Patent Publication Number: US-11660037-B2

Title: System for electrocardiographic signal acquisition and processing

Description:
FIELD 
     This application relates in general to electrocardiographic monitoring and, in particular, to a system for electrocardiographic signal acquisition and processing. 
     BACKGROUND 
     The first electrocardiogram (ECG) was invented by a Dutch physiologist, Willem Einthoven, in 1903, who used a string galvanometer to measure the electrical activity of the heart. Generations of physicians around the world have since used ECGs, in various forms, to diagnose heart problems and other potential medical concerns. Although the basic principles underlying Dr. Einthoven&#39;s original work, including his naming of various waveform deflections (Einthoven&#39;s triangle), are still applicable today, ECG machines have evolved from his original three-lead ECG, to ECGs with unipolar leads connected to a central reference terminal starting in 1934, to augmented unipolar leads beginning in 1942, and finally to the 12-lead ECG standardized by the American Heart Association in 1954 and still in use today. Further advances in portability and computerized interpretation have been made, yet the electronic design of the ECG recording apparatuses has remained fundamentally the same for much of the past 40 years. 
     Essentially, an ECG measures the electrical signals emitted by the heart as generated by the propagation of the action potentials that trigger depolarization of heart fibers. Physiologically, transmembrane ionic currents are generated within the heart during cardiac activation and recovery sequences. Cardiac depolarization originates high in the right atrium in the sinoatrial (SA) node before spreading leftward towards the left atrium and inferiorly towards the atrioventricular (AV) node. After a delay occasioned by the AV node, the depolarization impulse transits the Bundle of His and moves into the right and left bundle branches and Purkinje fibers to activate the right and left ventricles. 
     During each cardiac cycle, the ionic currents create an electrical field in and around the heart that can be detected by ECG electrodes placed on the skin. Cardiac electrical activity is then visually represented in an ECG trace by PQRSTU-waveforms. The P-wave represents atrial electrical activity, and the QRSTU components represent ventricular electrical activity. Specifically, a P-wave represents atrial depolarization, which causes atrial contraction. 
     P-wave analysis based on ECG monitoring is critical to accurate cardiac rhythm diagnosis and focuses on localizing the sites of origin and pathways of arrhythmic conditions. P-wave analysis is also used in the diagnosis of other medical disorders, including imbalance of blood chemistry. Cardiac arrhythmias are defined by the morphology of P-waves and their relationship to QRS intervals. For instance, atrial fibrillation (AF), an abnormally rapid heart rhythm, can be confirmed by an absence of P-waves and an irregular ventricular rate. Similarly, sinoatrial block is characterized by a delay in the onset of P-waves, while junctional rhythm, an abnormal heart rhythm resulting from impulses coming from a locus of tissue in the area of the AV node, usually presents without P-waves or with inverted P-waves. Also, the amplitudes of P-waves are valuable for diagnosis. The presence of broad, notched P-waves can indicate left atrial enlargement. Conversely, the presence of tall, peaked P-waves can indicate right atrial enlargement. Finally, P-waves with increased amplitude can indicate hypokalemia, caused by low blood potassium, whereas P-waves with decreased amplitude can indicate hyperkalemia, caused by elevated blood potassium. 
     Cardiac rhythm disorders may present with lightheadedness, fainting, chest pain, hypoxia, syncope, palpitations, and congestive heart failure (CHF), yet rhythm disorders are often sporadic in occurrence and may not show up in-clinic during a conventional 12-second ECG. Continuous ECG monitoring with P-wave-centric action potential acquisition over an extended period is more apt to capture sporadic cardiac events. However, recording sufficient ECG and related physiological data over an extended period remains a significant challenge, despite an over 40-year history of ambulatory ECG monitoring efforts combined with no appreciable improvement in P-wave acquisition techniques since Dr. Einthoven&#39;s original pioneering work over a 110 years ago. 
     Electrocardiographic monitoring over an extended period provides a physician with the kinds of data essential to identifying the underlying cause of sporadic cardiac conditions, especially rhythm disorders, and other physiological events of potential concern. A 30-day observation period is considered the “gold standard” of monitoring, yet a 14-day observation period is currently pitched as being achievable by conventional ECG monitoring approaches. Realizing a 30-day observation period has proven unworkable with existing ECG monitoring systems, which are arduous to employ; cumbersome, uncomfortable and not user-friendly to the patient; and costly to manufacture and deploy. Still, if a patient&#39;s ECG could be recorded in an ambulatory setting over a prolonged time periods, particularly for more than 14 days, thereby allowing the patient to engage in activities of daily living, the chances of acquiring meaningful medical information and capturing an abnormal event while the patient is engaged in normal activities are greatly improved. 
     The location of the atria and their low amplitude, low frequency content electrical signals make P-waves difficult to sense, particularly through ambulatory ECG monitoring. The atria are located posteriorly within the chest, and their physical distance from the skin surface adversely affects current strength and signal fidelity. Cardiac electrical potentials measured dermally have an amplitude of only one-percent of the amplitude of transmembrane electrical potentials. The distance between the heart and ECG electrodes reduces the magnitude of electrical potentials in proportion to the square of change in distance, which compounds the problem of sensing low amplitude P-waves. Moreover, the tissues and structures that lie between the activation regions within the heart and the body&#39;s surface alter the cardiac electrical field due to changes in the electrical resistivity of adjacent tissues. Thus, surface electrical potentials, when even capable of being accurately detected, are smoothed over in aspect and bear only a general spatial relationship to actual underlying cardiac events, thereby complicating diagnosis. Conventional 12-lead ECGs attempt to compensate for weak P-wave signals by monitoring the heart from multiple perspectives and angles, while conventional ambulatory ECGs primarily focus on monitoring higher amplitude ventricular activity that can be readily sensed. Both approaches are unsatisfactory with respect to the P-wave and the accurate, medically actionable diagnosis of the myriad cardiac rhythm disorders that exist. 
     Additionally, maintaining continual contact between ECG electrodes and the skin after a day or two of ambulatory ECG monitoring has been a problem. Time, dirt, moisture, and other environmental contaminants, as well as perspiration, skin oil, and dead skin cells from the patient&#39;s body, can get between an ECG electrode&#39;s non-conductive adhesive and the skin&#39;s surface. These factors adversely affect electrode adhesion and the quality of cardiac signal recordings. Furthermore, the physical movements of the patient and their clothing impart various compressional, tensile, bending, and torsional forces on the contact point of an ECG electrode, especially over long recording times, and an inflexibly fastened ECG electrode will be prone to becoming dislodged. Moreover, dislodgment may occur unbeknownst to the patient, making the ECG recordings worthless. Further, some patients may have skin that is susceptible to itching or irritation, and the wearing of ECG electrodes can aggravate such skin conditions. Thus, a patient may want or need to periodically remove or replace ECG electrodes during a long-term ECG monitoring period, whether to replace a dislodged electrode, reestablish better adhesion, alleviate itching or irritation, allow for cleansing of the skin, allow for showering and exercise, or for other purpose. Such replacement or slight alteration in electrode location actually facilitates the goal of recording the ECG signal for long periods of time. 
     Conventionally, multi-week or multi-month monitoring can be performed by implantable ECG monitors, such as the Reveal LINQ insertable cardiac monitor, manufactured by Medtronic, Inc., Minneapolis, Minn. This monitor can detect and record paroxysmal or asymptomatic arrhythmias for up to three years. However, like all forms of implantable medical device (IMD), use of this monitor requires invasive surgical implantation, which significantly increases costs; requires ongoing follow up by a physician throughout the period of implantation; requires specialized equipment to retrieve monitoring data; and carries complications attendant to all surgery, including risks of infection, injury or death. 
     Holter monitors are widely used for extended ECG monitoring. Typically, they are often used for only 24-48 hours. A typical Holter monitor is a wearable and portable version of an ECG that include cables for each electrode placed on the skin and a separate battery-powered ECG recorder. The leads are placed in the anterior thoracic region in a manner similar to what is done with an in-clinic standard ECG machine using electrode locations that are not specifically intended for optimal P-wave capture. The duration of monitoring depends on the sensing and storage capabilities of the monitor. A “looping” Holter (or event) monitor can operate for a longer period of time by overwriting older ECG tracings, thence “recycling” storage in favor of extended operation, yet at the risk of losing event data. Although capable of extended ECG monitoring, Holter monitors are cumbersome, expensive and typically only available by medical prescription, which limits their usability. Further, the skill required to properly place the electrodes on the patient&#39;s chest precludes a patient from replacing or removing the sensing leads and usually involves moving the patient from the physician office to a specialized center within the hospital or clinic. 
     U.S. Pat. No. 8,460,189, to Libbus et al. (“Libbus”) discloses an adherent wearable cardiac monitor that includes at least two measurement electrodes and an accelerometer. The device includes a reusable electronics module and a disposable adherent patch that includes the electrodes. ECG monitoring can be conducted using multiple disposable patches adhered to different locations on the patient&#39;s body. The device includes a processor configured to control collection and transmission of data from ECG circuitry, including generating and processing of ECG signals and data acquired from two or more electrodes. The ECG circuitry can be coupled to the electrodes in many ways to define an ECG vector, and the orientation of the ECG vector can be determined in response to the polarity of the measurement electrodes and orientation of the electrode measurement axis. The accelerometer can be used to determine the orientation of the measurement electrodes in each of the locations. The ECG signals measured at different locations can be rotated based on the accelerometer data to modify amplitude and direction of the ECG features to approximate a standard ECG vector. The signals recorded at different locations can be combined by summing a scaled version of each signal. Libbus further discloses that inner ECG electrodes may be positioned near outer electrodes to increase the voltage of measured ECG signals. However, Libbus treats ECG signal acquisition as the measurement of a simple aggregate directional data signal without differentiating between the distinct kinds of cardiac electrical activities presented with an ECG waveform, particularly atrial (P-wave) activity. 
     The ZIO XT Patch and ZIO Event Card devices, manufactured by iRhythm Tech., Inc., San Francisco, Calif., are wearable monitoring devices that are typically worn on the upper left pectoral region to respectively provide continuous and looping ECG recording. The location is used to simulate surgically implanted monitors, but without specifically enhancing P-wave capture. Both of these devices are prescription-only and for single patient use. The ZIO XT Patch device is limited to a 14-day period, while the electrodes only of the ZIO Event Card device can be worn for up to 30 days. The ZIO XT Patch device combines both electronic recordation components and physical electrodes into a unitary assembly that adheres to the patient&#39;s skin. The ZIO XT Patch device uses adhesive sufficiently strong to support the weight of both the monitor and the electrodes over an extended period and to resist disadherence from the patient&#39;s body, albeit at the cost of disallowing removal or relocation during the monitoring period. The ZIO Event Card device is a form of downsized Holter monitor with a recorder component that must be removed temporarily during baths or other activities that could damage the non-waterproof electronics. Both devices represent compromises between length of wear and quality of ECG monitoring, especially with respect to ease of long term use, female-friendly fit, and quality of cardiac electrical potential signals, especially atrial (P-wave) signals. 
     Therefore, a need remains for a low cost extended wear continuously recording ECG monitor attuned to capturing low amplitude cardiac action potential propagation for arrhythmia diagnosis, particularly atrial activation P-waves, and practicably capable of being worn for a long period of time, especially in patient&#39;s whose breast anatomy or size can interfere with signal quality in both women and men. 
     SUMMARY 
     Physiological monitoring can be provided through a lightweight wearable monitor that includes two components, a flexible extended wear electrode patch and a reusable monitor recorder that removably snaps into a receptacle on the electrode patch. The wearable monitor sits centrally (in the midline) on the patient&#39;s chest along the sternum oriented top-to-bottom. The ECG electrodes on the electrode patch are tailored to be positioned axially along the midline of the sternum for capturing action potential propagation in an orientation that corresponds to the aVF lead used in a conventional 12-lead ECG that is used to sense positive or upright P-waves. The placement of the wearable monitor in a location at the sternal midline (or immediately to either side of the sternum), with its unique narrow “hourglass”-like shape, significantly improves the ability of the wearable monitor to cutaneously sense cardiac electrical potential signals, particularly the P-wave (or atrial activity) and, to a lesser extent, the QRS interval signals indicating ventricular activity in the ECG waveforms. In addition, the monitor recorder includes an ECG sensing circuit that measures raw cutaneous electrical signals using a driven reference containing power supply noise and system noise to the reference lead, which is critical to preserving the characteristics of low amplitude cardiac action potentials, particularly P-waves. 
     Moreover, the electrocardiography monitor offers superior patient comfort, convenience and user-friendliness. The electrode patch is specifically designed for ease of use by a patient (or caregiver); assistance by professional medical personnel is not required. The patient is free to replace the electrode patch at any time and need not wait for a doctor&#39;s appointment to have a new electrode patch placed. Patients can easily be taught to find the familiar physical landmarks on the body necessary for proper placement of the electrode patch. Empowering patients with the knowledge to place the electrode patch in the right place ensures that the ECG electrodes will be correctly positioned on the skin, no matter the number of times that the electrode patch is replaced. In addition, the monitor recorder operates automatically and the patient only need snap the monitor recorder into place on the electrode patch to initiate ECG monitoring. Thus, the synergistic combination of the electrode patch and monitor recorder makes the use of the electrocardiography monitor a reliable and virtually foolproof way to monitor a patient&#39;s ECG and physiology for an extended, or even open-ended, period of time. 
     In one embodiment, a system for electrocardiographic signal acquisition and processing is provided. The system includes a monitor that includes: a housing adapted to interface to two electrocardiographic electrodes; the two electrocardiographic electrodes, one of the electrocardiographic electrodes including an inline resistor; an electrocardiographic front end circuit under a control of a microcontroller and configured to sense cardiac action potentials through the two electrocardiographic electrodes and to output an analog electrocardiographic signal, the electrocardiographic front end circuit further including: two leads configured to sense the potentials through the ECG sensing electrodes; at least a portion of a passive input filter stage, the at least the portion of including a coupling capacitor, a termination resistor, and filter capacitor through which at least some of a current of the sensed electrocardiographic potentials passes in a direct sequence; the microcontroller operable to execute under modular micro program control as specified in firmware and further operable to acquire samples of the output analog electrocardiographic signal; and a memory electrically interfaced with the microcontroller and operable to store the samples. 
     In a further embodiment, a multi-part system for electrocardiographic data acquisition and processing is provided. The system includes an extended wear electrode patch and a monitor recorder. The extended wear electrode patch includes a flexible backing including stretchable material defined as an elongated strip with a narrow longitudinal midsection; a pair of electrocardiographic electrodes included on a contact surface of each end of the flexible backing, each electrocardiographic electrode conductively exposed for dermal adhesion and adapted to be positioned axially along a midline of a sternum for capturing action potential propagation, one of the electrocardiographic electrodes including an inline resistor; a non-conductive receptacle affixed to a non-contacting surface of the flexible backing and including an electro mechanical docking interface; and a pair of flexible circuit traces affixed at each end of the flexible backing with each circuit trace connecting one of the electrocardiographic electrodes to the docking interface. The monitor recorder includes a housing adapted to be coupled to the pair of the electrocardiographic electrodes via the electromechanical docking interface; an electrocardiographic front end circuit under a control of a microcontroller and configured to sense cardiac action potentials through the pair of the electrocardiographic electrodes and to output an analog electrocardiographic signal; the microcontroller operable to execute under modular micro program control as specified in firmware and further operable to acquire samples of the output analog electrocardiographic signal; and a memory electrically interfaced with the microcontroller and operable to store the samples. The electrocardiographic front end circuit further includes: two leads configured to sense the potentials through the ECG sensing electrodes; at least a portion of a passive input filter stage, the at least the portion of the stage including a coupling capacitor, a termination resistor, and filter capacitor through which at least some of a current of the sensed electrocardiographic potentials passes in a direct sequence. 
     The foregoing aspects enhance ECG monitoring performance and quality by facilitating long-term ECG recording, which is critical to accurate arrhythmia and cardiac rhythm disorder diagnoses. 
     The monitoring patch is especially suited to the female anatomy, although also easily used over the male sternum. The narrow longitudinal midsection can fit nicely within the inter-mammary cleft of the breasts without inducing discomfort, whereas conventional patch electrodes are wide and, if adhered between the breasts, would cause chafing, irritation, discomfort, and annoyance, leading to low patient compliance. 
     In addition, the foregoing aspects enhance comfort in women (and certain men), but not irritation of the breasts, by placing the monitoring patch in the best location possible for optimizing the recording of cardiac signals from the atrium, particularly P-waves, which is another feature critical to proper arrhythmia and cardiac rhythm disorder diagnoses. 
     Still other embodiments will become readily apparent to those skilled in the art from the following detailed description, wherein are described embodiments by way of illustrating the best mode contemplated. As will be realized, other and different embodiments are possible and the embodiments&#39; several details are capable of modifications in various obvious respects, all without departing from their spirit and the scope. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1  and  2    are diagrams showing, by way of examples, an extended wear electrocardiography monitor, including an extended wear electrode patch, in accordance with one embodiment, respectively fitted to the sternal region of a female patient and a male patient. 
         FIG.  3    is a front anatomical view showing, by way of illustration, the locations of the heart and lungs within the rib cage of an adult human. 
         FIG.  4    is a perspective view showing an extended wear electrode patch in accordance with one embodiment with a monitor recorder inserted. 
         FIG.  5    is a perspective view showing the monitor recorder of  FIG.  4   . 
         FIG.  6    is a perspective view showing the extended wear electrode patch of  FIG.  4    without a monitor recorder inserted. 
         FIG.  7    is a bottom plan view of the monitor recorder of  FIG.  4   . 
         FIG.  8    is a top view showing the flexible circuit of the extended wear electrode patch of  FIG.  4   . 
         FIG.  9    is a functional block diagram showing the component architecture of the circuitry of the monitor recorder of  FIG.  4   . 
         FIG.  10    is a functional block diagram showing the circuitry of the extended wear electrode patch of  FIG.  4   . 
         FIG.  11    is a schematic diagram showing the ECG front end circuit of the circuitry of the monitor recorder of  FIG.  9   . 
         FIG.  12    is a flow diagram showing a monitor recorder-implemented method for monitoring ECG data for use in the monitor recorder of  FIG.  4   . 
         FIG.  13    is a graph showing, by way of example, a typical ECG waveform. 
         FIG.  14    is a functional block diagram showing the signal processing functionality of the microcontroller. 
         FIG.  15    is a functional block diagram showing the operations performed by the download station. 
         FIGS.  16 A-C  are functional block diagrams respectively showing practical uses of the extended wear electrocardiography monitors of  FIGS.  1  and  2   . 
         FIG.  17    is a perspective view of an extended wear electrode patch with a flexile wire electrode assembly in accordance with a still further embodiment. 
         FIG.  18    is perspective view of the flexile wire electrode assembly from  FIG.  17   , with a layer of insulating material shielding a bare distal wire around the midsection of the flexible backing. 
         FIG.  19    is a bottom view of the flexile wire electrode assembly as shown in  FIG.  17   . 
         FIG.  20    is a bottom view of a flexile wire electrode assembly in accordance with a still yet further embodiment. 
         FIG.  21    is a perspective view showing the longitudinal midsection of the flexible backing of the electrode assembly from  FIG.  17   . 
     
    
    
     DETAILED DESCRIPTION 
     ECG and physiological monitoring can be provided through a wearable ambulatory monitor that includes two components, a flexible extended wear electrode patch and a removable reusable (or single use) monitor recorder. Both the electrode patch and the monitor recorder are optimized to capture electrical signals from the propagation of low amplitude, relatively low frequency content cardiac action potentials, particularly the P-waves generated during atrial activation.  FIGS.  1  and  2    are diagrams showing, by way of examples, an extended wear electrocardiography monitor  12 , including a monitor recorder  14 , in accordance with one embodiment, respectively fitted to the sternal region of a female patient  10  and a male patient  11 . The wearable monitor  12  sits centrally, positioned axially along the sternal midline  16 , on the patient&#39;s chest along the sternum  13  and oriented top-to-bottom with the monitor recorder  14  preferably situated towards the patient&#39;s head. In a further embodiment, the orientation of the wearable monitor  12  can be corrected post-monitoring, as further described infra, for instance, if the wearable monitor  12  is inadvertently fitted upside down. 
     The electrode patch  15  is shaped to fit comfortably and conformal to the contours of the patient&#39;s chest approximately centered on the sternal midline  16  (or immediately to either side of the sternum  13 ). The distal end of the electrode patch  15 , under which a lower or inferior pole (ECG electrode) is adhered, extends towards the Xiphoid process and lower sternum and, depending upon the patient&#39;s build, may straddle the region over the Xiphoid process and lower sternum. The proximal end of the electrode patch  15 , located under the monitor recorder  14 , under which an upper or superior pole (ECG electrode) is adhered, is below the manubrium and, depending upon patient&#39;s build, may straddle the region over the manubrium. 
     During ECG monitoring, the amplitude and strength of action potentials sensed on the body&#39;s surface are affected to varying degrees by cardiac, cellular, extracellular, vector of current flow, and physical factors, like obesity, dermatitis, large breasts, and high impedance skin, as can occur in dark-skinned individuals. Sensing along the sternal midline  16  (or immediately to either side of the sternum  13 ) significantly improves the ability of the wearable monitor  12  to cutaneously sense cardiac electric signals, particularly the P-wave (or atrial activity) and, to a lesser extent, the QRS interval signals in the ECG waveforms that indicate ventricular activity by countering some of the effects of these factors. 
     The ability to sense low amplitude, low frequency content body surface potentials is directly related to the location of ECG electrodes on the skin&#39;s surface and the ability of the sensing circuitry to capture these electrical signals.  FIG.  3    is a front anatomical view showing, by way of illustration, the locations of the heart  4  and lungs  5  within the rib cage of an adult human. Depending upon their placement locations on the chest, ECG electrodes may be separated from activation regions within the heart  4  by differing combinations of internal tissues and body structures, including heart muscle, intracardiac blood, the pericardium, intrathoracic blood and fluids, the lungs  5 , skeletal muscle, bone structure, subcutaneous fat, and the skin, plus any contaminants present between the skin&#39;s surface and electrode signal pickups. The degree of amplitude degradation of cardiac transmembrane potentials increases with the number of tissue boundaries between the heart  4  and the skin&#39;s surface that are encountered. The cardiac electrical field is degraded each time the transmembrane potentials encounter a physical boundary separating adjoining tissues due to differences in the respective tissues&#39; electrical resistances. In addition, other non-spatial factors, such as pericardial effusion, emphysema or fluid accumulation in the lungs, as further explained infra, can further degrade body surface potentials. 
     Internal tissues and body structures can adversely affect the current strength and signal fidelity of all body surface potentials, yet low amplitude cardiac action potentials, particularly the P-wave with a normative amplitude of less than 0.25 microvolts (mV) and a normative duration of less than 120 milliseconds (ms), are most apt to be negatively impacted. The atria  6  are generally located posteriorly within the thoracic cavity (with the exception of the anterior right atrium and right atrial appendage), and, physically, the left atrium constitutes the portion of the heart  4  furthest away from the surface of the skin on the chest. Conversely, the ventricles  7 , which generate larger amplitude signals, generally are located anteriorly with the anterior right ventricle and most of the left ventricle situated relatively close to the skin surface on the chest, which contributes to the relatively stronger amplitudes of ventricular waveforms. Thus, the quality of P-waves (and other already-low amplitude action potential signals) is more susceptible to weakening from intervening tissues and structures than the waveforms associated with ventricular activation. 
     The importance of the positioning of ECG electrodes along the sternal midline  15  has largely been overlooked by conventional approaches to ECG monitoring, in part due to the inability of their sensing circuitry to reliably detect low amplitude, low frequency content electrical signals, particularly in P-waves. In turn, that inability to keenly sense P-waves has motivated ECG electrode placement in other non-sternal midline thoracic locations, where the QRSTU components that represent ventricular electrical activity are more readily detectable by their sensing circuitry than P-waves. In addition, ECG electrode placement along the sternal midline  15  presents major patient wearability challenges, such as fitting a monitoring ensemble within the narrow confines of the inter-mammary cleft between the breasts, that to large extent drive physical packaging concerns, which can be incompatible with ECG monitors intended for placement, say, in the upper pectoral region or other non-sternal midline thoracic locations. In contrast, the wearable monitor  12  uses an electrode patch  15  that is specifically intended for extended wear placement in a location at the sternal midline  16  (or immediately to either side of the sternum  13 ). When combined with a monitor recorder  14  that uses sensing circuitry optimized to preserve the characteristics of low amplitude cardiac action potentials, especially those signals from the atria, as further described infra with reference to  FIG.  11   , the electrode patch  15  helps to significantly improve atrial activation (P-wave) sensing through placement in a body location that robustly minimizes the effects of tissue and body structure. 
     Referring back to  FIGS.  1  and  2   , the placement of the wearable monitor  12  in the region of the sternal midline  13  puts the ECG electrodes of the electrode patch  15  in locations better adapted to sensing and recording low amplitude cardiac action potentials during atrial propagation (P-wave signals) than placement in other locations, such as the upper left pectoral region, as commonly seen in most conventional ambulatory ECG monitors. The sternum  13  overlies the right atrium of the heart  4 . As a result, action potential signals have to travel through fewer layers of tissue and structure to reach the ECG electrodes of the electrode patch  15  on the body&#39;s surface along the sternal midline  13  when compared to other monitoring locations, a distinction that is of critical importance when capturing low frequency content electrical signals, such as P-waves. 
     Moreover, cardiac action potential propagation travels simultaneously along a north-to-south and right-to-left vector, beginning high in the right atrium and ultimately ending in the posterior and lateral region of the left ventricle. Cardiac depolarization originates high in the right atrium in the SA node before concurrently spreading leftward towards the left atrium and inferiorly towards the AV node. The ECG electrodes of the electrode patch  15  are placed with the upper or superior pole (ECG electrode) along the sternal midline  13  in the region of the manubrium and the lower or inferior pole (ECG electrode) along the sternal midline  13  in the region of the Xiphoid process  9  and lower sternum. The ECG electrodes are placed primarily in a north-to-south orientation along the sternum  13  that corresponds to the north-to-south waveform vector exhibited during atrial activation. This orientation corresponds to the aVF lead used in a conventional 12-lead ECG that is used to sense positive or upright P-waves. 
     Furthermore, the thoracic region underlying the sternum  13  along the midline  16  between the manubrium  8  and Xiphoid process  9  is relatively free of lung tissue, musculature, and other internal body structures that could occlude the electrical signal path between the heart  4 , particularly the atria, and ECG electrodes placed on the surface of the skin. Fewer obstructions means that cardiac electrical potentials encounter fewer boundaries between different tissues. As a result, when compared to other thoracic ECG sensing locations, the cardiac electrical field is less altered when sensed dermally along the sternal midline  13 . As well, the proximity of the sternal midline  16  to the ventricles  7  facilitates sensing of right ventricular activity and provides superior recordation of the QRS interval, again, in part due to the relatively clear electrical path between the heart  4  and the skin surface. 
     Finally, non-spatial factors can affect transmembrane action potential shape and conductivity. For instance, myocardial ischemia, an acute cardiac condition, can cause a transient increase in blood perfusion in the lungs  5 . The perfused blood can significantly increase electrical resistance across the lungs  5  and therefore degrade transmission of the cardiac electrical field to the skin&#39;s surface. However, the placement of the wearable monitor  12  along the sternal midline  16  in the inter-mammary cleft between the breasts is relatively resilient to the adverse effects to cardiac action potential degradation caused by ischemic conditions as the body surface potentials from a location relatively clear of underlying lung tissue and fat help compensate for the loss of signal amplitude and content. The monitor recorder  14  is thus able to record the P-wave morphology that may be compromised by myocardial ischemia and therefore make diagnosis of the specific arrhythmias that can be associated with myocardial ischemia more difficult. 
     During use, the electrode patch  15  is first adhered to the skin along the sternal midline  16  (or immediately to either side of the sternum  13 ). A monitor recorder  14  is then snapped into place on the electrode patch  15  using an electro mechanical docking interface to initiate ECG monitoring.  FIG.  4    is a perspective view showing an extended wear electrode patch  15  in accordance with one embodiment with a monitor recorder  14  inserted. The body of the electrode patch  15  is preferably constructed using a flexible backing  20  formed as an elongated strip  21  of wrap knit or similar stretchable material about 145 mm long and 32 mm at the widest point with a narrow longitudinal mid-section  23  evenly tapering inward from both sides. A pair of cut-outs  22  between the distal and proximal ends of the electrode patch  15  create a narrow longitudinal midsection  23  or “isthmus” and defines an elongated “hourglass”-like shape, when viewed from above, such as described in commonly-assigned U.S. Design Patent No. D744,659, issued Dec. 1, 2015, the disclosure of which is incorporated by reference. The upper part of the “hourglass” is sized to allow an electrically non-conductive receptacle  25 , sits on top of the outward-facing surface of the electrode patch  15 , to be affixed to the electrode patch  15  with an ECG electrode placed underneath on the patient-facing underside, or contact, surface of the electrode patch  15 ; the upper part of the “hourglass” has a longer and wider profile (but still rounded and tapered to fit comfortably between the breasts) than the lower part of the “hourglass,” which is sized primarily to allow just the placement of an ECG electrode of appropriate shape and surface area to record the P-wave and the QRS signals sufficiently given the inter-electrode spacing. 
     The electrode patch  15  incorporates features that significantly improve wearability, performance, and patient comfort throughout an extended monitoring period. The entire electrode patch  15  is lightweight in construction, which allows the patch to be resilient to disadhesing or falling off and, critically, to avoid creating distracting discomfort to the patient, even when the patient is asleep. In contrast, the weight of a heavy ECG monitor impedes patient mobility and will cause the monitor to constantly tug downwards and press on the patient&#39;s body that can generate skin inflammation with frequent adjustments by the patient needed to maintain comfort. 
     During everyday wear, the electrode patch  15  is subjected to pushing, pulling, and torsional movements, including compressional and torsional forces when the patient bends forward, or tensile and torsional forces when the patient leans backwards. To counter these stress forces, the electrode patch  15  incorporates crimp and strain reliefs, such as described in commonly-assigned U.S. Pat. No. 9,545,204, issued Jan. 17, 2017, the disclosure of which is incorporated by reference. In addition, the cut-outs  22  and longitudinal midsection  23  help minimize interference with and discomfort to breast tissue, particularly in women (and gynecomastic men). The cut-outs  22  and longitudinal midsection  23  further allow better conformity of the electrode patch  15  to sternal bowing and to the narrow isthmus of flat skin that can occur along the bottom of the inter-mammary cleft between the breasts, especially in buxom women. The cut-outs  22  and narrow and flexible longitudinal midsection  23  help the electrode patch  15  fit nicely between a pair of female breasts in the inter-mammary cleft. In one embodiment, the cut-outs  22  can be graduated to form the longitudinal midsection  23  as a narrow in-between stem or isthmus portion about 7 mm wide. In a still further embodiment, tabs  24  can respectively extend an additional 8 mm to 12 mm beyond the distal and proximal ends of the flexible backing  20  to facilitate with adhering the electrode patch  15  to or removing the electrode patch  15  from the sternum  13 . These tabs preferably lack adhesive on the underside, or contact, surface of the electrode patch  15 . Still other shapes, cut-outs and conformities to the electrode patch  15  are possible. 
     The monitor recorder  14  removably and reusably snaps into an electrically non-conductive receptacle  25  during use. The monitor recorder  14  contains electronic circuitry for recording and storing the patient&#39;s electrocardiography as sensed via a pair of ECG electrodes provided on the electrode patch  15 , as further described infra beginning with reference to  FIG.  9   . The non-conductive receptacle  25  is provided on the top surface of the flexible backing  20  with a retention catch  26  and tension clip  27  molded into the non-conductive receptacle  25  to conformably receive and securely hold the monitor recorder  14  in place. 
     The monitor recorder  14  includes a sealed housing that snaps into place in the non-conductive receptacle  25 .  FIG.  5    is a perspective view showing the monitor recorder  14  of  FIG.  4   . The sealed housing  50  of the monitor recorder  14  intentionally has a rounded isosceles trapezoidal-like shape  52 , when viewed from above, such as described in commonly-assigned U.S. Design Pat. No. D717,955, issued Nov. 18, 2014, the disclosure of which is incorporated by reference. The edges  51  along the top and bottom surfaces are rounded for patient comfort. The sealed housing  50  is approximately 47 mm long, 23 mm wide at the widest point, and 7 mm high, excluding a patient-operable tactile-feedback button  55 . The sealed housing  50  can be molded out of polycarbonate, ABS, or an alloy of those two materials. The button  55  is waterproof and the button&#39;s top outer surface is molded silicon rubber or similar soft pliable material. A retention detent  53  and tension detent  54  are molded along the edges of the top surface of the housing  50  to respectively engage the retention catch  26  and the tension clip  27  molded into non-conductive receptacle  25 . Other shapes, features, and conformities of the sealed housing  50  are possible. 
     The electrode patch  15  is intended to be disposable, while the monitor recorder  14  is designed for reuse and can be transferred to successive electrode patches  15  to ensure continuity of monitoring, if so desired. The monitor recorder  14  can be used only once, but single use effectively wastes the synergistic benefits provided by the combination of the disposable electrode patch and reusable monitor recorder, as further explained infra with reference to  FIGS.  16 A-C . The placement of the wearable monitor  12  in a location at the sternal midline  16  (or immediately to either side of the sternum  13 ) benefits long-term extended wear by removing the requirement that ECG electrodes be continually placed in the same spots on the skin throughout the monitoring period. Instead, the patient is free to place an electrode patch  15  anywhere within the general region of the sternum  13 . 
     As a result, at any point during ECG monitoring, the patient&#39;s skin is able to recover from the wearing of an electrode patch  15 , which increases patient comfort and satisfaction, while the monitor recorder  14  ensures ECG monitoring continuity with minimal effort. A monitor recorder  14  is merely unsnapped from a worn out electrode patch  15 , the worn out electrode patch  15  is removed from the skin, a new electrode patch  15  is adhered to the skin, possibly in a new spot immediately adjacent to the earlier location, and the same monitor recorder  14  is snapped into the new electrode patch  15  to reinitiate and continue the ECG monitoring. 
     During use, the electrode patch  15  is first adhered to the skin in the sternal region.  FIG.  6    is a perspective view showing the extended wear electrode patch  15  of  FIG.  4    without a monitor recorder  14  inserted. A flexible circuit  32  is adhered to each end of the flexible backing  20 . A distal circuit trace  33  from the distal end  30  of the flexible backing  20  and a proximal circuit trace (not shown) from the proximal end  31  of the flexible backing  20  electrically couple ECG electrodes (not shown) with a pair of electrical pads  34 . In a further embodiment, the distal and proximal circuit traces are replaced with interlaced or sewn-in flexible wires, as further described infra beginning with reference to  FIG.  17   . The electrical pads  34  are provided within a moisture-resistant seal  35  formed on the bottom surface of the non-conductive receptacle  25 . When the monitor recorder  14  is securely received into the non-conductive receptacle  25 , that is, snapped into place, the electrical pads  34  interface to electrical contacts (not shown) protruding from the bottom surface of the monitor recorder  14 . The moisture-resistant seal  35  enables the monitor recorder  14  to be worn at all times, even during showering or other activities that could expose the monitor recorder  14  to moisture or adverse conditions. 
     In addition, a battery compartment  36  is formed on the bottom surface of the non-conductive receptacle  25 . A pair of battery leads (not shown) from the battery compartment  36  to another pair of the electrical pads  34  electrically interface the battery to the monitor recorder  14 . The battery contained within the battery compartment  35  is a direct current (DC) power cell and can be replaceable, rechargeable or disposable. 
     The monitor recorder  14  draws power externally from the battery provided in the non-conductive receptacle  25 , thereby uniquely obviating the need for the monitor recorder  14  to carry a dedicated power source.  FIG.  7    is a bottom plan view of the monitor recorder  14  of  FIG.  4   . A cavity  58  is formed on the bottom surface of the sealed housing  50  to accommodate the upward projection of the battery compartment  36  from the bottom surface of the non-conductive receptacle  25 , when the monitor recorder  14  is secured in place on the non-conductive receptacle  25 . A set of electrical contacts  56  protrude from the bottom surface of the sealed housing  50  and are arranged in alignment with the electrical pads  34  provided on the bottom surface of the non-conductive receptacle  25  to establish electrical connections between the electrode patch  15  and the monitor recorder  14 . In addition, a seal coupling  57  circumferentially surrounds the set of electrical contacts  56  and securely mates with the moisture-resistant seal  35  formed on the bottom surface of the non-conductive receptacle  25 . The battery contained within the battery compartment  36  can be replaceable, rechargeable or disposable. In a further embodiment, the ECG sensing circuitry of the monitor recorder  14  can be supplemented with additional sensors, including an SpO 2  sensor, a blood pressure sensor, a temperature sensor, respiratory rate sensor, a glucose sensor, an air flow sensor, and a volumetric pressure sensor, which can be incorporated directly into the monitor recorder  14  or onto the non-conductive receptacle  25 . 
     The placement of the flexible backing  20  on the sternal midline  16  (or immediately to either side of the sternum  13 ) also helps to minimize the side-to-side movement of the wearable monitor  12  in the left- and right-handed directions during wear. However, the wearable monitor  12  is still susceptible to pushing, pulling, and torqueing movements, including compressional and torsional forces when the patient bends forward, and tensile and torsional forces when the patient leans backwards or twists. To counter the dislodgment of the flexible backing  20  due to compressional and torsional forces, a layer of non-irritating adhesive, such as hydrocolloid, is provided at least partially on the underside, or contact, surface of the flexible backing  20 , but only on the distal end  30  and the proximal end  31 . As a result, the underside, or contact surface of the longitudinal midsection  23  does not have an adhesive layer and remains free to move relative to the skin. Thus, the longitudinal midsection  23  forms a crimp relief that respectively facilitates compression and twisting of the flexible backing  20  in response to compressional and torsional forces. Other forms of flexible backing crimp reliefs are possible. 
     Unlike the flexible backing  20 , the flexible circuit  32  is only able to bend and cannot stretch in a planar direction. The flexible circuit  32  can be provided either above or below the flexible backing  20 .  FIG.  8    is a top view showing the flexible circuit  32  of the extended wear electrode patch  15  of  FIG.  4    when mounted above the flexible backing  20 . A distal ECG electrode  38  and proximal ECG electrode  39  are respectively coupled to the distal and proximal ends of the flexible circuit  32  to serve as electrode signal pickups. The flexible circuit  32  preferably does not extend to the outside edges of the flexible backing  20 , thereby avoiding gouging or discomforting the patient&#39;s skin during extended wear, such as when sleeping on the side. During wear, the ECG electrodes  38 ,  39  must remain in continual contact with the skin. A strain relief  40  is defined in the flexible circuit  32  at a location that is partially underneath the battery compartment  36  when the flexible circuit  32  is affixed to the flexible backing  20 . The strain relief  40  is laterally extendable to counter dislodgment of the ECG electrodes  38 ,  39  due to bending, tensile and torsional forces. A pair of strain relief cutouts  41  partially extend transversely from each opposite side of the flexible circuit  32  and continue longitudinally towards each other to define in ‘S’-shaped pattern, when viewed from above. The strain relief respectively facilitates longitudinal extension and twisting of the flexible circuit  32  in response to tensile and torsional forces. Other forms of circuit board strain relief are possible. 
     ECG monitoring and other functions performed by the monitor recorder  14  are provided through a micro controlled architecture.  FIG.  9    is a functional block diagram showing the component architecture of the circuitry  60  of the monitor recorder  14  of  FIG.  4   . The circuitry  60  is externally powered through a battery provided in the non-conductive receptacle  25  (shown in  FIG.  6   ). Both power and raw ECG signals, which originate in the pair of ECG electrodes  38 ,  39  (shown in  FIG.  8   ) on the distal and proximal ends of the electrode patch  15 , are received through an external connector  65  that mates with a corresponding physical connector on the electrode patch  15 . The external connector  65  includes the set of electrical contacts  56  that protrude from the bottom surface of the sealed housing  50  and which physically and electrically interface with the set of pads  34  provided on the bottom surface of the non-conductive receptacle  25 . The external connector includes electrical contacts  56  for data download, microcontroller communications, power, analog inputs, and a peripheral expansion port. The arrangement of the pins on the electrical connector  65  of the monitor recorder  14  and the device into which the monitor recorder  14  is attached, whether an electrode patch  15  or download station (not shown), follow the same electrical pin assignment convention to facilitate interoperability. The external connector  65  also serves as a physical interface to a download station that permits the retrieval of stored ECG monitoring data, communication with the monitor recorder  14 , and performance of other functions. The download station is further described infra with reference to  FIG.  15   . 
     Operation of the circuitry  60  of the monitor recorder  14  is managed by a microcontroller  61 , such as the EFM32 Tiny Gecko 32-bit microcontroller, manufactured by Silicon Laboratories Inc., Austin, Tex. The microcontroller  61  has flexible energy management modes and includes a direct memory access controller and built-in analog-to-digital and digital-to-analog converters (ADC and DAC, respectively). The microcontroller  61  also includes a program memory unit containing internal flash memory that is readable and writeable. The internal flash memory can also be programmed externally. The microcontroller  61  operates under modular micro program control as specified in firmware stored in the internal flash memory. The functionality and firmware modules relating to signal processing by the microcontroller  61  are further described infra with reference to  FIG.  14   . The microcontroller  61  draws power externally from the battery provided on the electrode patch  15  via a pair of the electrical contacts  56 . The microcontroller  61  connects to the ECG front end circuit  63  that measures raw cutaneous electrical signals using a driven reference that eliminates common mode noise, as further described infra with reference to  FIG.  11   . 
     The circuitry  60  of the monitor recorder  14  also includes a flash memory  62 , which the microcontroller  61  uses for storing ECG monitoring data and other physiology and information. The flash memory  62  also draws power externally from the battery provided on the electrode patch  15  via a pair of the electrical contacts  56 . Data is stored in a serial flash memory circuit, which supports read, erase and program operations over a communications bus. The flash memory  62  enables the microcontroller  61  to store digitized ECG data. The communications bus further enables the flash memory  62  to be directly accessed externally over the external connector  65  when the monitor recorder  14  is interfaced to a download station. 
     The microcontroller  61  includes functionality that enables the acquisition of samples of analog ECG signals, which are converted into a digital representation, as further described infra with reference to  FIG.  14   . In one mode, the microcontroller  61  will acquire, sample, digitize, signal process, and store digitized ECG data into available storage locations in the flash memory  62  until all memory storage locations are filled, after which the digitized ECG data needs to be downloaded or erased to restore memory capacity. Data download or erasure can also occur before all storage locations are filled, which would free up memory space sooner, albeit at the cost of possibly interrupting monitoring while downloading or erasure is performed. In another mode, the microcontroller  61  can include a loop recorder feature that will overwrite the oldest stored data once all storage locations are filled, albeit at the cost of potentially losing the stored data that was overwritten, if not previously downloaded. Still other modes of data storage and capacity recovery are possible. 
     The circuitry  60  of the monitor recorder  14  further includes an actigraphy sensor  64  implemented as a 3-axis accelerometer. The accelerometer may be configured to generate interrupt signals to the microcontroller  61  by independent initial wake up and free fall events, as well as by device position. In addition, the actigraphy provided by the accelerometer can be used during post-monitoring analysis to correct the orientation of the monitor recorder  14  if, for instance, the monitor recorder  14  has been inadvertently installed upside down, that is, with the monitor recorder  14  oriented on the electrode patch  15  towards the patient&#39;s feet, as well as for other event occurrence analyses. 
     The microcontroller  61  includes an expansion port that also utilizes the communications bus. External devices, separately drawing power externally from the battery provided on the electrode patch  15  or other source, can interface to the microcontroller  61  over the expansion port in half duplex mode. For instance, an external physiology sensor can be provided as part of the circuitry  60  of the monitor recorder  14 , or can be provided on the electrode patch  15  with communication with the microcontroller  61  provided over one of the electrical contacts  56 . The physiology sensor can include an SpO 2  sensor, blood pressure sensor, temperature sensor, respiratory rate sensor, glucose sensor, airflow sensor, volumetric pressure sensing, or other types of sensor or telemetric input sources. In a further embodiment, a wireless interface for interfacing with other wearable (or implantable) physiology monitors, as well as data offload and programming, can be provided as part of the circuitry  60  of the monitor recorder  14 , or can be provided on the electrode patch  15  with communication with the microcontroller  61  provided over one of the electrical contacts  56 . 
     Finally, the circuitry  60  of the monitor recorder  14  includes patient-interfaceable components, including a tactile feedback button  66 , which a patient can press to mark events or to perform other functions, and a buzzer  67 , such as a speaker, magnetic resonator or piezoelectric buzzer. The buzzer  67  can be used by the microcontroller  61  to output feedback to a patient such as to confirm power up and initiation of ECG monitoring. Still other components as part of the circuitry  60  of the monitor recorder  14  are possible. 
     While the monitor recorder  14  operates under micro control, most of the electrical components of the electrode patch  15  operate passively.  FIG.  10    is a functional block diagram showing the circuitry  70  of the extended wear electrode patch  15  of  FIG.  4   . The circuitry  70  of the electrode patch  15  is electrically coupled with the circuitry  60  of the monitor recorder  14  through an external connector  74 . The external connector  74  is terminated through the set of pads  34  provided on the bottom of the non-conductive receptacle  25 , which electrically mate to corresponding electrical contacts  56  protruding from the bottom surface of the sealed housing  50  to electrically interface the monitor recorder  14  to the electrode patch  15 . 
     The circuitry  70  of the electrode patch  15  performs three primary functions. First, a battery  71  is provided in a battery compartment formed on the bottom surface of the non-conductive receptacle  25 . The battery  71  is electrically interfaced to the circuitry  60  of the monitor recorder  14  as a source of external power. The unique provisioning of the battery  71  on the electrode patch  15  provides several advantages. First, the locating of the battery  71  physically on the electrode patch  15  lowers the center of gravity of the overall wearable monitor  12  and thereby helps to minimize shear forces and the effects of movements of the patient and clothing. Moreover, the housing  50  of the monitor recorder  14  is sealed against moisture and providing power externally avoids having to either periodically open the housing  50  for the battery replacement, which also creates the potential for moisture intrusion and human error, or to recharge the battery, which can potentially take the monitor recorder  14  off line for hours at a time. In addition, the electrode patch  15  is intended to be disposable, while the monitor recorder  14  is a reusable component. Each time that the electrode patch  15  is replaced, a fresh battery is provided for the use of the monitor recorder  14 , which enhances ECG monitoring performance quality and duration of use. Also, the architecture of the monitor recorder  14  is open, in that other physiology sensors or components can be added by virtue of the expansion port of the microcontroller  61 . Requiring those additional sensors or components to draw power from a source external to the monitor recorder  14  keeps power considerations independent of the monitor recorder  14 . This approach also enables a battery of higher capacity to be introduced when needed to support the additional sensors or components without effecting the monitor recorders circuitry  60 . 
     Second, the pair of ECG electrodes  38 ,  39  respectively provided on the distal and proximal ends of the flexible circuit  32  are electrically coupled to the set of pads  34  provided on the bottom of the non-conductive receptacle  25  by way of their respective circuit traces  33 ,  37 . The signal ECG electrode  39  includes a protection circuit  72 , which is an inline resistor that protects the patient from excessive leakage current should the front end circuit fail. 
     Last, in a further embodiment, the circuitry  70  of the electrode patch  15  includes a cryptographic circuit  73  to authenticate an electrode patch  15  for use with a monitor recorder  14 . The cryptographic circuit  73  includes a device capable of secure authentication and validation. The cryptographic device  73  ensures that only genuine, non-expired, safe, and authenticated electrode patches  15  are permitted to provide monitoring data to a monitor recorder  14  and for a specific patient. 
     The ECG front end circuit  63  measures raw cutaneous electrical signals using a driven reference that effectively reduces common mode noise, power supply noise and system noise, which is critical to preserving the characteristics of low amplitude cardiac action potentials, especially those signals from the atria.  FIG.  11    is a schematic diagram  80  showing the ECG front end circuit  63  of the circuitry  60  of the monitor recorder  14  of  FIG.  9   . The ECG front end circuit  63  senses body surface potentials through a signal lead (“S 1 ”) and reference lead (“REF”) that are respectively connected to the ECG electrodes of the electrode patch  15 . Power is provided to the ECG front end circuit  63  through a pair of DC power leads (“VCC” and “GND”). An analog ECG signal (“ECG”) representative of the electrical activity of the patient&#39;s heart over time is output, which the micro controller  11  converts to digital representation and filters, as further described infra. 
     The ECG front end circuit  63  is organized into five stages, a passive input filter stage  81 , a unity gain voltage follower stage  82 , a passive high pass filtering stage  83 , a voltage amplification and active filtering stage  84 , and an anti-aliasing passive filter stage  85 , plus a reference generator. Each of these stages and the reference generator will now be described. 
     The passive input filter stage  81  includes the parasitic impedance of the ECG electrodes  38 ,  39  (shown in  FIG.  8   ), the protection resistor that is included as part of the protection circuit  72  of the ECG electrode  39  (shown in  FIG.  10   ), an AC coupling capacitor  87 , a termination resistor  88 , and filter capacitor  89 . This stage passively shifts the frequency response poles downward there is a high electrode impedance from the patient on the signal lead S 1  and reference lead REF, which reduces high frequency noise. 
     The unity gain voltage follower stage  82  provides a unity voltage gain that allows current amplification by an Operational Amplifier (“Op Amp”)  90 . In this stage, the voltage stays the same as the input, but more current is available to feed additional stages. This configuration allows a very high input impedance, so as not to disrupt the body surface potentials or the filtering effect of the previous stage. 
     The passive high pass filtering stage  83  is a high pass filter that removes baseline wander and any offset generated from the previous stage. Adding an AC coupling capacitor  91  after the Op Amp  90  allows the use of lower cost components, while increasing signal fidelity. 
     The voltage amplification and active filtering stage  84  amplifies the voltage of the input signal through Op Amp  92 , while applying a low pass filter. The DC bias of the input signal is automatically centered in the highest performance input region of the Op Amp  92  because of the AC coupling capacitor  91 . 
     The anti-aliasing passive filter stage  85  provides an anti-aliasing low pass filter. When the microcontroller  61  acquires a sample of the analog input signal, a disruption in the signal occurs as a sample and hold capacitor that is internal to the microcontroller  61  is charged to supply signal for acquisition. 
     The reference generator in subcircuit  86  drives a driven reference containing power supply noise and system noise to the reference lead REF. A coupling capacitor  87  is included on the signal lead S 1  and a pair of resistors  93   a ,  93   b  inject system noise into the reference lead REF. The reference generator is connected directly to the patient, thereby avoiding the thermal noise of the protection resistor that is included as part of the protection circuit  72 . 
     In contrast, conventional ECG lead configurations try to balance signal and reference lead connections. The conventional approach suffers from the introduction of differential thermal noise, lower input common mode rejection, increased power supply noise, increased system noise, and differential voltages between the patient reference and the reference used on the device that can obscure, at times, extremely, low amplitude body surface potentials. 
     Here, the parasitic impedance of the ECG electrodes  38 ,  39 , the protection resistor that is included as part of the protection circuit  72  and the coupling capacitor  87  allow the reference lead REF to be connected directly to the skin&#39;s surface without any further components. As a result, the differential thermal noise problem caused by pairing protection resistors to signal and reference leads, as used in conventional approaches, is avoided. 
     The monitor recorder  14  continuously monitors the patient&#39;s heart rate and physiology.  FIG.  12    is a flow diagram showing a monitor recorder-implemented method  100  for monitoring ECG data for use in the monitor recorder  14  of  FIG.  4   . Initially, upon being connected to the set of pads  34  provided with the non-conductive receptacle  25  when the monitor recorder  14  is snapped into place, the microcontroller  61  executes a power up sequence (step  101 ). During the power up sequence, the voltage of the battery  71  is checked, the state of the flash memory  62  is confirmed, both in terms of operability check and available capacity, and microcontroller operation is diagnostically confirmed. In a further embodiment, an authentication procedure between the microcontroller  61  and the electrode patch  15  are also performed. 
     Following satisfactory completion of the power up sequence, an iterative processing loop (steps  102 - 110 ) is continually executed by the microcontroller  61 . During each iteration (step  102 ) of the processing loop, the ECG frontend  63  (shown in  FIG.  9   ) continually senses the cutaneous ECG electrical signals (step  103 ) via the ECG electrodes  38 ,  29  and is optimized to maintain the integrity of the P-wave. A sample of the ECG signal is read (step  104 ) by the microcontroller  61  by sampling the analog ECG signal that is output by the ECG front end circuit  63 .  FIG.  13    is a graph showing, by way of example, a typical ECG waveform  120 . The x-axis represents time in approximate units of tenths of a second. They-axis represents cutaneous electrical signal strength in approximate units of millivolts. The P-wave  121  has a smooth, normally upward, that is, positive, waveform that indicates atrial depolarization. The QRS complex often begins with the downward deflection of a Q-wave  122 , followed by a larger upward deflection of an R-wave  123 , and terminated with a downward waveform of the S-wave  124 , collectively representative of ventricular depolarization. The T-wave  125  is normally a modest upward waveform, representative of ventricular depolarization, while the U-wave  126 , often not directly observable, indicates the recovery period of the Purkinje conduction fibers. 
     Sampling of the R-to-R interval enables heart rate information derivation. For instance, the R-to-R interval represents the ventricular rate and rhythm, while the P-to-P interval represents the atrial rate and rhythm. Importantly, the PR interval is indicative of atrioventricular (AV) conduction time and abnormalities in the PR interval can reveal underlying heart disorders, thus representing another reason why the P-wave quality achievable by the ambulatory electrocardiography monitoring patch optimized for capturing low amplitude cardiac action potential propagation described herein is medically unique and important. The long-term observation of these ECG indicia, as provided through extended wear of the wearable monitor  12 , provides valuable insights to the patient&#39;s cardiac function symptoms, and overall well-being. 
     Referring back to  FIG.  12   , each sampled ECG signal, in quantized and digitized form, is processed by signal processing modules as specified in firmware (step  105 ), as described infra, and temporarily staged in a buffer (step  106 ), pending compression preparatory to storage in the flash memory  62  (step  107 ). Following compression, the compressed ECG digitized sample is again buffered (step  108 ), then written to the flash memory  62  (step  109 ) using the communications bus. Processing continues (step  110 ), so long as the monitoring recorder  14  remains connected to the electrode patch  15  (and storage space remains available in the flash memory  62 ), after which the processing loop is exited (step  110 ) and execution terminates. Still other operations and steps are possible. 
     The microcontroller  61  operates under modular micro program control as specified in firmware, and the program control includes processing of the analog ECG signal output by the ECG front end circuit  63 .  FIG.  14    is a functional block diagram showing the signal processing functionality  130  of the microcontroller  61 . The microcontroller  61  operates under modular micro program control as specified in firmware  132 . The firmware modules  132  include high and low pass filtering  133 , and compression  134 . Other modules are possible. The microcontroller  61  has a built-in ADC, although ADC functionality could also be provided in the firmware  132 . 
     The ECG front end circuit  63  first outputs an analog ECG signal, which the ADC  131  acquires, samples and converts into an uncompressed digital representation. The microcontroller  61  includes one or more firmware modules  133  that perform filtering. In one embodiment, three low pass filters and two high pass filters are used. Following filtering, the digital representation of the cardiac activation wave front amplitudes are compressed by a compression module  134  before being written out to storage  135 . 
     The download station executes a communications or offload program (“Offload”) or similar program that interacts with the monitor recorder  14  via the external connector  65  to retrieve the stored ECG monitoring data.  FIG.  15    is a functional block diagram showing the operations  140  performed by the download station. The download station could be a server, personal computer, tablet or handheld computer, smart mobile device, or purpose-built programmer designed specific to the task of interfacing with a monitor recorder  14 . Still other forms of download station are possible, including download stations connected through wireless interfacing using, for instance, a smart phone connected to the monitor recorder  14  through Bluetooth or Wi-Fi. 
     The download station is responsible for offloading stored ECG monitoring data from a monitor recorder  14  and includes an electro mechanical docking interface by which the monitor recorder  14  is connected at the external connector  65 . The download station operates under programmable control as specified in software  141 . The stored ECG monitoring data retrieved from storage  142  on a monitor recorder  14  is first decompressed by a decompression module  143 , which converts the stored ECG monitoring data back into an uncompressed digital representation more suited to signal processing than a compressed signal. The retrieved ECG monitoring data may be stored into local storage for archival purposes, either in original compressed form, or as uncompressed. 
     The download station can include an array of filtering modules. For instance, a set of phase distortion filtering tools  144  may be provided, where corresponding software filters can be provided for each filter implemented in the firmware executed by the microcontroller  61 . The digital signals are run through the software filters in a reverse direction to remove phase distortion. For instance, a 45 Hertz high pass filter in firmware may have a matching reverse 45 Hertz high pass filter in software. Most of the phase distortion is corrected, that is, canceled to eliminate noise at the set frequency, but data at other frequencies in the waveform remain unaltered. As well, bidirectional impulse infinite response (IIR) high pass filters and reverse direction (symmetric) IIR low pass filters can be provided. Data is run through these filters first in a forward direction, then in a reverse direction, which generates a square of the response and cancels out any phase distortion. This type of signal processing is particularly helpful with improving the display of the ST-segment by removing low frequency noise. 
     An automatic gain control (AGC) module  145  can also be provided to adjust the digital signals to a usable level based on peak or average signal level or other metric. AGC is particularly critical to single-lead ECG monitors, where physical factors, such as the tilt of the heart, can affect the electrical field generated. On three-lead Holter monitors, the leads are oriented in vertical, horizontal and diagonal directions. As a result, the horizontal and diagonal leads may be higher amplitude and ECG interpretation will be based on one or both of the higher amplitude leads. In contrast, the electrocardiography monitor  12  has only a single lead that is oriented in the vertical direction, so variations in amplitude will be wider than available with multi-lead monitors, which have alternate leads to fall back upon. 
     In addition, AGC may be necessary to maintain compatibility with existing ECG interpretation software, which is typically calibrated for multi-lead ECG monitors for viewing signals over a narrow range of amplitudes. Through the AGC module  145 , the gain of signals recorded by the monitor recorder  14  of the electrocardiography monitor  12  can be attenuated up (or down) to work with FDA-approved commercially available ECG interpretation. 
     AGC can be implemented in a fixed fashion that is uniformly applied to all signals in an ECG recording, adjusted as appropriate on a recording-by-recording basis. Typically, a fixed AGC value is calculated based on how an ECG recording is received to preserve the amplitude relationship between the signals. Alternatively, AGC can be varied dynamically throughout an ECG recording, where signals in different segments of an ECG recording are amplified up (or down) by differing amounts of gain. 
     Typically, the monitor recorder  14  will record a high resolution, low frequency signal for the P-wave segment. However, for some patients, the result may still be a visually small signal. Although high resolution is present, the unaided eye will normally be unable to discern the P-wave segment. Therefore, gaining the signal is critical to visually depicting P-wave detail. This technique works most efficaciously with a raw signal with low noise and high resolution, as generated by the monitor recorder  14 . Automatic gain control applied to a high noise signal will only exacerbate noise content and be self-defeating. 
     Finally, the download station can include filtering modules specifically intended to enhance P-wave content. For instance, a P-wave base boost filter  146 , which is a form of pre-emphasis filter, can be applied to the signal to restore missing frequency content or to correct phase distortion. Still other filters and types of signal processing are possible. 
     Conventional ECG monitors, like Holter monitors, invariably require specialized training on proper placement of leads and on the operation of recording apparatuses, plus support equipment purpose-built to retrieve, convert, and store ECG monitoring data. In contrast, the electrocardiography monitor  12  simplifies monitoring from end to end, starting with placement, then with use, and finally with data retrieval.  FIGS.  16 A-C  are functional block diagrams respectively showing practical uses 150, 160, 170 of the extended wear electrocardiography monitors  12  of  FIGS.  1  and  2   . The combination of a flexible extended wear electrode patch and a removable reusable (or single use) monitor recorder empowers physicians and patients alike with the ability to readily perform long-term ambulatory monitoring of the ECG and physiology. 
     Especially when compared to existing Holter-type monitors and monitoring patches placed in the upper pectoral region, the electrocardiography monitor  12  offers superior patient comfort, convenience and user-friendliness. To start, the electrode patch  15  is specifically designed for ease of use by a patient (or caregiver); assistance by professional medical personnel is not required. Moreover, the patient is free to replace the electrode patch  15  at any time and need not wait for a doctor&#39;s appointment to have a new electrode patch  15  placed. In addition, the monitor recorder  14  operates automatically and the patient only need snap the monitor recorder  14  into place on the electrode patch  15  to initiate ECG monitoring. Thus, the synergistic combination of the electrode patch  15  and monitor recorder  14  makes the use of the electrocardiography monitor  12  a reliable and virtually foolproof way to monitor a patient&#39;s ECG and physiology for an extended, or even open-ended, period of time. 
     In simplest form, extended wear monitoring can be performed by using the same monitor recorder  14  inserted into a succession of fresh new electrode patches  15 . As needed, the electrode patch  15  can be replaced by the patient (or caregiver) with a fresh new electrode patch  15  throughout the overall monitoring period. Referring first to  FIG.  16 A , at the outset of monitoring, a patient adheres a new electrode patch  15  in a location at the sternal midline  16  (or immediately to either side of the sternum  13 ) oriented top-to-bottom (step  151 ). The placement of the wearable monitor in a location at the sternal midline (or immediately to either side of the sternum), with its unique narrow “hourglass”-like shape, significantly improves the ability of the wearable monitor to cutaneously sense cardiac electrical potential signals, particularly the P-wave (or atrial activity) and, to a lesser extent, the QRS interval signals indicating ventricular activity in the ECG waveforms. 
     Placement involves simply adhering the electrode patch  15  on the skin along the sternal midline  16  (or immediately to either side of the sternum  13 ). Patients can easily be taught to find the physical landmarks on the body necessary for proper placement of the electrode patch  15 . The physical landmarks are locations on the surface of the body that are already familiar to patients, including the inter-mammary cleft between the breasts above the manubrium (particularly easily locatable by women and gynecomastic men), the sternal notch immediately above the manubrium, and the Xiphoid process located at the bottom of the sternum. Empowering patients with the knowledge to place the electrode patch  15  in the right place ensures that the ECG electrodes will be correctly positioned on the skin, no matter the number of times that the electrode patch  15  is replaced. 
     A monitor recorder  14  is snapped into the non-conductive receptacle  25  on the outward-facing surface of the electrode patch  15  (step  152 ). The monitor recorder  14  draws power externally from a battery provided in the non-conductive receptacle  25 . In addition, the battery is replaced each time that a fresh new electrode patch  15  is placed on the skin, which ensures that the monitor recorder  14  is always operating with a fresh power supply and minimizing the chances of a loss of monitoring continuity due to a depleted battery source. 
     By default, the monitor recorder  14  automatically initiates monitoring upon sensing body surface potentials through the pair of ECG electrodes (step  153 ). In a further embodiment, the monitor recorder  14  can be configured for manual operation, such as by using the tactile feedback button  66  on the outside of the sealed housing  50 , or other user-operable control. In an even further embodiment, the monitor recorder  14  can be configured for remotely-controlled operation by equipping the monitor recorder  14  with a wireless transceiver, such as described in commonly-assigned U.S. Pat. No. 9,433,367, issued Sep. 6, 2016, the disclosure of which is incorporated by reference. The wireless transceiver allows wearable or mobile communications devices to wirelessly interface with the monitor recorder  14 . 
     A key feature of the extended wear electrocardiography monitor  12  is the ability to monitor ECG and physiological data for an extended period of time, which can be well in excess of the 14 days currently pitched as being achievable by conventional ECG monitoring approaches. In a further embodiment, ECG monitoring can even be performed over an open-ended time period, as further explained infra. The monitor recorder  14  is reusable and, if so desired, can be transferred to successive electrode patches  15  to ensure continuity of monitoring. At any point during ECG monitoring, a patient (or caregiver) can remove the monitor recorder  14  (step  154 ) and replace the electrode patch  15  currently being worn with a fresh new electrode patch  15  (step  151 ). The electrode patch  15  may need to be replaced for any number of reasons. For instance, the electrode patch  15  may be starting to come off after a period of wear or the patient may have skin that is susceptible to itching or irritation. The wearing of ECG electrodes can aggravate such skin conditions. Thus, a patient may want or need to periodically remove or replace ECG electrodes during a long-term ECG monitoring period, whether to replace a dislodged electrode, reestablish better adhesion, alleviate itching or irritation, allow for cleansing of the skin, allow for showering and exercise, or for other purpose. 
     Following replacement, the monitor recorder  14  is again snapped into the electrode patch  15  (step  152 ) and monitoring resumes (step  153 ). The ability to transfer the same monitor recorder  14  to successive electrode patches  15  during a period of extended wear monitoring is advantageous not to just diagnose cardiac rhythm disorders and other physiological events of potential concern, but to do extremely long term monitoring, such as following up on cardiac surgery, ablation procedures, or medical device implantation. In these cases, several weeks of monitoring or more may be needed. In addition, some IMDs, such as pacemakers or implantable cardioverter defibrillators, incorporate a loop recorder that will capture cardiac events over a fixed time window. If the telemetry recorded by the IMD is not downloaded in time, cardiac events that occurred at a time preceding the fixed time window will be overwritten by the IMD and therefore lost. The monitor recorder  14  provides continuity of monitoring that acts to prevent loss of cardiac event data. In a further embodiment, the firmware executed by the microcontroller  61  of the monitor recorder  14  can be optimized for minimal power consumption and additional flash memory for storing monitoring data can be added to achieve a multi-week monitor recorder  14  that can be snapped into a fresh new electrode patch  15  every seven days, or other interval, for weeks or even months on end. 
     Upon the conclusion of monitoring, the monitor recorder  14  is removed (step  154 ) and recorded ECG and physiological telemetry are downloaded (step  155 ). For instance, a download station can be physically interfaced to the external connector  65  of the monitor recorder  14  to initiate and conduct downloading, as described supra with reference to  FIG.  15   . 
     In a further embodiment, the monitoring period can be of indeterminate duration. Referring next to  FIG.  16 B , a similar series of operations are followed with respect to replacement of electrode patches  15 , reinsertion of the same monitor recorder  14 , and eventual download of ECG and physiological telemetry (steps  161 - 165 ), as described supra with reference to  FIG.  16 A . However, the flash memory  62  (shown in  FIG.  9   ) in the circuitry  60  of the monitor recorder  14  has a finite capacity. Following successful downloading of stored data, the flash memory  62  can be cleared to restore storage capacity and monitoring can resume once more, either by first adhering a new electrode patch  15  (step  161 ) or by snapping the monitor recorder  14  into an already-adhered electrode patch  15  (step  162 ). The foregoing expanded series of operations, to include reuse of the same monitor recorder  14  following data download, allows monitoring to continue indefinitely and without the kinds of interruptions that often affect conventional approaches, including the retrieval of monitoring data only by first making an appointment with a medical professional. 
     In a still further embodiment, when the monitor recorder  14  is equipped with a wireless transceiver, the use of a download station can be skipped. Referring last to  FIG.  16 C , a similar series of operations are followed with respect to replacement of electrode patches  15  and reinsertion of the same monitor recorder  14  (steps  171 - 174 ), as described supra with reference to  FIG.  16 A . However, recorded ECG and physiological telemetry are downloaded wirelessly (step  175 ), such as described in commonly-assigned U.S. Pat. No. 9,433,367, cited supra. The recorded ECG and physiological telemetry can even be downloaded wirelessly directly from a monitor recorder  14  during monitoring while still snapped into the non-conductive receptacle  25  on the electrode patch  15 . The wireless interfacing enables monitoring to continue for an open-ended period of time, as the downloading of the recorded ECG and physiological telemetry will continually free up onboard storage space. Further, wireless interfacing simplifies patient use, as the patient (or caregiver) only need worry about placing (and replacing) electrode patches  15  and inserting the monitor recorder  14 . Still other forms of practical use of the extended wear electrocardiography monitors  12  are possible. 
     The circuit trace and ECG electrodes components of the electrode patch  15  can be structurally simplified. In a still further embodiment, the flexible circuit  32  (shown in  FIG.  5   ) and distal ECG electrode  38  and proximal ECG electrode  39  (shown in  FIG.  6   ) are replaced with a pair of interlaced flexile wires. The interlacing of flexile wires through the flexible backing  20  reduces both manufacturing costs and environmental impact, as further described infra. The flexible circuit and ECG electrodes are replaced with a pair of flexile wires that serve as both electrode circuit traces and electrode signal pickups.  FIG.  17    is a perspective view  180  of an extended wear electrode patch  15  with a flexile wire electrode assembly in accordance with a still further embodiment. The flexible backing  20  maintains the unique narrow “hourglass”-like shape that aids long term extended wear, particularly in women, as described supra with reference to  FIG.  4   . For clarity, the non-conductive receptacle  25  is omitted to show the exposed battery printed circuit board  182  that is adhered underneath the non-conductive receptacle  25  to the proximal end  31  of the flexible backing  20 . Instead of employing flexible circuits, a pair of flexile wires are separately interlaced or sewn into the flexible backing  20  to serve as circuit connections for an anode electrode lead and for a cathode electrode lead. 
     To form a distal electrode assembly, a distal wire  181  is interlaced into the distal end  30  of the flexible backing  20 , continues along an axial path through the narrow longitudinal midsection of the elongated strip, and electrically connects to the battery printed circuit board  182  on the proximal end  31  of the flexible backing  20 . The distal wire  181  is connected to the battery printed circuit board  182  by stripping the distal wire  181  of insulation, if applicable, and interlacing or sewing the uninsulated end of the distal wire  181  directly into an exposed circuit trace  183 . The distal wire-to-battery printed circuit board connection can be made, for instance, by back stitching the distal wire  181  back and forth across the edge of the battery printed circuit board  182 . Similarly, to form a proximal electrode assembly, a proximal wire (not shown) is interlaced into the proximal end  31  of the flexible backing  20 . The proximal wire is connected to the battery printed circuit board  182  by stripping the proximal wire of insulation, if applicable, and interlacing or sewing the uninsulated end of the proximal wire directly into an exposed circuit trace  184 . The resulting flexile wire connections both establish electrical connections and help to affix the battery printed circuit board  182  to the flexible backing  20 . 
     The battery printed circuit board  182  is provided with a battery compartment  36 . A set of electrical pads  34  are formed on the battery printed circuit board  182 . The electrical pads  34  electrically interface the battery printed circuit board  182  with a monitor recorder  14  when fitted into the non-conductive receptacle  25 . The battery compartment  36  contains a spring  185  and a clasp  186 , or similar assembly, to hold a battery (not shown) in place and electrically interfaces the battery to the electrical pads  34  through a pair battery leads  187  for powering the electrocardiography monitor  14 . Other types of battery compartment are possible. The battery contained within the battery compartment  36  can be replaceable, rechargeable, or disposable. 
     In a yet further embodiment, the circuit board and non-conductive receptacle  25  are replaced by a combined housing that includes a battery compartment and a plurality of electrical pads. The housing can be affixed to the proximal end of the elongated strip through the interlacing or sewing of the flexile wires or other wires or threads. 
     The core of the flexile wires may be made from a solid, stranded, or braided conductive metal or metal compounds. In general, a solid wire will be less flexible than a stranded wire with the same total cross-sectional area, but will provide more mechanical rigidity than the stranded wire. The conductive core may be copper, aluminum, silver, or other material. The pair of the flexile wires may be provided as insulated wire. In one embodiment, the flexile wires are made from a magnet wire from Belden Cable, catalogue number 8051, with a solid core of AWG 22 with bare copper as conductor material and insulated by polyurethane or nylon. Still other types of flexile wires are possible. In a further embodiment, conductive ink or graphene can be used to print electrical connections, either in combination with or in place of the flexile wires. 
     In a still further embodiment, the flexile wires are uninsulated.  FIG.  18    is perspective view of the flexile wire electrode assembly from  FIG.  17   , with a layer of insulating material  189  shielding a bare uninsulated distal wire  181  around the midsection on the contact side of the flexible backing. On the contact side of the proximal and distal ends of the flexible backing, only the portions of the flexile wires serving as electrode signal pickups are electrically exposed and the rest of the flexile wire on the contact side outside of the proximal and distal ends are shielded from electrical contact. The bare uninsulated distal wire  181  may be insulated using a layer of plastic, rubber-like polymers, or varnish, or by an additional layer of gauze or adhesive (or non-adhesive) gel. The bare uninsulated wire  181  on the non-contact side of the flexible backing may be insulated or can simply be left uninsulated. 
     Both end portions of the pair of flexile wires are typically placed uninsulated on the contact surface of the flexible backing  20  to form a pair of electrode signal pickups.  FIG.  19    is a bottom view  190  of the flexile wire electrode assembly as shown in  FIG.  17   . When adhered to the skin during use, the uninsulated end portions of the distal wire  181  and the proximal wire  191  enable the monitor recorder  14  to measure dermal electrical potential differentials. At the proximal and distal ends of the flexible backing  20 , the uninsulated end portions of the flexile wires may be configured into an appropriate pattern to provide an electrode signal pickup, which would typically be a spiral shape formed by guiding the flexile wire along an inwardly spiraling pattern. The surface area of the electrode pickups can also be variable, such as by selectively removing some or all of the insulation on the contact surface. For example, an electrode signal pickup arranged by sewing insulated flexile wire in a spiral pattern could have a crescent-shaped cutout of uninsulated flexile wire facing towards the signal source. 
     In a still yet further embodiment, the flexile wires are left freely riding on the contact surfaces on the distal and proximal ends of the flexible backing, rather than being interlaced into the ends of the flexible backing  20 .  FIG.  20    is a bottom view  200  of a flexile wire electrode assembly in accordance with a still yet further embodiment. The distal wire  181  is interlaced onto the midsection and extends an exposed end portion  192  onto the distal end  30 . The proximal wire  191  extends an exposed end portion  193  onto the proximal end  31 . The exposed end portions  192  and  193 , not shielded with insulation, are further embedded within an electrically conductive adhesive  201 . The adhesive  201  makes contact to skin during use and conducts skin electrical potentials to the monitor recorder  14  (not shown) via the flexile wires. The adhesive  201  can be formed from electrically conductive, non-irritating adhesive, such as hydrocolloid. 
     The distal wire  181  is interlaced or sewn through the longitudinal midsection of the flexible backing  20  and takes the place of the flexible circuit  32 .  FIG.  21    is a perspective view showing the longitudinal midsection of the flexible backing of the electrode assembly from  FIG.  17   . Various stitching patterns may be adopted to provide a proper combination of rigidity and flexibility. In simplest form, the distal wire  181  can be manually threaded through a plurality of holes provided at regularly-spaced intervals along an axial path defined between the battery printed circuit board  182  (not shown) and the distal end  30  of the flexible backing  20 . The distal wire  181  can be threaded through the plurality of holes by stitching the flexile wire as a single “thread.” Other types of stitching patterns or stitching of multiple “threads” could also be used, as well as using a sewing machine or similar device to machine-stitch the distal wire  181  into place, as further described infra. Further, the path of the distal wire  181  need not be limited to a straight line from the distal to the proximal end of the flexible backing  20 . 
     While the invention has been particularly shown and described as referenced to the embodiments thereof, those skilled in the art will understand that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope.