in the accompanying figures and described herein do not require the particular order shown, or sequential order, to achieve desirable results. Other embodiments may be within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method comprising:

monitoring a Wilson point in an electrocardiogram (ECG) monitor having a plurality of recording ECG leads each electrically connected to a body of a patient, the ECG monitor having a feedback ECG lead electrically connected to the body of the patient and to a feedback amplifier to compensate for noise on the plurality of recording ECG leads by injecting current into the body of the patient, each of the plurality of recording ECG leads having an associated lead-off circuit;

comparing the Wilson point to a threshold; and

providing a characterization of the comparison when the Wilson point is beyond the threshold.

2. The method of claim 1 , wherein the Wilson point is an average of voltages measured on each of the plurality of recording ECG leads.

3. The method of any of the preceding claims, wherein the feedback amplifier injects current into the body of the patient to drive the Wilson point towards a reference voltage.

4. The method of any of the preceding claims, wherein providing the

characterization of the comparison comprises one or more of generating an alarm, displaying the characterization of the comparison, storing data comprising the characterization of the comparison, loading data comprising the characterization of the comparison, and transmitting data comprising the characterization of the comparison.

5. The method of any of the preceding claims, wherein each of the associated lead- off circuits comprises a pull-down resistor connected to a negative voltage.

6. The method of any of the preceding claims, wherein each of the associated lead- off circuits comprises a pull-up resistor connected to a positive voltage.

7. The method of any of the preceding claims, wherein the providing the

characterization of the comparison occurs when the Wilson point is beyond the threshold for a predefined length of time.

8. The method of any of the preceding claims, wherein the threshold is

predetermined to correspond to the feedback amplifier being saturated.

9. The method of any of the preceding claims, further comprising continuously monitoring the Wilson point.

10. The method of any of the preceding claims, further comprising switching the feedback amplifier to a different ECG lead.

11. The method of claim 9, wherein the different ECG lead is selected from one of the plurality of recording ECG leads.

12. The method of any of claims 1 -4 or 6-11 , wherein each of the associated lead-off circuits provides a positive direct current source.

13. The method of any of claims 1 -5, or 7-11 , wherein each of the associated lead-off circuits provides a negative direct current source.

14. The method of any of claims 1 -4, or 7-11 , wherein each of the associated lead-off circuits provides an alternating current source.

15. The method of any of the preceding claims, wherein at least one of the monitoring, comparing, or providing, are implemented by at least one data processor.

16. A non-transitory computer program product storing instructions which, when executed by at least one data processor, results in operations to implement a method as in any of the preceding claims.

17. A system comprising:

at least one data processor;

a neutral drive amplifier; and

memory storing instructions which, when executed by the at least one data processor, results in operations to implement a method as in any of claims 1 to 15.

18. A system comprising:

a plurality of recording electrocardiogram (ECG) leads;

a feedback ECG lead;

at least one data processor; and