Abstract:
The invention relates to a method and apparatus for diagnosis of conductor anomalies, such as insulation failures, in an implantable medical device, such as an implantable cardioverter defibrillator (ICD), a pacemaker, or a neurostimulator. Insulation failures are detected and localized by identifying changes in electrical fields via surface (skin) potentials. Small variations in potential are detected along the course of the electrode near the site of insulation failure.

Description:
RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/689,189 filed Jun. 1, 2012, which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates, generally, to scientific and medical systems, apparatus and methods. More particularly, the invention relates to method and apparatus for diagnosis of conductor anomalies. Most particularly, the invention relates to a method and apparatus for diagnosis of conductor anomalies, such as insulation failures, in an implantable medical device, such as an implantable cardioverter defibrillator (ICD), a pacemaker, or a neurostimulator. Insulation failures are detected and localized by identifying changes in electrical fields via surface (skin) potentials. Small variations in potential are detected along the course of the electrode near the site of insulation failure. 
       BACKGROUND 
       [0003]    The long-term reliability and safety of implantable cardiac leads is a significant issue. Anomalies of conductors in implantable medical devices constitute a major cause of morbidity. Representative examples of such medical devices include, but are not limited to, pacemakers, vagal nerve stimulators, pain stimulators, neurostimulators, and implantable cardioverter defibrillators (ICDs). For example, early diagnosis of ICD lead conductor anomalies is important to reduce morbidity and/or mortality from loss of pacing, inappropriate ICD shocks, and/or ineffective treatment of ventricular tachycardia or fibrillation (ventricular fibrillation). The early diagnosis of conductor anomalies for implantable cardiac leads is a critically important step in reducing these issues and making ICDs safer. 
         [0004]    Multilumen ICD defibrillation electrodes or leads include one or more high-voltage conductors and one or more pace-sense conductors. The leads can be implanted as subcutaneous or intravascular leads. Insulation failures have been known to result in functional failure of the corresponding conductor. Functional failure of a pace-sense conductor may result in symptoms caused by loss of pacing functions for bradycardia, cardiac resynchronization, or antitachycardia pacing. Functional failure of a high-voltage conductor may result in fatal failure of cardioversion or defibrillation. 
         [0005]    Thus, one major goal is high sensitivity of diagnosis: identification of lead insulation failures at the subclinical stage, before they present as a clinical problem. A second major goal is high specificity: a false positive provisional clinical diagnosis of lead insulation failure may trigger patient anxiety and lead to potentially-avoidable diagnostic testing. A false positive clinical diagnosis of insulation failure results in unnecessary lead replacement, with corresponding expense and surgical risk. 
         [0006]    Insulation failures occur most commonly at three regions along the course of a pacemaker or ICD lead. The first region is within the pocket, caused either by abrasion of the lead insulation by pressure from the housing (“CAN”) of the pulse generator or twisting of the lead within the pocket. The second region is that between the clavicle and first rib, where the lead is subject to “clavicular crush.” The third region is the intracardiac region near the tricuspid valve. This third region is a particularly common site of insulation failure for the St. Jude Riata® lead which is subject to “insideout” insulation failure due to motion of the internal cables relative to the outer insulation. 
         [0007]    It is extremely difficult to detect and localize lead insulation failures on an ICD implanted in the chest of a patient. The taking of x-rays has been attempted to easily identify anomalies but has had extremely limited success, and essentially zero success where the anomaly is a lead insulation failure. For example,  FIG. 1  is an x-ray of an implanted ICD, the arrow showing a small area of insulation stress from a tight suture (taken from Radiography of Cardiac Conduction Devices: A Comprehensive Review by Amanda L. Aguilera, MD, Yulia V. Volokhina, D O and Kendra L. Fisher, MD, October 2011 RadioGraphics, 31, 1669-1682.). Whether this area of insulation stress has already led to an insulation failure or not is impossible to discern from this image. 
         [0008]    Due to the failure of x-ray diagnosis, the primary method in the prior art for monitoring pacemaker and ICD lead integrity is periodic measurement of electrical resistance, usually referred to as “impedance monitoring.” Impedance monitoring uses single pulses. Various methods well-known in the art provide a measure of impedance close to the direct-current resistance. 
         [0009]    However, another common issue is that insulation failures commonly present clinically without detected changes in impedance as measured by presently used methods. There are several possible explanations. One explanation is that the range of impedance in normally functioning leads may be wide. For example, it has been reported that high-voltage impedance in normally functioning high-voltage leads may fall approximately 30% from maximum measured values (Gunderson B D, Ellenbogen K A, Sachanandani H, Wohl B N, Kendall K T, Swerdlow C D. Lower impedance threshold provides earlier warning for high voltage lead fractures. Heart Rhythm 8:S19, 2011.). Similarly, the range of impedance for pace-sense leads can vary widely. A second explanation is that impedance is determined primarily by body tissue, so that even if an in-pocket insulation failure is present, a test pulse delivered from housing to the affected electrode may not detect the insulation failure unless dielectric breakdown is complete. 
         [0010]    The difficulty in detecting an insulation failure with present electrical testing may be appreciated from this example. Consider a fracture in the conductor leading to the SVC coil (SVC conductor). Such a fracture—in its initial stages—may have an impedance (to the body core) of 2 kΩ or more. The typical SVC coil has an impedance on the order of 60 Ω. Thus the parallel combination of the normal impedance and the “leakage” impedance (from the fracture) would result in an impedance reduction of 1.8Ω which is far lower than the typical (5-10Ω) serial impedance changes seen chronically. Similar difficulties are seen with insulation failures on pace/sense conductors as the tip and ring impedances change significantly with fibrosis and other chronic effects. 
         [0011]    In the circuit being measured, most of the resistance is at the electrode-tissue interface of the high-resistance tip electrode, and variations of up to 10% in this value are common. Each individual pace-sense conductor (for example, the conductor to the tip electrode or the ring electrode) contributes less than 10% to the measured resistance. In some ICD leads, this value is less than 3%. Thus even if the resistance in a single conductor doubled or tripled, the overall measured resistance will remain within the expected range. Measurements indicate that resistance exceeds the expected range until the conductor has lost most of its structural integrity. Thus, resistance remains within the expected range even when only a fraction of the conductor is intact. For this reason, resistance measurements are insensitive to partial loss of conductor integrity. Further, resistance measurements have limited specificity. A single, out-of-range value may be an artifact, and marked increases can occur at the electrode-myocardial interface. 
         [0012]    In addition to limited sensitivity, present methods for diagnosing lead conductor anomalies have limited specificity resulting in false positive diagnostics. Evaluation of false positive diagnostics adds cost and work to medical care and may contribute to patient anxiety. If a false-positive diagnostic is not diagnosed correctly, patients may be subject to unnecessary surgical lead replacement with its corresponding risks. In the only report on this subject, 23% of leads extracted for the clinical diagnosis of lead fracture tested normally after explant. 
         [0013]    Any clinical method for detecting conductor anomalies in implanted leads must make measurements while the conductor and lead are in the body. Typically, the measuring circuit includes the conductor-tissue interface in the body. Thus the measured values will depend both on the behavior of the conductor being evaluated and the conductor-tissue interface. 
         [0014]    Existing technology for diagnosis of conductor anomalies in an implantable medical device is believed to have significant limitations and shortcomings. What is desired are method and apparatus that could analyze and identify implantable cardiac lead conductor anomalies at the subclinical stage, before they present as a clinical problem, and do so with a high sensitivity and specificity that minimizes false positives for implantable cardiac lead conductor anomalies. 
       SUMMARY OF THE INVENTION 
       [0015]    The disclosed method and apparatus relates to the diagnosis of conductor anomalies, such as insulation failures, in an implantable medical device, such as an implantable cardioverter defibrillator (ICD), a pacemaker, or a neurostimulator. Insulation failures are detected and localized by identifying changes in electrical fields via surface (skin) potentials. Small variations in potential are detected along the course of the electrode near the site of insulation failure. Pulses delivered to the affected conductor result in the appearance of local electrical equi-potential lines, further resulting in a disturbance of local potentials recorded within the body or on the body surface via electrodes on the skin. 
         [0016]    One embodiment is a detection method for an insulation failure of a pacing conductor. An insulation failure is checked for in either the tip pacing-sensing electrode or the ring pace-sense electrode. Low amplitude test pulses with short duration &lt;1.5 ms are delivered in a “bipolar” fashion between the “tip” sense conductors and the “ring” sense conductors. Pulses may be delivered in the absolute refractory period after a paced or conducted beat and use a higher output at pulse generator change with background electrical noise. Due to the dipole effect of the closely-spaced “tip” pacing-sensing electrode and “ring” pacing-sensing electrode, the electrical potential lines are tightly located near these electrodes if the insulation around the conductors is intact. 
         [0017]    An embodiment is disclosed for detecting an insulation failure in either the tip or ring conductor. In operation, a single recording electrode is moved along the surface of the body parallel to the path of the implanted lead. Another embodiment can utilize multiple simultaneous recordings on the surface of the body, e.g., one over the surgical pocket and intravascular/intracardiac course of the lead. Another embodiment can utilize an array of electrodes deployed in fixed positions over the surface of the body. Further, although the embodiments described utilize the ICD&#39;s electronics as the source of test pulses, test pulses may also be delivered at pulse generator change using an external test device such as a programmer. 
         [0018]    In this embodiment, the patient is connected to a high-fidelity ECG unit. The high-fidelity ECG unit reliably displays bipolar transvenous pacing pulses, in order to detect the pacing pulses. The test pulse voltage is reduced below the pacing threshold to minimize pacing. The pacing rate is set to a rate faster than the spontaneous ventricular rate in the asynchronous (VOO mode) to provide more pulses to detect and to avoid most QRS complexes. A cutaneous electrode is moved along the surface of the body parallel to the path of the implanted lead. A local maximum voltage spike is searched for. If a local spike, of sufficient amplitude (e.g. &gt;1 mV) is located sufficiently far away from the tip of the lead then an insulation break is indicated. 
         [0019]    An embodiment is disclosed for detecting an insulation failure in defibrillation conductors in dual-coil systems. The ICD delivers a continuous high-frequency AC impedance test between the SVC and RV electrodes with the ICD housing turned off. The patient is connected to a high-fidelity ECG unit to detect the pacing pulses. The impedance testing high-frequency signal is turned on by initiating an impedance test. An electrode is moved along the surface of the body parallel to the path of the implanted lead. A local maximum voltage spike is searched for. If a local spike, of sufficient amplitude is located sufficiently far away from the tip of the lead then an insulation break is indicated. 
         [0020]    An embodiment is disclosed for detecting an insulation failure in defibrillation conductors in single-coil systems. The ICD is modified to deliver a continuous high-frequency AC impedance test between the RV electrodes and the ICD housing. The patient is connected to a high-fidelity ECG unit to detect the pacing pulses. The impedance testing high-frequency signal is turned on by initiating an impedance test. An electrode is moved along the path of the implanted lead and a local voltage null is searched for. Without any insulation break, there should be a large signal near the ICD can and a large signal near the RV coil. The voltage of this signal is expected to gradually decrease when moving from the can to the RV coil with a polarity inversion (and now voltage null) in between. If a local spike (away from the coil) is located then an insulation break is detected. 
         [0021]    In an embodiment, localization of the failure where the insulation failure is adjacent to the ICD housing is disclosed. Pulses or a continuous high-frequency alternating current are delivered over the three current paths corresponding to the three electrodes on the lead which include the pace-sense (tip-ring) dipole, the ring-RV coil dipole, and the tip-RV coil dipole. In no case should there be evidence of current flow near the pocket or directly under the clavicle (clavicular crush). By determining which dipole or dipoles do not result in an anomalous potential near the housing, identification of the remaining conductor can be made as having failed insulation. 
         [0022]    Another embodiment for localizing an insulation break is disclosed. Localization of the break can be accomplished via a high-resolution inverse-ECG system that is modified to provide potentials along the path of the lead with the suspected insulation break. The method is performed by modifying inverse-ECG system to calculate potentials along the lead. The next step is connecting the patient to the inverse-ECG system and then delivering pacing or AC impedance test stimuli. The steps further include locating maximum signals that are not at an electrode and localizing the insulation break to the region of maximum signal that are not adjacent to a stimulated electrode. 
         [0023]    In an embodiment, the test signals as generated in the embodiments of the methods about are generated from an implantable pulse generator. In another embodiment, an apparatus including an external test device, such as programmer module, is utilized with the implant device. The apparatus has an AC amplifier, an AC generator, a filter, a modulator and a digital meter. In an embodiment, the AC generator is configured to deliver a sine wave to the implanted leads during a procedure that involves disconnecting the leads from the pulse generator. Typically, this occurs for a device change due to battery depletion or infection. The frequency of the sine wave is chosen so that it is above the frequencies that affect cardiac cells. In embodiments, a voltage of is delivered to give a strong signal at the skin surface. 
         [0024]    In an embodiment, a number of ECG adhesive electrodes would be placed in the region of the implantable lead and be connected to a switch-box. The operator would then select the desired electrode, one at a time. The “reference” electrode would be a subcutaneous electrode in the device pocket. As the clinician moves the roving electrode, via the switch-box, the digital meter displays the signal at each skin location and is thus able to find the skin location with the strongest signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
           [0026]      FIG. 1  shows a radiograph or x-ray of an implantable cardioverter defibrillator (ICD) implanted in a human body where the lead has a small area of insulation stress from a tight suture. 
           [0027]      FIG. 2A  is a cross-sectional view of the Medtronic Quattro Secure® Lead, a multi-lumen ICD implantable cardiac lead. 
           [0028]      FIG. 2B  is a cross-sectional view of the Medtronic Sprint Fidelis® Lead, a multi-lumen ICD implantable cardiac lead. 
           [0029]      FIG. 3  is a cross-sectional view of the St. Jude Medical Riata® Lead, a multi-lumen ICD implantable cardiac lead. 
           [0030]      FIG. 4  illustrates regions within the human body associated with the implantation of an ICD and associated leads. 
           [0031]      FIG. 5  shows an implantable medical device in which an embodiment of the present invention may be practiced. It shows an ICD pulse generator connected to a patient&#39;s heart via a transvenous cardiac lead used for pacing and defibrillation. 
           [0032]      FIG. 6  shows an ICD pulse generator connected to a patient&#39;s heart via a transvenous cardiac lead used for pacing and defibrillation where the lead contains an insulation failure nearer the ICD. 
           [0033]      FIG. 7  shows an ICD pulse generator connected to a patient&#39;s heart via a transvenous cardiac lead used for pacing and defibrillation where the lead contains an insulation failure nearer the ICD and the method of checking for failure in either the tip or ring pace-sense conductors. 
           [0034]      FIG. 8  is a flowchart depicting the method of detecting an insulation failure in either the tip or ring pacing conductor. 
           [0035]      FIG. 9  is a flowchart depicting the method of detecting an insulation failure in defibrillation conductors in dual-coil systems. 
           [0036]      FIG. 10  is a flowchart depicting the method of detecting an insulation failure in defibrillation conductors in single-coil systems. 
           [0037]      FIG. 11  is a flowchart depicting the method of localizing an insulation failure. 
           [0038]      FIG. 12  illustrates an apparatus used to generate test signals and display the amplified signal amplitude. 
           [0039]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    The embodiments herein are directed to the diagnosis of lead or conductor anomalies, such as insulation failures, in an implantable medical device, such as pacemakers, vagal nerve stimulators, pain stimulators, neurostimulators, and implantable cardioverter defibrillators (ICDs). However, for clarity, discussion of lead or conductor anomalies will be made in reference to ICDs. However, those with skill in the art are cognizant of the fact that the methods and apparatus as disclosed herein are suitable for use with any one of the various implantable medical devices. 
         [0041]      FIGS. 2A, 2B, and 3  depict known multi-lumen ICD defibrillation electrodes or leads that have been diagnosed with lead conductor anomalies. While these are indicative of the type of leads that can be diagnosed, anomalies in any type of defibrillation electrodes or leads are capable of being diagnosed with the methods and apparatus as disclosed herein.  FIG. 2A  depicts the Medtronic Quattro Secure® Lead.  FIG. 2B  depicts the Medtronic Sprint Fidelis® Lead.  FIG. 3  depicts the St. Jude Medical Riata® Lead. The leads  10 , while having various constructions, have similar features. These similar features are identified with the same reference numbers in the figures. 
         [0042]    The implantable cardiac lead  10  is comprised of a lumen  12  and center inner pacing coil  14  surrounded by PTFE insulation  16 , a plurality of lumens  18  each containing at least one conductor  20  with each conductor  20  surrounded by ETFE insulation  22 , an outer insulating layer  24 , and a silicone insulation  26  disposed between the lumen  12  and the outer insulating layer  24 . The conductors  20  include a sense conductor  21 , a high voltage RV conductor  23 , and a high voltage SVC conductor  25 . The plurality of lumens  18  are disposed in the silicone insulation  26 . The conductors  20  carry electric current to the pace-sense electrodes  66 ,  68  high voltage RV coil  64 , and high voltage SVC coil  62 . 
         [0043]    As discussed above, and shown in  FIG. 4 , insulation failures most commonly occur at three regions along the course of a pacemaker or ICD lead  10 . The first region  42  is proximate the pocket, caused either by abrasion of the lead  10  insulation  24  by pressure from the housing (“CAN”) of the pulse generator or twisting of the lead  10  within the pocket. The second region  44  is that between the clavicle and first rib, where the lead  10  is subject to “clavicular crush.” The third region  46  is the intracardiac region near the tricuspid valve. This third region  46  is a particularly common site of insulation  24  failure for the St. Jude Riata® lead  10  which is subject to “inside-out” insulation failure due to motion of the internal conductors  20  relative to the outer insulation  24 . 
         [0044]      FIG. 5  depicts on ICD  52  implanted in the chest of a patient. The ICD  52  has an outer housing  54 , commonly referred to as a “CAN,” inner circuitry  56  and a battery  58 . Connection is made to the heart  60  via the lead  10 . The lead  10  can have an optional proximal defibrillation coil  62  which is near the superior vena cava (SVC) and is commonly referred to as the SVC coil  62 . The lead  10  also has a distal defibrillation coil  64  which is commonly referred to as the right. ventricular coil or RV coil  64 . Also shown is the optional “ring” pacing-sensing electrode  66 . Located at the distal end of the lead  10  is the “tip” pacing-sensing electrode  68 . 
         [0045]    The outer insulating layer  24  of the leads  10  is generally a polymer such as silicone, polyurethane, or a copolymer of silicone and polyurethane. Stress on the insulation  24  from anchoring sutures, outside-in abrasion from contact with the housing, or inside-out abrasion from movement of the cables within the lead  10  may result in insulation  24  breaches or failures. In addition, the insulation  24  can fail due to chemical reactions such as metal ion oxidation. 
         [0046]      FIG. 6  depicts a lead  10  insulation  24  failure at location  70 . When pulses are delivered to the affected conductor  20  the local electrical equi-potential lines appear approximately as those shown at  72 . This results in a disturbance of local potentials recorded within the body or on the body surface via electrodes on the skin. 
         [0047]      FIG. 7  depicts an example of a detection method for pacing conductors  14 . In this embodiment, an insulation failure is checked for in either the tip pacing-sensing electrode  68  or the ring pace-sense electrode  66 . Low amplitude (&lt;1 V) test pulses with duration &lt;1.5 ms are delivered in a “bipolar” fashion between the “tip” sense conductors  21  and the “ring” sense conductors  21 . Pulses may be delivered in the absolute refractory period after a paced or conducted beat. The test pulse may be delivered, under sedation in a surgical suite, using a higher output at pulse generator change with background electrical noise. Due to the dipole effect of the closely-spaced “tip” pacing-sensing electrode  68  and “ring” pacing-sensing electrode  66 , the electrical potential lines are tightly located near these electrodes  66 ,  68  if the insulation around the conductors is intact, as shown by the smaller ovals  74 . Note that these ovals in  FIG. 7  represent a simplification of the actual equi-potential lines which are typically positive (+) around the “ring” pacing-sensing electrode  66  and negative (−) around the “tip” pacing-sensing electrode  68 . 
         [0048]    The amplitude of the skin-surface potential during bipolar pacing is on the order of approximately 1 mV in a conventional lead and much weaker in other locations with high-fidelity ECG units. An example of a high-fidelity ECG unit is the GE MAC 5500 HD. Due to the high-frequency nature of the pacing pulse it is typically invisible on older ECG units due to their low-pass filtering. 
         [0049]      FIG. 8  depicts an embodiment of a method for detecting an insulation failure in either the tip or ring conductor  20 . Note that this method is able to be used for any pacing conductor such as endocardial electrodes in the right atrium, coronary sinus, or the left ventricle, as well as epicardial or subcutaneous electrodes. In operation, a single recording electrode is moved along the surface of the body parallel to the path of the implanted lead  10 . Visual confirmation of the path can be provided by, for example, radiographic methods, such as an x-ray taken in one or more views, or by CT scan. Another embodiment can utilize multiple simultaneous recordings on the surface of the body, e.g., one over the surgical pocket and intravascular/intracardiac course of the lead  10 . Another embodiment can utilize an array of electrodes deployed in fixed positions over the surface of the body. Further, although the embodiments described utilize the ICD&#39;s  52  electronics  56  as the source of test pulses, test pulses may also be delivered at pulse generator change using an external test device such as a programmer. 
         [0050]    The method shown in  FIG. 8  connects the patient to a high-fidelity ECG unit  80 . The high-fidelity ECG unit reliably displays bipolar transvenous pacing pulses, in order to detect the pacing pulses. The test pulse voltage is reduced below the pacing threshold  81  to minimize pacing. The pacing rate is set to a rate faster than the spontaneous ventricular rate  82  in the asynchronous (VOO mode) to provide more pulses to detect and to avoid most QRS complexes. A cutaneous electrode is moved along the surface of the body parallel to the path of the implanted lead  83 . Visual confirmation of the path can be provided by, for example, radiographic methods, such as an x-ray taken in one or more views, or by CT scan. The cutaneous electrode movement includes the loops of lead in the pocket which is a common site of insulation breach. A local maximum voltage spike is searched for  85 . If a local spike, of sufficient amplitude (e.g. &gt;1 mV) is located sufficiently far away from the tip of the lead then an insulation break is detected  86 . Else, an insulation break is not detected  87 . If the patient cannot tolerate a brief absence of pacing, then pacing is performed at a rate between the lower and upper rate limits (inclusive) with the voltage set to the maximum. 
         [0051]      FIG. 9  depicts an embodiment of a method for detecting an insulation failure in defibrillation conductors in dual-coil systems. The method can detect a possible insulation break in either one of the defibrillation conductors. The ICD is modified to deliver a continuous high-frequency AC impedance test between the SVC and RV electrodes with the ICD housing turned off  91 . The patient is connected to a high-fidelity ECG unit  92  such as a GE MAC 5500 HD in order to detect the pacing pulses. The impedance testing high-frequency signal is turned on by initiating an impedance test  93 . An electrode is moved along the surface of the body parallel to the path of the implanted lead  94 . Visual confirmation of the path can be provided by, for example, radiographic methods, such as an x-ray taken in one or more views, or by CT scan. The electrode movement includes the loops of lead in the pocket which is a common site of insulation breach. A local maximum voltage spike is searched for as shown at  95 . If a local spike, of sufficient amplitude is located sufficiently far away from the tip of the lead then an insulation break is detected as shown at  97 . Else, an insulation break is not detected as shown at  98 . 
         [0052]      FIG. 10  depicts an embodiment of a method for detecting an insulation failure in defibrillation conductors in single-coil systems. The method can detect a possible insulation break in either one of the defibrillation conductors. The ICD is modified to deliver a continuous high-frequency AC impedance test between the RV electrodes and the ICD housing  101 . The patient is connected to a high-fidelity ECG unit  102  such as a GE MAC 5500 HD in order to detect the pacing pulses. The impedance testing high-frequency signal is turned on by initiating an impedance test  103 . An electrode is moved along the path of the implanted lead  104 . Visual confirmation of the path can be provided by, for example, radiographic methods, such as an x-ray taken in one or more views, or by CT scan. A local voltage null is searched for at  105 . Without any insulation break, there should be a large signal near the ICD can and a large signal near the RV coil  108 . The voltage of this signal is expected to gradually decrease when moving from the can to the RV coil with a polarity inversion (and now voltage null) in between. If a local spike (away from the coil) is located then an insulation break is detected at  107 . Else, an insulation break is not detected at  108 . 
         [0053]    In the case of an insulation  24  failure adjacent to the ICD&#39;s  52  housing  54 , it is sometimes difficult to localize the failure using the above techniques of detecting a surface potential. In this type of scenario, pulses, or a continuous high-frequency alternating current are delivered over the three current paths corresponding to the three electrodes on the lead which include the pace-sense (tip-ring) dipole, the ring-RV coil dipole, and the tip-RV coil dipole. In no case should there be evidence of current flow near the pocket  42  or directly under the clavicle (clavicular crush)  44 . By determining which dipole or dipoles do not result in an anomalous potential near the housing, identification of the remaining conductor can be made as having failed insulation  24 . 
         [0054]      FIG. 11  depicts a method of localizing an insulation break. Once an insulation break is detected or indicated, localization of the break can be accomplished via a high-resolution inverse-ECG system such as the Cardio-Insight mapping system. The method requires that the system be modified to provide potentials along the path of the lead with the suspected insulation break. The method is performed by modifying inverse-ECG system to calculate potentials along the lead  110 . The next step is connecting the patient to the inverse-ECG system  112  and then delivering pacing or AC impedance test stimuli. The steps further include locating maximum signals that are not at an electrode  116  and localizing the insulation break to the region of maximum signal that are not adjacent to a stimulated electrode. 
         [0055]    Additionally, for the embodiments as disclosed herein, a baseline recording (or set of recordings) can be completed when the lead system becomes stable, which is typically between one and three months after implant. The sites of maximum potential in subsequent testing may then be compared with sites of maximum potential during subsequent testing either by the operator or by using custom software. 
         [0056]    The above descriptions depict the duration of the testing process as an embodiment of a single test setting (e.g. at pulse generator change or in outpatient follow up). However, in other embodiments, the methods can be practiced with a 24 hour to one week ambulatory recording period thus providing an extended time period that can be used to effectively identify intermittent lead insulation failure. In embodiments, the recording system may be activated by telemetry signals delivered by the pulse generator. 
         [0057]    In an embodiment, the test signals as generated in the embodiments of the methods about are generated from an implantable pulse generator. In another embodiment, as shown in  FIG. 12 , an apparatus including an external test device, such as programmer module, is utilized at implant and measurements from the cutaneous electrodes are analyzed in near real time using the external test device. The apparatus  120  has an AC amplifier  122 , an AC generator  124 , a filter  126 , a modulator  128 , and a digital meter  130 . One input of the AC amplifier is connected to the roving ECG electrode and the other input is connected to the reference ECG electrode. The AC amplifier output is fed into the filter  126 , which is depicted as a 10 kHz filter but it is not so limited. The filter  126  is output to the modulator  128  and to the digital meter  130 . The AC generator  124  is connected to the implanted leads. The AC generator  124  is depicted as a 10 kHz AC generator but it is not so limited. Additionally the AC generator  124  is connected to the filter  126 . 
         [0058]    In an embodiment, the test pulses are delivered through the implanted pulse generator. Measurements from the cutaneous electrodes are then stored in an ambulatory recording device or long-term electrocardiographic event recorder. In some embodiments, the recording device can be equipped with a larger electrode array as described herein with respect to other embodiments. After a specified monitoring period, e.g., 24 hour to one week to one month or longer ambulatory recording period, the stored measurements are downloaded to an external analysis unit. In embodiments, the recording system may be activated by telemetry signals delivered by the pulse generator. It is apparent to those skilled in the art that the ambulatory recording period can be any timeframe as specified by the physician that is sufficient to accomplish the purposes herein. 
         [0059]    In an embodiment, the 10 kHz AC generator  124  is configured to deliver a 10 V sine wave to the implanted leads  10  during a procedure that involves disconnecting the leads  10  from the pulse generator. Typically, this occurs for a device change due to battery depletion or infection. The 10 kHz frequency is chosen as it is above the frequencies that affect cardiac cells. In certain embodiments, frequencies of 5 to 25 kHz are used but, in other embodiments, frequencies of 1 to 250 kHz can be used. Care should be taken at lower frequencies because cardiac capture or collapse can occur with sufficient amplitudes. In embodiments, a voltage of 5 V AC is delivered to give a strong signal at the skin surface. In other embodiments, voltages of 1 to 20 V AC are provided. 
         [0060]    In an embodiment, the signal from the recording electrode or electrodes is fed thru the amplifier  122  and then filtered by a narrow-band filter  126 . The filter  126  can be either an analog or digital notch filter. On another embodiment, the filter can be a synchronous filter using the signal  132  for synchronization from the generator  124 . Extremely high signal-to-noise ratios are thus achieved. The generator frequency and the filter frequency should be the same. 
         [0061]    In this embodiment, the roving electrode would not be physically moved. Rather, a number of ECG adhesive electrodes would be placed in the region of the implantable lead  10  and be connected to a switch-box. The operator or a computerized system would then select the desired electrode, one at a time. The “reference” electrode would be a subcutaneous electrode in the device pocket  42 . 
         [0062]    The amplified signal is then modulated down to a lower frequency, e.g., 100 Hz, so that the signal can be readily processed by a conventional ECG unit. The digital meter  130  displays the amplified signal amplitude. As the clinician moves the roving electrode, via the switch-box, the digital meter  130  displays the signal at each skin location and is thus able to find the skin location with the strongest signal. Additionally, an ECG unit is not required as the digital meter  130  displays the signals. 
         [0063]    In an alternative embodiment, the signal generator generates a 90 Hz sine wave which is easy to differentiate from utility 50/60 Hz interference and has the advantage of passing readily thru any ECG unit. It does have the disadvantage of limited driving voltage as such frequencies can easily cause cardiac collapse. 
         [0064]    The following patents and applications, the disclosures of which are incorporated by reference in this case (other than claims and express definitions), are prior art attempts by common inventors to solve the problem at issue: U.S. Pat. No. 8,352,033 (&#39;033) to Kroll, issued Jan. 8, 2013; U.S. patent application Ser. No. 13/735,599 to Kroll, filed on Jan. 7, 2013 which is a continuation of &#39;033; and U.S. patent application Ser. No. 12/868,056 to Swerdlow, filed on Aug. 25, 2010. 
         [0065]    The following provisional applications, the disclosures of which are incorporated by reference in this case (other than claims and express definitions), are related to each other: U.S. Patent Application 61/689,191 to Kroll and Swerdlow, filed on Jun. 1, 2012; U.S. Patent Application 61/689,189 to Kroll and Swerdlow, filed on Jun. 1, 2012; and U.S. Patent Application 61/733,713 to Kroll and Swerdlow, filed on Dec. 5, 2012. 
         [0066]    The values noted above are example embodiments and should not be read as limiting the scope of this invention. Those skilled in the art will recognize that the above values may be adjusted to practice the invention as necessary depending on the electrode implantable cardiac lead technology used and the physical characteristics of the patient. 
         [0067]    While the present invention has been described with reference to certain embodiments, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the spirit and scope of the invention as defined by the appended claims.