Abstract:
Method and apparatus for diagnosis of conductor anomalies, such as partial conductor failures, in an implantable lead for an implantable medical device are disclosed. In various embodiments, small changes in the lead impedance are identified by the use of a small circuit element that is incorporated as part of the distal end of the implantable lead. In various embodiments, the small circuit element is electrically connected to a lead conductor and/or electrode of the implantable lead. Methods of diagnosing conductor anomalies in accordance with these embodiments generate measured values that depend only on the impedance of the conductors and electrodes of the lead, and not on the behavior of the conductor-tissue interface and other body tissues.

Description:
RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/817,667 filed Apr. 30, 2013, 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, such as partial conductor failures, in an implantable lead for an implantable medical device by identifying small changes in the lead impedance by the use of a small circuit element that is connected near the distal end of the lead. 
       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). “Multilumen” ICD defibrillation leads include one or more high-voltage conductors and one or more pace-sense conductors. The leads can be implanted as subcutaneously or intravascularly. The lead body consists of a flexible, insulating cylinder having three to six longitudinal lumens configured in parallel. Conductors are disposed in the lumens providing an electrical pathway between the proximal terminals to small pace-sense electrodes and larger shock coil electrodes. 
         [0004]    Early diagnosis of ICD lead conductor anomalies is important to reduce morbidity and/or mortality. Anomalies may occur in either the pace-sense or shock components. The most common presentation of pace-sense failures is oversensing of rapid, nonphysiological signals. Prior to the implementation of diagnostics that incorporated oversensing, pace-sense fractures presented most commonly as inappropriate shocks, despite daily, automated measurements of pacing impedance. Functional failure of a pace-sense conductor or oversensing also results in inhibition of bradycardia pacing, cardiac resynchronization, or antitachycardia pacing. Lead insulation failure or fractures may also present with loss of capture. High-voltage conductor failures present with potentially fatal failed defibrillation or cardioversion. Thus, early diagnosis of ICD lead conductor anomalies is a critically important step in reducing morbidity and/or mortality from the failure 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. 
         [0005]    Thus, one major goal is high sensitivity of diagnosis: identification of lead conductor 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]    Currently, the primary method, of which various methods are well-known in the art, for monitoring pacemaker and ICD lead integrity is periodic measurement of electrical resistance, commonly referred to as “impedance monitoring.” The current art impedance monitoring methods use single pulses and provide a value of impedance close to the direct-current resistance. 
         [0007]    For pace-sense conductors, the circuit being measured comprises the connection between the implantable pulse generator header and the lead, the conductors to the tip and ring electrodes, and the electrode-myocardial interface. 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. Thus, even if the pace-sense resistance in a single conductor doubled or tripled, the overall measured resistance remains within the expected range. 
         [0008]    In fact, with presently-used methods, abnormal increases in impedance occur before diagnostic oversensing—and usually shocks—in only 28% of confirmed Fidelis fractures. For example, the normal resistance of the ring connecting cable (in a Riata® lead) is only 13Ω. This connection has two cables in parallel, with each cable having a normal resistance of 26Ω. In the case of one of the two cables being completely severed, the impedance thus only increase to 26Ω while the impedance seen by the conventional lead impedance “monitoring”—which includes the impedance of the electrode-electrolyte impedance and the return through another electrode—is around 500Ω to 2 kΩ and is thus undetectable. The normally sensed impedance will typically vary by hundreds of ohms during the course of the implant and thus a change of 13Ω is nominal and is easily missed. 
         [0009]    Therefore, measurements indicate that resistance does not exceed the expected range until the conductor has lost most of its structural integrity and resistance remains within the expected range even when only a fraction of the conductor is intact. 
         [0010]    Further, resistance measurements have limited specificity. A single, out-of-range value may be an artifact, and marked, persistent increases can occur at the electrode-myocardial interface in normally functioning leads, thus resulting in over diagnosis of lead failures. 
         [0011]    Hafelinger et al. (U.S. Pat. No. 5,003,975) and Cinbis et al. (U.S. Pat. No. 5,897,577) summarize some of these methods, which include measurements made directly using either a single pacing pulse or a single independent pulse used only for measuring resistance. McVenes et al. (U.S. Pat. No. 5,741,311) describes use of a longer burst of alternating current at a single frequency. The purpose of these longer (about 100 ms) pulses is to drive the system to a steady-state condition that is not achieved by single, short (less than 1 ms) pacing pulse. Schuelke et al. (U.S. Pat. No. 5,755,742) describes a method for measuring resistance of defibrillation electrodes by applying a test voltage applied to a different excitation current pathway. Kroll et al. (U.S. Pat. No. 5,944,746) describes an automated method for periodic measurement of the resistance of the high-voltage (defibrillating) coil in ICD electrodes. Gunderson et al. (U.S. Pat. No. 7,047,083) describes a method and system for automated periodic measurements of resistance in conductors attached to an ICD or pacemaker. However, these types of “impedance monitoring,” which return values close to direct current resistance, identify lead anomalies before inappropriate shocks in only about a third of ICD patients who have conductor fractures. 
         [0012]    A newer method for monitoring ICD lead integrity is based on the response of ICD pulse generators to electrical “noise” signals associated with lead conductor fractures. These nonphysiological signals have specific characteristics that differentiate them from true cardiac signals such as high variability and, at times, nonphysiologically-rapid rates. If these signals are of sufficient amplitude and exceed the ICD&#39;s dynamically-changing sensing threshold, the ICD oversenses them. Repetitive oversensing of nonphysiologically-short intervals may indicate lead conductor fracture even if lead resistance is normal. Gunderson et al. (U.S. Pat. No. 7,289,851) described a Lead-Integrity Alert that incorporates both ICD-based measures of oversensing based on the nonphysiologically-rapid rate of sensed signals and periodic measurements of resistance. This method, combined with automatic ICD reprogramming, improves warning time before inappropriate shocks caused by lead-related oversensing. Nevertheless, approximately 25% of patients receive less than 3 days of warning, and some receive almost no warning. 
         [0013]    Additionally, the Gunderson Lead Integrity Alert method detects only some lead-noise signals. It cannot detect a lead anomaly unless it generates signals that are both fast enough and of sufficient amplitude to be classified as nonphysiological oversensing. Thus, it will not detect a lead anomaly if it does not generate “noise signals” or if it generates only low-amplitude noise signals, or signals that do not occur at a fast enough rate. 
         [0014]    Gunderson et al. (U.S. Pat. No. 7,369,893) describes a method for withholding delivery of ICD shocks if ventricular fibrillation is detected from analysis of the pace-sense lead, but is not confirmed by analysis of the high-voltage lead. Although not yet evaluated in patients, this method is expected to further reduce unnecessary shocks. However, it requires sufficient oversensing to result in inappropriate detection of ventricular fibrillation clinically. Thus, it does not provide early diagnosis of conductor anomalies. Withholding shocks for ventricular fibrillation detected on the near-field electrogram has an inherent risk of withholding life-saving therapy, however small, and is, thus, not the preferred approach to diagnosis conductor fracture. Like the Lead-Integrity Alert method, it is not applicable to intraoperative diagnosis or to pacemakers and neurostimulators. 
         [0015]    Comparable limitations apply to measuring impedance of the high-voltage lead components. The circuit measured comprises the connection between the implantable pulse generator header and lead, high-voltage conductor cables, shock electrodes, blood in the right atrium and ventricle, and—providing the CAN or ICD housing is included—heart, lung, and chest wall. High-voltage cables and shock electrodes have low impedance (˜1Ω), and the overall circuit typically has resistance in the range of 30Ω to 80Ω. Thus, impedance measurements are correspondingly insensitive to significant changes in cable or coil impedance. 
         [0016]    The difficulty in detecting a partial failure with present electrical testing may also be appreciated from the following example. Consider a fracture in the conductor leading to the RV coil (RV conductor). The typical SVC coil has an impedance on the order of 60Ω through to another electrode or the CAN, while the cable connection impedance is only about 1.5Ω (from the combination of two cables in parallel, each having impedance of 3Ω). Thus, the complete fracture of one of the two cables would result in an impedance increase of only 1.5Ω, which is far lower than the typical (5Ω to 10Ω) serial impedance changes seen chronically. 
         [0017]    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. 
         [0018]    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. 
         [0019]    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 
       [0020]    The disclosed method and apparatus relates to the diagnosis of conductor anomalies, such as partial conductor failures, in an implantable lead for use with an implantable medical device, such as an implantable cardioverter defibrillator (ICD), a pacemaker, or a neurostimulator. The disclosed method and apparatus removes the conductor-tissue interface in the body and body tissues from the measuring circuit by the incorporation of a small circuit element as part of the distal end of the implantable lead. In various embodiments, the small circuit element is electrically connected to a lead conductor and/or electrode of the implantable lead. In various embodiments, methods of diagnosing conductor anomalies will generate measured values that depend only on the impedance of the conductors and electrodes of the lead, and not on the behavior of the conductor-tissue interface and other body tissues. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    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: 
           [0022]      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 stress from a tight suture. 
           [0023]      FIG. 2A  is a cross-sectional view of the Medtronic Quattro Secure® Lead, a multi-lumen ICD implantable cardiac lead. 
           [0024]      FIG. 2B  is a cross-sectional view of the Medtronic Sprint Fidelis® Lead, a multi-lumen ICD implantable cardiac lead. 
           [0025]      FIG. 3  is a cross-sectional view of the St. Jude Medical Riata® Lead, a multi-lumen ICD implantable cardiac lead. 
           [0026]      FIG. 4  illustrates regions within the human body associated with the implantation of an ICD and associated leads. 
           [0027]      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. 
           [0028]      FIG. 6  shows a partial schematic of a pacemaker or ICD lead with the diode embodiment of the invention. 
           [0029]      FIG. 7  shows a partial schematic of a pacemaker or ICD lead with the capacitor embodiment of the invention. 
           [0030]      FIG. 8  shows a partial schematic of a pacemaker or ICD lead with the diode embodiment of the invention along with typical MRI protection components. 
           [0031]      FIG. 9  shows a partial schematic of an ICD lead with the diode embodiment of the invention. 
           [0032]      FIG. 10  depicts a plot of the impedance spectra of a Ring cable. 
           [0033]      FIG. 11  shows a partial schematic of an ICD lead with the diode embodiment of the invention along with typical MRI protection components. 
           [0034]      FIG. 12  shows a partial schematic of an ICD lead with a solid state embodiment of the invention. 
       
    
    
       [0035]    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 
       [0036]    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. 
         [0037]      FIG. 1  depicts a radiograph or x-ray of an implantable cardioverter defibrillator (ICD) implanted in a human body where the lead has a small area of stress from a tight suture. 
         [0038]      FIGS. 2A ,  2 B 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. 
         [0039]    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 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  (see  FIG. 5 ). 
         [0040]    As shown in  FIG. 4 , lead 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 . 
         [0041]      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 . 
         [0042]      FIG. 6  depicts an embodiment of a circuit  70  in accordance with one aspect of the invention using a diode  72  as a small circuit element near the end of the lead to aid in identifying small changes in the lead impedance. Shown is a partial schematic of a pacemaker or ICD lead  74 . The tip  76  and ring  78  of the lead  74  are used for both sensing and pacing. The sensed electrogram (EGM) signals are of low voltage (typically &lt;30 mV) and thus the diode  72  will not conduct sensing current so it will not interfere with the sensing signals. Pacing is done with a negative voltage on the tip  76  (so-called “cathodal” pacing) and the diode  72  will be back-biased during this pacing pulse. Hence, the diode  72  will not interfere with pacing functions. Resistor  73  (shown as 13Ω) and resistor  75  (shown as 35Ω) are not discrete resistors but rather represent the resistances of the ring and tip conductors respectively. 
         [0043]    To determine an accurate conductor resistance measurement, a short positive pulse (e.g. 5 V) is fed to the tip  76  with respect to the ring  78  while the current is being monitored. The diode  72  will conduct this pulse and the conductor resistance can be accurately determined by Ohm&#39;s law. In the embodiment shown, resistor  73  can be 13Ω and resistor  75  can be 35Ω, but resistor  73 ,  75  are not so limited. The voltage used for the calculation will be the test voltage less the diode “drop” to allow for the voltage loss in the diode, e.g., 4.3 V=5 V—0.7 V. For example, in the situation where one of the two cables in the ring connection is broken then the total resistance will be increased from 48Ω to 61Ω.  FIG. 7  depicts an embodiment of a circuit  80  in accordance with another aspect of the invention using a capacitor  82  as a small circuit element near the end of the lead to aid in identifying small changes in the lead impedance. Shown is a partial schematic of a pacemaker or ICD lead  84 . Useful values for the capacitor  82  are 100 pF to 100 nF. The tip  86  and ring  88  of such a lead  84  are used for both sensing and pacing. The sensed electrogram (EGM) signals are of low frequency (typically &lt;100 Hz) and the source impedance is about 1 KΩ. The time constant for the resulting low-pass filter is thus ranging from 100 ns to 100 μs and thus the capacitor  82  will not attenuate the sensing signals. 
         [0044]    The pacing pulse is typically about 500 μs and the relevant resistance is that of the conductors which is about 48Ω total, where the value of the first resistor  83  is about 13Ω and the value of the second resistor  85  is about 35Ω. However, the values of the resistors  83 ,  85  are not so limited. Thus, the low-pass time constant seen by the pacing pulse is &lt;5 μs and as such is immaterial to the pacing pulse. 
         [0045]    To determine an accurate conductor resistance measurement, a moderate frequency signal is fed to the tip  86  with respect to the ring  88  while the current is being monitored. With a 100 nF capacitor  82  a 160 KHz signal is used. The capacitor impedance is then: 
         [0000]        Z   c =1/(2π fC )=10Ω.
 
         [0000]    Thus, changes of connection resistance of, for example, 13Ω, are easily detected. The approximate propagation speed V in an implantable cardiac lead, without anomalies, was determined using the following equation: 
         [0000]    
       
      
       V=c/√κ 
      
     
         [0000]    where c is the speed of light in a vacuum (30 cm/ns) and κ is the relative permeability of the insulator compared to a vacuum. The relative permeability is about 3 for silicone and about 2.5 for ETFE. For silicone insulation this results in an approximate propagation velocity of 17.3 cm/ns. This is approximate because the ICD implantable cardiac lead is more complex than a classic coaxial cable having a central conductor. 
         [0046]    For an ICD implantable cardiac lead having a common length of 65 cm, the “round-trip” travel distance for wave propagation is 130 cm. The frequency is determined by: 
         [0000]    
       
      
       f=V/λ 
      
     
         [0000]    where λ is the wavelength of the test frequency. Thus, with a propagation velocity of 17.3 cm/ns, a frequency of about 66 MHz of corresponds to a ¼ wavelength over the implantable cardiac lead. At this frequency, transmission line effects dominate and a short (at the tip) appears to be an open circuit at the source. Hence, test frequencies must be much lower than this or transmission line corrections must be made. It is noted that for a 160 KHz test signal the wavelength is very long compared to the lead so this does not present an issue. 
         [0047]    Alternatively, a smaller 10 nF capacitor  82  could be used with a test frequency of 1.6 MHz without problems. A 1 nF capacitor  82  could also be used with 16 MHz without resorting to transmission line corrections. A 100 pF capacitor  82  would require a test frequency on the order of 100 MHz and this can still be used albeit with transmission line corrections. 
         [0048]      FIG. 8  depicts an embodiment of a circuit  90  in accordance with another aspect of the invention using the diode embodiment  70  in conjunction with typical MRI protection components as a small circuit element near the end of the lead  94  to aid in identifying small changes in the lead impedance. In another embodiment, the capacitor embodiment  80  can be used in conjunction with typical MRI protection components near the end of the lead  94 . Shown is a partial schematic of a pacemaker or ICD lead  94 . A well-known approach to limiting the conduction of the MRI RF signal is to insert an LC “tank” circuit  91  between the lead conductor and the electrode. Since this is distal to the impedance test component—a capacitor (as in  FIG. 7 ) or diode  72 —it will not affect the conductor impedance measurement. 
         [0049]      FIG. 9  depicts an embodiment of an ICD lead  100  using the diode embodiment  70  to aid in identifying small changes in the lead impedance. Shown is a partial schematic of an ICD lead  100 . Conductor impedance measurements are performed as described earlier. The main phase pulse (up to 900 V) of a defibrillation shock is preferable positive (on the RV coil  106  with respect to the body) and thus the diode  72  will not conduct. For the negative (2 nd  phase), the diode  72  will conduct and pass some current thru the diode  72  and the ring  78  electrode. This is expected to be acceptable as some popular ICD leads (so-called “integrated bipolar” type) dispense with the ring electrode altogether and thus deliver current to the ring anatomical position—near the tip—without causing significant performance problems. 
         [0050]    The embodiment of  FIG. 9  has two advantages over the “integrated bipolar” lead. The first is that less than one-quarter of the energy is shared with the ring as the diode blocks the much stronger positive phase. The peak current (important for electroporation stunning) is also cut in half or more. Second, after the defibrillation shock is delivered, the ring is quickly available for normal “true bipolar” sensing. 
         [0051]    In another embodiment, a capacitor embodiment  80  could be used for the RV coil conductor impedance measurement. The capacitance ranges discussed earlier are acceptable and would not conduct sufficient energy or current to cause damage. While there would be a short high-current spike, its duration would be too short to cause electroporation stunning 
         [0052]    In general, the input impedance Z in  of a transmission line, with an excitation wavelength of, is given by: 
         [0000]    
       
         
           
             
               Z 
               
                 i 
                  
                 
                     
                 
                  
                 n 
               
             
             = 
             
               
                 Z 
                 0 
               
                
               
                 
                   
                     Z 
                     L 
                   
                   + 
                   
                     
                       jZ 
                       0 
                     
                      
                     
                       tan 
                        
                       
                         ( 
                         
                           β 
                            
                           
                               
                           
                            
                           l 
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     Z 
                     0 
                   
                   + 
                   
                     
                       jZ 
                       L 
                     
                      
                     
                       tan 
                        
                       
                         ( 
                         
                           β 
                            
                           
                               
                           
                            
                           l 
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
       
         
           
             where =2π/Z L  is the load impedance, and Z 0  is the characteristic impedance of the transmission line. 
           
         
       
     
         [0054]    For the special case where a frequency with a multiple of a half-wavelength (of the lead) is used, the tangent terms become 0 and the equation reduces to: 
         [0000]      Z in =Z L    
         [0000]    so that the input impedance equals the load impedance. 
         [0055]    Modern ICD leads are complex and hence their impedance spectra are more complex than the ideal simple transmission line. 
         [0056]    The plot of the impedance spectra of a Ring cable is shown in  FIG. 10 . It will be seen that the real impedance climbs towards a peak near the ¼ wavelength frequency of about 67 MHz. The real impedance then decreases and fluctuates between 50Ω and 100Ω. The imaginary part goes negative at about the same point and has a negative peak at about ½ wavelength (about 133 MHz). It then fluctuates between 20Ω and 60Ω. 
         [0057]    Thus, for a practical system, the impedance spectra would be determined for the ICD lead (in physiologic saline) both with and without a broken cable. In actual clinical use, the spectra best matching the findings would indicate whether a cable was broken or not. 
         [0058]      FIG. 11  depicts an embodiment of an ICD lead  110  using the diode embodiment  70  along with typical MRI protection components  91  to aid in identifying small changes in the lead impedance. Shown is a partial schematic of an ICD lead  110 . This embodiment depicts the combination of the impedance-testing diode  72  for both the tip-ring conductor pair and the RV coil-ring conductor pair. Because of the diode  72  configuration, this approach has the disadvantage of conducting some of the negative (2 nd ) phase current thru the tip. However, the MRI LC tank  91  would be expected to limit this current to a low value. 
         [0059]      FIG. 12  depicts a solid state implementation  120  of an embodiment of the invention. The diode  121  and capacitor  123  scavenge a small amount of charge from the pacing pulses. If testing is required and there have been no pacing pulses, then a few are delivered to charge up the distal capacitor which then powers the digital circuit shown in the block diagram of  FIG. 12 . 
         [0060]    For lead testing, special sequence of pulses can be delivered to the pacing tip which would not have any pacing effect. For example, ten pulses each of 10 μs in duration. This pulse sequence is recognized by the digital block  122  which then gives complementary drives to the two enhancement mode MOSFETs  124 ,  126  shown. In parallel they function as a resistor and provide a low impedance path between the tip  78  and the ring  76  for a short period of time to allow an accurate impedance measurement. 
         [0061]    In one embodiment, the short period of time is typically &lt;1 second so as to not interfere with pacing and sensing. This time duration is determined by a small capacitor connected to the digital circuit (not shown) or alternatively by using an integrated capacitor within the digital circuit. 
         [0062]    It will be understood that other variations of a small circuit element in accordance with the various embodiments of the present invention may be used to accomplish a similar function. While the small circuit element may be a single passive circuit element, or a small combination of active circuit elements, these embodiments of the small circuit element feature a limited circuit functionality that may be easily incorporated into the distal end of a lead without the overhead, expense, size or power consumption of a larger control circuit or microcontroller. Alternatively, elements of different ones of the various embodiments of the small circuit element may be combined. 
         [0063]    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. 
         [0064]    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.