Author + information
- Received May 6, 2002
- Revision received August 6, 2002
- Accepted August 19, 2002
- Published online January 1, 2003.
- Kenneth A Ellenbogen, MD, FACC*,* (, )
- Mark A Wood, MD, FACC*,
- Richard K Shepard, MD*,
- Henry F Clemo, MD, PhD, FACC*,
- Tim Vaughn, BS†,
- Keith Holloman, BA†,
- Marlene Dow, RN†,
- Jeanne Leffler, RN*,
- Athula Abeyratne, PhD† and
- Dave Verness, BS, PE, CQE†
- ↵*Reprint requests and correspondence:
Dr. Kenneth A. Ellenbogen, Medical College of Virginia, P.O. Box 980053, Richmond, Virginia 23398-0053, USA.
Objectives This study evaluated the long-term reliability of an implantable cardioverter defibrillator (ICD) lead to determine the incidence, clinical presentation, and management of lead failure.
Background Despite recent advances in ICD technology, the long-term reliability of ICD leads remains a significant problem.
Methods Concern about long-term reliability of coaxial polyurethane ICD leads caused us to systematically study all patients implanted with Medtronic (Minneapolis, Minnesota) 6936 lead at our institution. We performed follow-up of 74 patients with 76 ICD leads that were implanted from February 28, 1995 to September 8, 1997. Thirty-seven patients underwent routine clinical ICD follow-up testing and ventricular fibrillation induction to determine the status of their ICD lead after a mean follow-up of 68.6 ± 8.2 months.
Results The lead survival analysis shows a cumulative failure probability of 37% (confidence interval, 24% to 54%) at 68.6 months. Six patients demonstrated a previously undescribed mode of ICD lead failure: prolonged oversensing immediately after shock therapy. The use of short interval counters to monitor nonphysiologic R-R intervals and the measurement of ring-to-coil impedance detected early lead failures in five patients.
Conclusions This analysis shows: 1) problems with ICD leads may not become apparent until late during follow-up and may become a significant late problem, 2) a “signature” mode of lead failure for the 6936 consisting of oversensing of electrical noise following shocks, 3) early detection of lead failure with a short interval counter algorithm or measurement of ring-to-coil impedance may be clinically useful.
The implantable cardioverter defibrillator (ICD) has become the standard of care for treatment of patients with life-threatening or potentially life-threatening ventricular tachyarrhythmias (1,2). With the increased implantation of ICDs, concern about the long-term reliability of ICD leads has become an increasing concern (3–18). Implantable cardioverter defibrillator leads are significantly more complex than pacemaker leads and, as a result, may be inherently more susceptible to failure. Long-term reliability of ICD leads has become the “Achilles heel” of defibrillator therapy.
Implantable cardioverter defibrillator lead dysfunction may result in failure of the ICD to deliver therapy for ventricular tachycardia and, thus, result in syncope or sudden death. Lead dysfunction may also result in inappropriate shocks and subsequent psychological distress, need for operative revision or removal resulting in additional morbidity and mortality, and increased health care costs. Implantable cardioverter defibrillator lead failure may initially be clinically silent, and early detection before clinical presentation with inappropriate shocks or sudden death is important. Patients with ICD leads demonstrating an increased failure rate also need closer follow-up. Finally, lessons learned from ICD leads with high failure rates may help engineers design better and more reliable leads.
We noted a unique mode of failure of an ICD lead during clinical follow-up that led us to perform a systematic evaluation of all patients at our institution with this lead. The purpose of this investigation was to determine the mode, incidence, and time course of failure of patients with the Medtronic 6936 ICD lead. Finally, we sought to determine if any electrophysiologic measurements could predict the development of lead failure.
The study population consisted of 74 patients who underwent a total of 76 Medtronic 6936 ICD lead implantations from February 28, 1995 to September 8, 1997. The 6936 defibrillation lead is a coaxial, tripolar active fixation lead with true bipolar sensing. It is made with a polyurethane 55D insulation covering the inner conductor and a polyurethane 80A covering the middle conductor and serving as the outer insulation (Fig. 1). Patients underwent implantation of an ICD at the Medical College of Virginia for an accepted clinical indication. Routine ICD testing of pacing and sensing thresholds and defibrillation thresholds (DFT) was performed at the time of implantation. Patients were followed in ICD clinic every three to six months at which time they underwent routine device evaluation. Evaluation in ICD clinic consisted of interrogation of the ICD with retrieval of all stored events and intracardiac electrograms, measurement of pacing and sensing thresholds, measurement of lead impedance, recordings of real time electrograms, evaluation of nonsustained ventricular tachycardia counters assessing nonphysiologic events, and measurement of lead impedance and noninvasive telemetry of shocking impedance. Routine annual ICD testing of DFTs was not performed. Each patient was seen within 24 to 48 h after delivery of ICD therapy to have the device interrogated, and lead impedances and pacing and sensing thresholds checked.
All surviving leads underwent a follow-up evaluation of their ICD system during the time period of July 2001 to February 2002, which consisted of induction of ventricular fibrillation for determining DFT and high-voltage impedance, measurement of pacing and sensing thresholds, measurement of lead impedances, recording of real-time electrograms, and mechanical maneuvers (e.g., isometric exercises, etc.) to provoke evidence of sensing problems. Additionally, with the use of custom translation software, all patients with Medtronic GEM models (7227, 7229, 7271, 7273) underwent telemetry of the sensing integrity counter, which collects data on the number of short R-R ventricular intervals between follow-up appointments. The short interval or sensing integrity counter tracks the number of nonpaced R-R intervals less than 140 ms between clinic visits (19). The translation software also stores the results of ring-to-right-ventricle coil lead impedance measurements for the past year. This measurement reports the minimal value every week based on daily measurements. In a true bipolar lead, normal measurements are 26 to 40 Ω. In an integrated bipolar lead, where the ring is the coil, the normal measurement is approximately 4 Ω. In theory, as the inner insulation fails on a bipolar ICD lead, the ring-to-coil impedance is expected to fall.
Patients who moved out of state had all follow-ups done by their local electrophysiologist except for ventricular fibrillation induction, and the results were forwarded to one of the investigators for review. All patients who were deceased had their medical records reviewed and their family and local doctors contacted to obtain details about the events at the time of death.
Oversensing was defined as the detection of inappropriate electrical signals by the pulse generator’s sense amplifier. Undersensing was defined as the failure to sense a signal visible on the surface electrocardiogram. Metal ion oxidation (MIO) was defined when examination of the pacing leads by the manufacturer confirmed this. Returned product analysis consists of electrical testing to determine the continuity of the conductor coils, and microscopic examination to assess the presence of insulation breaching. Metal ion oxidation is a type of insulation failure that occurs with polyurethane leads caused by degradation of the polyurethane on the inside of the lead initiated by the migration of ions from the conductor wire.
All variables were reported as mean ± SD. Kaplan-Meier product limit method was used to compute survival estimates of chronic lead performance of the 6936 leads. The Greenwood method was used to compute the 95% confidence interval (CI) for survival at each failure time of the leads. The log-rank method was used to compare the 6936 lead survival curves for different vein access methods. Continuous variables between groups were compared using the ttest.
We studied 74 patients with 76 ICD leads. The mean age was 62 ± 16 years. Fifty-eight patients were men, 16 were women. The mean ejection fraction was 34 ± 11%. Sixty-five percent of patients had an ischemic cardiomyopathy, 23% had a nonischemic cardiomyopathy, and 12% had other forms of heart disease. Twenty patients died during a mean follow-up of 2.9 ± 1.5 years (range, 0.1 to 5.2 years). Fourteen patients died from congestive heart failure, five patients died from noncardiac causes, and the cause of death is unknown in one patient. None of the deaths were related to ICD lead failure.
From May 1995 to July 2001, all ICD patients underwent routine follow-up. During this period of time, ICD lead failures were detected in 14 patients. The failure mechanism(s) detected in these patients are shown in Figure 2. Of the 14 patients who had evidence of ICD lead failure, six patients presented with postshock oversensing, one patient presented with noise on the rate/sensing electrogram during sinus rhythm, one patient presented with high pacing impedance, two patients presented with high shocking impedance, one patient presented with poor sensing of R waves during sinus rhythm, one patient presented with low pacing impedance (<200 Ω), and two patients presented with abnormal impedances for both the rate/sensing and high-voltage measurements. Four additional patients had their leads removed because of infection or heart transplantation.
Unique mode of failure
A unique mode of ICD lead failure was noted during clinical follow-up. A total of six patients demonstrated oversensing after an appropriate shock (n = 2) for ventricular tachycardia and after an inappropriate shock (n = 4) (Fig. 3). In each case, multiple additional inappropriate shocks were delivered.
From June 2001 until July 2002, 37 patients had ICD follow-up testing. Twenty-six patients had normal results of follow-up ICD testing (Fig. 2). In no patient did provocative maneuvers demonstrate evidence of lead failure. Four patients had increases in high-voltage impedance of 28% to 31% and are being followed up every three months. Two patients had normal lead measurements, but were unable to be defibrillated by their ICD system due to increased DFTs. The results of statistical comparisons between patients at follow-up with ICD lead failure and those without ICD lead failure are shown in Table 1. Changes in pacing or sensing thresholds or lead impedance were not helpful in predicting or detecting lead problems.
The time course of lead failures is shown by the Kaplan-Meier curves in Figure 4. The cumulative lead survival was 98.6% at one year (CI: 90.7% to 99.8%), 95.8% at two years (CI: 87.5% to 98.6%), 94.2% at three years (85.3% to 97.8%), 90.7% at four years (CI: 80.3% to 95.7%), and 80.9% at five years (CI: 68.0% to 89.1%). The estimated cumulative failure probability at 68.6 months is 37% (CI: 24% to 54%). The survival of 6936 ICD leads implanted by a cephalic vein cutdown approach or by a subclavian access approach showed no difference (p = 0.972).
A total of five failures were detected during electrophysiologic testing and additional follow-up. Two lead failures were detected by follow-up ICD testing. One patient demonstrated oversensing after an appropriate ventricular tachycardia shock and increasing numbers of nonsustained ventricular tachycardia episodes (140 to 160 ms), and one patient showed noise after cardioversion in sinus rhythm and a decrease in ring-to-coil impedance.
Three additional failures were detected by routine clinical follow-up. In these patients short interval counters and/or ring-to-coil impedance measurements predicted lead failure (Fig. 5). One patient had a short interval counter >2,400 and a decrease in ring-to-coil impedance from 30 to 9 Ω in the period five months before the time of ICD testing. One month later the short interval counter was >10,000, and a decrease in coil-to-tip impedance was noted. One episode of oversensing was noted, but no therapy was delivered.
Another patient had a variable ring-to-coil impedance with a normal short interval count. Three months later the short interval count reached 3,700, and the ring-to-coil impedance dropped from 30 to 8 Ω. No episodes of ventricular tachycardia were detected. This patient underwent ICD lead extraction.
One patient had normal ICD testing, but one week later the short interval count reached 3,500. One month later oversensing resulting in six inappropriate shocks, and lead extraction was performed.
The results of lead analysis for the seven returned leads are shown in Table 2. There was an excellent correlation between the presumed clinical diagnosis and the results of analysis of the failed ICD leads. Six of the seven patients had evidence of MIO from lead analysis, and this was predicted based on the finding of oversensing during sinus rhythm.
Management of ICD lead-related problems
Fifteen patients underwent laser lead extraction and replacement with a new ICD lead. Two patients had their ICD leads capped and a new ICD lead implanted. One patient had a new sensing lead implanted, and one patient had a new ICD system implanted in the right pectoral region.
The major findings from our study are that transvenous ICD lead failure: 1) may occur at any time after implantation with a high cumulative failure rate requiring more careful follow-up and testing; 2) a new mode of lead failure, noise after a shock, may be a specific sign of coaxial polyurethane ICD lead failure; and 3) measurements from short interval counters or ring-to-coil impedance may be helpful for detecting problematic leads before lead failure is detected by inappropriate shocks. The Medtronic 6936 ICD lead can be safely removed by laser lead extraction.
The high failure rate of the Medtronic 6936 ICD lead observed in our study is similar to that reported by others, including the manufacturer (10,18,20). Luria et al. (10)and Hauser et al. (18)have both reported failure rates for the 6936 and other similar coaxial polyurethane ICD leads of close to 20% at four years. The major presentation of lead failure was due to oversensing in all three studies. The reliability of this estimate is decreased due to inconsistent follow-up and loss of patients over time to death from heart failure and other causes. It is likely that we have also underestimated the number of lead failures, as follow-up testing has identified three patients with high shocking lead impedance undergoing intensified follow-up.
Careful study of the Kaplan-Meier curve shows that the increasing failure rate is noted primarily during long-term follow-up. A lead database that follows large numbers of patients until death is necessary to detect lead problems that occur primarily during late follow-up or with such a low incidence that investigators with small numbers of patients may not detect a problem. It is possible with continued long-term follow-up that many ICD leads will demonstrate more late problems.
Failure of pacing or defibrillation leads due to breakdown of polyurethane is not a new finding (21–23). Polyurethane breakdown, in most cases due to MIO, has been shown to be the mode of failure for most coaxial polyurethane pacing leads. Polyurethane polymers and the production of complex coaxial leads seem to be the combination that results in the highest incidence of lead failures. The middle insulation layer of the 6936 is 80A polyurethane, which is the same polyurethane that was used to manufacture Medtronic 4004 and 4012 leads. The insulation degradation presumably occurs over time in high-stress areas.
This study identifies a new presentation for polyurethane lead failure. In two patients appropriate shock therapy was delivered for ventricular tachycardia, followed by oversensing for several seconds immediately after shock. In four patients oversensing was also demonstrated intermittently during sinus rhythm and after an inappropriate shock. A possible explanation is this mode of failure represents a noncontact defect between the pace/sense ring conductor and the right ventricular high voltage conductor. It is likely that this problem remains clinically “silent” and is not exposed until a high-voltage shock is delivered. After a shock, the pace/sense conductor is hyperpolarized, and disturbance of the polarization potential due to cyclical loading during the cardiac cycle leads to electrical transients resulting in redetection and a succession of shocks. This polarization potential may linger for 10 min or longer. This can be confirmed by measurement of a decrease in the ring-coil impedance. Another possible cause for these observations is that the metal-to-metal contact occurs after powerful muscle contractions where one conductor has a large amount of postshock polarization voltage. The exposed conductors then generate high-voltage artifacts as they scrape together. This mode of failure may be a “signature” for 6936-lead failure and had not been previously reported.
The short interval counter or sensing integrity counter keeps track of the number of short nonpaced intervals less than 140 ms. The programmer displays the number of short ventricular intervals that have occurred since the stored data was cleared or the device implanted. This diagnostic proved useful for predicting early lead failure. If the use of the short interval counter to predict lead failure is confirmed in prospective studies, it may be particularly useful for diagnosing lead failures before they become clinically manifest. Detection of a high short interval counter could lead to early lead extraction or closer follow-up. While ring-to-coil impedance is the earliest method of detection, it may not provide a complete assessment of lead integrity. However, a ring-to-coil impedance drop confirmed subsequently by a rise in short interval counter data may be a useful indicator of MIO breach.
Our present study does provide some useful practical information that can be used to guide follow-up of ICD patients. We feel that given the high incidence of late failure of patients with the 6936 ICD lead should undergo more frequent clinic follow-up visits, especially looking for episodes of nonsustained ventricular tachycardia with nonphysiologic intervals. In patients with ICD, pulse generators capable of measuring short interval counters or lead-to-coil impedance, review of this data should be useful to predict eventual ICD lead failure. Routine ventricular fibrillation induction, perhaps on a yearly basis, may also be worthwhile to look for postshock oversensing. We do not perform routine chest radiography during follow-up, so we cannot comment on routine chest radiograph tests to detect lead failures. Changes in pacing or sensing thresholds did not predict lead failures, but the small number of patients with lead failures at the time of testing and the use of different device models calculating impedance in different ways made that data difficult to interpret.
Our findings raise important issues regarding the costs and alternative approaches to patients who have “prophylactic” ICDs. Many of these patients will have a relatively long life span where lead problems (particularly in patients who have multiple transvenous leads) may become a prominent concern. Lead problems result in considerable morbidity and add substantially to health care costs. Ideally, ICD leads with excellent long-term reliability will help leads, but the complex structure and requirements for such leads likely limit the ease with which this goal may be achieved. Consideration of novel approaches to these patients such as “leadless” ICDs deserves further attention (24).
In summary, ICD lead failures may occur late during follow-up after the lead is implanted. A new mode of ICD lead failure is described, as well as the measurements from the short interval counter and the ring-to-coil impedance, to predict lead failure. Finally, this study highlights the importance of a skilled electrophysiologist performing continued careful clinical follow-up of ICD leads to determine lead long-term reliability.
- confidence interval
- defibrillation threshold
- implantable cardioverter defibrillator
- metal ion oxidation
- Received May 6, 2002.
- Revision received August 6, 2002.
- Accepted August 19, 2002.
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