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Pace-sense conductors comprise a pacing coil to the tip electrode and cable to the ring-electrode. Implantable cardioverter-defibrillator (ICD) lead-monitoring diagnostics include pacing impedance (direct current resistance [DCR]) and measures of oversensing. How they change as fractures progress is unknown.To characterize the relationship between oversensing, impedance, and structural changes in ICD leads developing pace-sense conductor fractures.We performed bending tests on 39 leads connected to ICD generators in an electrolyte bath with simulated electrograms. DCR was recorded every 3 minutes; electrograms were telemetered continuously. Twenty-two leads were tested to develop partial or complete fracture criteria confirmed by imaging, using DCR or DCR variability measured by standard deviation (σInitial oversensing occurred with partial pacing coil fracture vs complete ring cable fracture and correlated with bending-induced DCR peaks. These peaks were too small to be detected by clinical impedance measurements and were characterized by small increases in σIn conductor fracture, early oversensing corresponds to partial pacing coil fracture or complete ring cable fracture and correlates with transient bending-induced impedance increases, which are detected by impedance variability but too small to trigger clinical impedance alerts. This explains why clinical oversensing alerts provide more warning for device-detected ventricular fibrillation than impedance alerts and suggests how to improve impedance diagnostics based on short-term variability.

Implantable cardioverter-defibrillators (ICDs) incorporate automated, lead-monitoring alerts (alerts) and other diagnostics to detect defibrillation lead failure (LF) and minimize its adverse clinical consequences. Partial conductor fractures cause oversensing, but pacing or high-voltage alerts for high impedance detect only complete conductor fracture. In both pacing and high-voltage insulation breaches, low-impedance alerts require complete breach with metal-to-metal contact. Oversensing alerts for pace-sense LF also require complete breach, but not metal-to metal contact. Electrograms (EGMs) from leads with confirmed fractures have characteristics findings. In insulation breach, however, oversensed EGMs reflect characteristics of the source signal. Oversensing alerts that operate on the sensing channel analyze R-R intervals for 2 patterns typical of LF but uncommon in other conditions: a rapidly increasing count of "nonphysiological" short intervals and rapid "nonsustained tachycardias." These alerts are sensitive but nonspecific. Alerts that compare sensing and shock channels define oversensing as sensed events that do not correlate temporally with EGMs on the shock channel. Their performance depends on implementation. Specific advantages and limitations are reviewed. Most ICDs measure impedance using subthreshold pulses. Patterns in impedance trends provide diagnostic information, whether or not an alert is triggered. Gradual increases in impedance do not indicate structural LF, but they may cause failed defibrillation if shock impedance is high enough. Because impedance-threshold alerts are insensitive, normal impedance trends never exclude LF, but an abrupt increase that triggers an alert almost always indicates a header connection issue or LF. Methods for discriminating connection issues from LF are reviewed.

A 25-year-old runner received a single-lead, VDD pacemaker after ablation of AV nodal reentrant tachycardia complicated by intermittent AV block. The rate-adaptive AV delay algorithm (RAAV), which shortens the sensed AV interval (SAV) at faster atrial rates, was programmed to provide a physiologic SAV with exercise. She developed repetitive, atypical, long-RP pacemaker-mediated tachycardia (PMT) because the RAAV shortened the antegrade SAV and retrograde conduction occurred over the slow AV nodal pathway. PMT was refractory to usual programming solutions. Using high-density electroanatomic mapping, we were able to ablate the retrograde limb of PMT without further damaging AV conduction.

Background Implantable cardioverter-defibrillators (ICDs) use low-voltage measures of shock impedance (LVSZ) to monitor integrity of leads. Objective To determine the separation distance between conductors required for LVSZ to detect insulation breaches that produce short circuits during shocks, causing failed defibrillation. Methods We simulated in-pocket insulation breaches between the ICD generator (CAN) and cables to the distal coil of 10 leads from 2 manufacturers. The ICD and lead were placed in an electrolyte bath. Polystyrene sheets were used to control the breach-CAN separation. We determined both the maximum lead-CAN separation for shorts during 800 V shocks and the shock strength at which shorts occurred for a fixed separation. We also calculated breach impedance and measured it using a low-voltage instrument. Results The maximum breach-CAN separation for shorting was 350–500 μm for all leads. The minimum shock strength to short varied from 650 to 771 V (24–32 J). LVSZ never triggered a warning, even with no separation between the cable's inner insulation and the CAN. Using low-voltage pulses, breach impedance was measured at approximately 500–1000 Ω. Conclusion LVSZ is insensitive to insulation breaches that cause life-threatening, shorted shocks. The explanation likely relates to impedance differences between ionic conduction during LVSZ measurements and free-electron conduction in plasma discharges.

SecureSense is an implantable cardioverter defibrillator algorithm that differentiates lead-related oversensing from ventricular tachycardia/ventricular fibrillation by continuous comparison between the near-field (NF) and the far-field (FF) electrogram. If lead noise is identified, inappropriate therapy is withheld. Undersensing on the FF channel could result in inappropriate inhibition of life-saving therapy. Thus, the device automatically switches SecureSense to passive mode if undersensing on the FF channel is suspected. We report here the first cases of inappropriate automatic SecureSense deactivation due to misdiagnosed FF undersensing in pacemaker-dependent patients. Physicians should be aware that SecureSense does not withhold an inappropriate therapy for sustained oversensing in pacemaker-dependent patients.

Since it was introduced 40 years ago, the implantable cardioverter-defibrillator (ICD) has become a mainstay of therapy for patients at risk for life-threatening ventricular tachycardia (VT) and fibrillation (VF). This chapter explores challenges and opportunities for future ICD therapy. Historically, transvenous leads have been the weak link in ICD systems. We review improved engineering standards for transvenous leads and methods to prevent proarrhythmia from ICD lead dislodgements, as well advantages and limitations of subcutaneous and substernal lead systems and their potential integration with leadless pacemakers. We consider better ways to ensure that necessary therapy is delivered for VT/VF while delivering fewer unnecessary and inappropriate shocks. Remote monitoring of ICD patients has become the standard of practice. We approach potential improvements in remote patient management related both to alerts generated in the ICD and potentially more computationally intensive analysis performed after device data is transmitted to servers. Perhaps the greatest unmet need is prediction and prevention of VT/VF. We review avenues for improvement both in long-term risk stratification to optimize utilization of ICDs and in short-term prediction of VT/VF to permit development of device-based preventive therapies.

Objectives This study reports the sensing and arrhythmia detection performance of a novel extravascular (EV) implantable cardioverter-defibrillator (ICD) in a first-in-human pilot study. Background The EV ICD lead is implanted in the substernal space, resulting in novel sensing and detection challenges. It uses a programmable sensing profile with new or modified discrimination of oversensing and of ventricular tachycardia (VT) from supraventricular tachycardia (SVT). Methods Electrograms were post-processed from induced ventricular fibrillation (VF) at implant to determine virtual detection times for each programmable sensitivity and the least-sensitive safe sensitivity setting. In ambulatory patients, programmed sensitivity provided at least a twofold safety margin for detecting induced VF. Noise discrimination was stress tested, and the effects of source, posture, and lead maturation were determined on electrogram amplitude. Telemetry Holter monitors were used to quantify undersensing and oversensing. Results In 20 patients at implant, the least-sensitive safe sensitivity for VF detection ranged from 0.1 to 0.6 mV. Seventeen patients were followed up for a total of 16.6 patient-years. Electrogram amplitudes were stable over time, but there were significant differences among postures and sensing vectors. For the primary sensing vector, the weighted oversensing and undersensing rates were 1.03% and 0.40% respectively, on a beat-to-beat basis. Oversensing did not cause inappropriate therapy in patients with in situ leads. Oversensing discriminators withheld VF detection in 4 of 5 spontaneous, sustained oversensed episodes. SVT-VT discriminators correctly classified 93% of 128 sustained SVTs in monitor zones. Conclusions In the EV ICD pilot study, oversensing did not cause inappropriate therapy during ambulatory follow-up of stable leads.

Impedance is the ratio of voltage to current in an electrical circuit. Cardiovascular implantable electronic devices measure impedance to assess the structural integrity electrical performance of leads, typically using subthreshold pulses. We review determinants of impedance, how it is measured, variation in clinically measured pacing and high-voltage impedance and impedance trends as a diagnostic for lead failure and lead-device connection problems. We consider the differential diagnosis of abnormal impedance and the approach to the challenging problem of a single, abnormal impedance measurement. Present impedance provides a specific but insensitive diagnostic. For pacing circuits, we review the complementary roles of impedance and more sensitive oversensing diagnostics. Shock circuits lack a sensitive diagnostic. This deficiency is particularly important for insulation breaches, which may go undetected and present with short circuits during therapeutic shocks. We consider new methods for measuring impedance that may increase sensitivity for insulation breaches.

A 73-year-old man with coronary artery disease, hypertension, and depressed ventricular function presents after receiving multiple shocks from his single chamber ICD. On physical examination, the blood pressure is 148/73, with heart rate 86. The jugular venous distension measures 8 cm, lungs are clear, gallops are absent, the abdomen is unremarkable, and trace lower extremity edema is noted. Diagnosis and management are discussed.

A 24-year-old woman with complex congenital heart disease, depressed systemic ventricular function, and congestive heart failure previously received an InSync II Marquis™ 7289 CRT-D utilizing a Medtronic Sprint Fidelis 6949 right ventricular defibrillation lead, 4193 left ventricular lead, and a Novus 5076 right atrial lead. While in hospital on telemetry, she develops recurrent episodes of pacing failure, ventricular arrhythmias, and shocks. Diagnosis and problem solving are discussed.

A 64-year-old man with ischemic cardiomyopathy, refractory congestive heart failure (New York Heart Association Class III), and chronic atrial fibrillation undergoes implantation of an InSync Sentry™ 7299 CRT-D defibrillator. The morning after the implantation, he receives a shock while shaking his cardiologist’s hand. Diagnosis and plan of action are discussed.

A 62-year-old man with ischemic cardiomyopathy (NYHA Class II, left-ventricular ejection fraction 29%) underwent implantation of a prophylactic dual-chamber ICD (Atlas DR V-242) 2 years ago. During a febrile illness, he received six shocks without premonitory symptoms. Diagnosis and management are discussed.

A 78-year-old man with ischemic cardiomyopathy (NYHA Class IV, left-ventricular ejection fraction 14%) and renal failure underwent implantation of a prophylactic, cardiac-resynchronization ICD in 2005. He had sinus rhythm with first degree AV block and left bundle branch block. His heart failure improved to Class III and ejection fraction improved to 21%. Bradycardia pacing parameter changes are discussed.

A 68-year-old man with ischemic cardiomyopathy (NYHA Class II, left-ventricular ejection fraction 26%) underwent implantation of a prophylactic dual-chamber ICD (Medtronic EnTrust™ D154ATG) 2 years ago. His medications include lisinopril 20 mg daily, carvedilol 12.5 mg daily, simvastatin 40 ms daily, and aspirin 325 mg daily. The ICD was programmed for single zone detection. Six months ago, he had presyncope followed by a shock. ICD interrogation showed ineffective antitachycardia pacing followed by a successful shock for VT with AV dissociation at cycle length 240 ms. Diagnosis and management are discussed.

A 59-year-old man with ischemic cardiomyopathy had a single-chamber Medtronic Entrust™ ICD and Sprint Fidelis™ Model 6949 dual-coil true bipolar lead implanted over 2 years ago after out-of-hospital VF. His NYHA Class is III and left ventricular ejection fraction is 34%. After an episode of monomorphic VT storm a year ago, he has been treated with amiodarone 200 mg per day to decrease the frequency of VT. He now presents for a routine follow-up visit. The pacing threshold is 0.5 V at 0.4 ms, and the pacing lead impedance is stable at 560 Ω. One episode of device-detected VT is stored in the ICD. Diagnosis and management steps are discussed.

The 78-year-old man described in previous case 169 is followed. No programming was performed at an August 2007 visit. Instead, amiodarone therapy was initiated to prevent asymptomatic VT requiring termination by antitachycardia pacing. Six weeks later, on September 28, 2007, the patient developed rapid palpitations after receiving bad news. Within a few minutes, he experienced dyspnea and his typical angina. He used nitroglycerin spray twice without relief. Twenty minutes later he collapsed and died. Steps that could have been taken to prevent the patient’s death are discussed.

A 76 year-old man with prior inferior wall myocardial infarction (NYHA Class II, left-ventricular ejection fraction 41%, first degree AV block, and narrow QRS) underwent implantation of a dual-chamber ICD (Medtronic EnTrust™ D154ATG) for sustained monomorphic VT requiring cardioversion. He was treated with sotalol 120 mg q 12 h to reduce the frequency of VT. An episode is retrieved from the ICD’s SVT log with programmed detection and bradycardia pacing parameters. Diagnosis is discussed.

International audience

The subcutaneous implantable cardioverter-defibrillator system (S-ICD) has been an important alternative to transvenous defibrillators for certain clinical situations, such as when there is a venous access problem or high risk of bacterial endocarditis. This device reliably detects ventricular tachyarrhythmias and effectively delivers shocks. However, oversensing is more common with the S-ICD compared with transvenous systems. We present a case of a patient in whom a change in the rhythm from sinus rhythm to slow ventricular tachycardia (VT) resulted in reduced R-wave sensing and T- and P-wave oversensing.

BACKGROUND Implantable cardioverter defibrillators (ICDs) must establish a balance between delivering appropriate shocks for ventricular tachyarrhythmias and withholding inappropriate shocks for lead-related oversensing ("noise"). To improve the specificity of ICD therapy, manufacturers have developed proprietary algorithms that detect lead noise. The SecureSenseTM RV Lead Noise discrimination (St. Jude Medical, St. Paul, MN, USA) algorithm is designed to differentiate oversensing due to lead failure from ventricular tachyarrhythmias and withhold therapies in the presence of sustained lead-related oversensing. METHODS AND RESULTS We report 5 patients in whom appropriate ICD therapy was withheld due to the operation of the SecureSense algorithm and explain the mechanism for inhibition of therapy in each case. Limitations of algorithms designed to increase ICD therapy specificity, especially for the SecureSense algorithm, are analyzed. CONCLUSION The SecureSense algorithm can withhold appropriate therapies for ventricular arrhythmias due to design and programming limitations. Electrophysiologists should have a thorough understanding of the SecureSense algorithm before routinely programming it and understand the implications for ventricular arrhythmia misclassification.

Ensuring sensing and detection of ventricular tachycardia (VT) and ventricular fibrillation (VF) was a prerequisite for the clinical trials that established the survival benefit of implantable cardioverter defibrillators (ICDs). However, for decades, a high incidence of unnecessary shocks limited patients' and physicians' acceptance of ICD therapy. Oversensing, misclassification of supraventricular tachycardia (SVT) as VT, and self-terminating VT accounted for the vast majority of unnecessary shocks. Medtronic ICDs utilize sensitive baseline settings with minimal blanking periods to ensure accurate sensing of VF, VT, and SVT electrograms. Programmable algorithms reject oversensing caused by far-field R waves, T waves, and non-physiologic signals caused by lead failure. A robust hierarchy of SVT-VT discriminators minimize misclassification of SVT as VT. These features, combined with evidence-based programming, have reduced the 1‑year inappropriate shock rate to 1.5 % for dual-/triple-chamber ICDs and to 2.5 % for single-chamber ICDs.

Diaphragmatic myopotential oversensing (DMO) causes inhibition of pacing and inappropriate detection of ventricular fibrillation in implantable cardioverter defibrillators (ICDs). It occurs almost exclusively with integrated bipolar leads and is extremely rare with dedicated bipolar leads. If DMO cannot be corrected by reducing programmed sensitivity, ventricular lead revision is often required. The new Low Frequency Attenuation (LFA) filter in St. Jude Medical ICDs (St. Jude Medical, Sylmar, CA, USA) alters the sensing bandpass to reduce T-wave oversensing. This paper aims to present the LFA filter as a reversible cause of DMO. Unnecessary lead revision can be avoided by the simple programming solution of deactivating this LFA filter.

The implantable-cardioverter defibrillator (ICD) lead is the most vulnerable component of the ICD system. Despite advanced engineering design, sophisticated manufacturing techniques, and extensive bench, pre-clinical, and clinical testing, lead failure (LF) remains the Achilles' heel of the ICD system. ICD LF has a broad range of adverse outcomes, ranging from intermittent inappropriate pacing to proarrhythmia leading to patient mortality. ICD LF is often considered in the context of design or construction defects, but is more appropriately considered in the context of the finite service life of a mechanical component placed in chemically stressful environment and subjected to continuous mechanical stresses. This clinical review summarizes LF mechanisms, assessment, and differential diagnosis of LF, including lead diagnostics, recent prominent lead recalls, and management of LF and functioning, but recalled leads. Despite recent advances in lead technology, physicians will likely continue to need to understand how to manage patients with transvenous ICD leads.

Background Ventricular fibrillation (VF) is induced during implantable cardioverter-defibrillator (ICD) implantation to ensure that the ICD will sense, detect, and defibrillate VF. ICD implant guidelines state that the amplitude of the sinus rhythm R wave recorded from the ventricular electrogram should have amplitude ≥5 mV. No study has tested the relationship between sinus rhythm R-wave amplitude and VF sensing using modern, transvenous sensing electrodes. Objective The goal of this study was to determine whether there is a sinus rhythm R-wave amplitude cutoff that can be used to determine which patients are not at risk of VF undersensing. Methods A retrospective analysis of induced and spontaneous VF episodes from 2 clinical trials with 2022 patients was performed. Episodes with undersensing during the initial detection of VF were identified, and the distribution of sinus rhythm R-wave amplitudes for patients with and without VF undersensing was analyzed. Results Only 3% of analyzed induced VF episodes were considered to have VF undersensing, and none had clinically significant detection delays. There was no correlation between device-measured, rectified sinus rhythm R-wave amplitude and VF undersensing at the time of implantation or during follow-up, although Conclusion We analyzed true bipolar sensing of induced VF or spontaneous ventricular tachycardia/VF detected in the ICD VF zone. Sensing of VF was so reliable that clinically significant undersensing did not occur. Our findings do not support any recommended minimum sinus rhythm R wave to ensure reliable sensing of VF or the necessity of inducing VF to verify sensing for rectified sinus rhythm R-waves with amplitude ≥3 mV.

Introduction Recognition of implantable cardioverter defibrillator (ICD) lead malfunction before occurrence of life threatening complications is crucial. We aimed to assess the effectiveness of remote monitoring associated or not with a lead noise alert for early detection of ICD lead failure. Methods From October 2013 to April 2017, a median of 1,224 (578-1,958) ICD patients were remotely monitored with comprehensive analysis of all transmitted materials. ICD lead failure and subsequent device interventions were prospectively collected in patients with (RMLN) and without (RM) a lead noise alert (Abbott Secure Sense™ or Medtronic Lead Integrity Alert™) in their remote monitoring system. Results During a follow-up of 4,457 patient years, 64 lead failures were diagnosed. Sixty-one (95%) of the diagnoses were made before any clinical complication occurred. Inappropriate shocks were delivered in only one patient of each group (3%), with an annual rate of 0.04%. All high voltage conductor failures were identified remotely by a dedicated impedance alert in 10 patients. Pace-sense component failures were correctly identified by a dedicated alert in 77% (17 of 22) of the RMLN group versus 25% (8 of 32) of the RM group (P = 0.002). The absence of a lead noise alert was associated with a 16-fold increase in the likelihood of initiating either a shock or ATP (OR: 16.0, 95% CI 1.8-143.3; P = 0.01). Conclusion ICD remote monitoring with systematic review of all transmitted data is associated with a very low rate of inappropriate shocks related to lead failure. Dedicated noise alerts further reduce inappropriate detection of ventricular arrhythmias.

Objectives This study sought to develop and evaluate an algorithm for early diagnosis of dislodged implantable cardioverter-defibrillator (ICD) leads. Background Dislodged defibrillation leads may sense atrial and ventricular electrograms (EGMs), triggering shocks in the vulnerable period that induce ventricular fibrillation (VF). Methods We developed a 2-step algorithm by using experimental lead dislodgements (LDs) at ICD implantation and a control dataset of newly implanted, in situ leads. Step 1 consisted of an alert triggered by abrupt decrease in R-wave amplitude and increase in pacing threshold. Step 2 withheld therapy based on ventricular EGM evidence of LD identified from experimental LD behavior. We estimated the algorithm’s performance using a registry dataset of 3,624 new implantations and an atrial dislodgement dataset of 14 LDs at the atrium. Results In the registry dataset, the algorithm identified 20 of 21 radiographic LDs (95%) at a median of 11 days before clinical diagnosis. Step 1 had positive predictive values of 57% for radiographic LD and 77% for surgical revision. The false positive rate was 0.4% after step 1 and ≤0.2% after step 2. In the atrial dislodgement dataset, step 1 identified all 14 LDs; step 2 would have prevented inappropriate therapy in all 7 patients with stored EGMs at LD, including 2 patients with fatal, shock-induced VF. Conclusions An ICD algorithm can facilitate early diagnosis of defibrillation LD. Additional data are needed to determine the safety of withholding shocks based on EGM evidence of LD.

Accurate atrial sensing during tachycardia is essential for reliable determination of atrial rate, which is a critical component of interval-based dual-chamber supraventricular tachycardia (SVT)–VT discrimination algorithms.1 Thus, atrial sensing in dual-chamber implantable cardioverter-defibrillators (ICDs) must be reliable at fast ventricular rates, a technically challenging requirement. Determining the atrial rate during rapidly conducted atrial flutter (AF) requires accurate sensing of low-amplitude signals (Figure I in the Data Supplement). The required high atrial sensitivity increases the risk of atrial oversensing, especially of far-field R waves. Pacemakers rely on a long postventricular atrial blanking period (PVAB) to prevent oversensing of far-field R waves, but ICDs cannot: for a fixed PVAB, the blanked fraction of the cardiac cycle increases with the ventricular rate. For example, a 100-ms PVAB blanks 10% of the cycle at a bradycardia pacing cycle length of 1000 ms, but 33% during rapidly conducted AF with ventricular cycle length of 300 ms. PVABs longer than ≈80 ms produce clinically significant, functional atrial undersensing in rapidly conducted AF,2 and any PVAB may cause undersensing of alternate atrial electrograms in AF with 2:1 atrioventricular conduction. The inherent trade-off between atrial oversensing and undersensing is problematic. Both cause errors in dual-chamber SVT–VT discrimination algorithms.2–6 In contemporary ICDs, ventricular electrogram morphology algorithms mitigate this problem but do not eliminate it. ### Determinants of Far-Field R Waves #### General Considerations In the absence of atrial lead dislodgement, the amplitude of far-field R wave depends on 4 variables. The first is the amplitude of the ventricular electrogram in ventricular myocardium. Thus, there is a correlation between far-field R–wave oversensing and left ventricular hypertrophy.7 The second is interelectrode spacing.8 Closely spaced electrodes (1.1 mm) minimize far-field R waves in comparison with conventional 10-mm spacing, but they also reduce the amplitude of atrial electrograms during short-term follow-up9,10 and …

Mark Josephson dedicated his career to the prevention of premature sudden cardiac death (SCD). Toward that goal, he was an early adopter of the implantable cardioverter defibrillator (ICD) and indefatigable advocate for better ICD technology,1 both as a clinical tool and as living laboratory to study SCD in ambulatory patients. With characteristic intellectual integrity and analytical rigour, he sought an honest and balanced appraisal of the life-saving benefits and serious complications of this unique therapy.

Background— The Lead Integrity Alert (LIA) was developed for Medtronic implantable cardioverter defibrillators to reduce inappropriate shocks for rapid oversensing caused by conductor fractures and reported for Medtronic Fidelis conductor fractures. The goal of this study was to compare the performance of LIA with conventional impedance monitoring for identifying lead system events (LSEs) and lead failures (LFs) in lead families that differ from Fidelis. Methods and Results— We analyzed data from 12 793 LIA enabled implantable cardioverter-defibrillator and lead combinations including 6123 St. Jude Riata or Durata, 5114 Boston Scientific Endotak, and 1556 Fidelis combinations followed in the CareLink remote monitoring network for LSEs and LFs. Each alert was adjudicated based on implantable cardioverter-defibrillator stored electrograms/diagnostics and clinical data as an LSE or non–lead system event by 2 physicians after reviewing the electrograms and clinical data. During 13 562 patient-years of LIA follow-up, there were 179 adjudicated alerts, of which 84 were LSEs (including 65 LFs) and 95 were non–lead system events. LIA identified >66% more LSE and >67% more LF compared with conventional impedance monitoring and did not differ by lead family for LSE ( P =0.573) or LF ( P =0.332). Isolated spikes on electrogram were associated more often with LF in St. Jude leads (71%) compared with Endotak (9%; P P P Conclusions— LIA markedly increased the detection rate of LSE compared with conventional impedance monitoring.

Clinical report A 78-year-old woman had a cardiac resynchronization implantable cardioverter-defibrillator implanted in January 2008 for nonischemic cardiomyopathy with New York Heart Association class III heart failure, left ventricular ejection fraction 22%, and left bundle branch block. A St Jude Medical Model 7121 Durata right ventricular lead (St Jude Medical, St Paul, MN) and a Medtronic Model D224TRK Consulta generator (Medtronic, Minneapolis, MN) were implanted. Three months later, ejection fraction improved to 47% and heart failure improved to class I. In July 2010, a combination of recent short intervals ≤140 ms and rapid, repetitive oversensing triggered the Lead Integrity Alert. Interrogation showed an inappropriate detection of ventricular tachycardia treated with antitachycardia pacing. Stored electrograms showed nonphysiological signals, some of which occurred synchronously with the cardiac cycle, timing with the T wave. Pacing threshold was 1.0 V, the R wave was stable at 10 mV, and both pacing and high-voltage impedances were within nominal ranges. However, the weekly minimum pacing impedance decreased by approximately 150 Ω (from 470 to 320 Ω) in February 2010. Differential real-time and telemetry Holter recordings isolated the nonphysiological signals to the cable to the ring electrode rather than the helix to the tip electrode (Figure 1A). Corresponding recordings from shock channels determined that simultaneous nonphysiological signals were present on the distal coil, but not the proximal coil. Additional diagnostic steps excluded other causes of oversensing. Initially, the patient declined lead replacement. But when frequent oversensing caused the percentage of biventricular pacing to decrease below 90% in late 2012, lead replacement was recommended to preserve resynchronization. Cinefluoroscopy (Figure 1B) showed no exteriorized cables or mechanical lead-lead interaction.