Inappropriate disabling of an ICD noise-detection algorithm in pacemaker-dependent patients.

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.

Towards eradication of inappropriate therapies for ICD lead failure by combining comprehensive remote monitoring and lead noise alerts.

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.

Mechanisms of Undersensing by a Noise Detection Algorithm That Utilizes Far-Field Electrograms With Near-Field Bandpass Filtering.

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.

Diaphragmatic Myopotential Oversensing Caused by Change in Implantable Cardioverter Defibrillator Sensing Bandpass Filter.

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.

Polarity-Dependent, Reduced or No-Output Shocks in Implantable Cardioverter-Defibrillators.

Recently, polarity-dependent shock failures were reported in implantable cardioverter-defibrillators caused by structural failure in the high-voltage feedthrough. Short circuits may occur when the right ventricular coil is cathodal for phase 1 of biphasic shocks (cathodal shock). This viewpoint proposes a mechanism for observed polarity dependence and considers whether the same mechanism may apply in other shock-induced, short circuits. Implantable cardioverter-defibrillator connections to the lead traverse feedthroughs into the hermetically sealed housing ("Can"). The feedthrough comprises 2 concentric, conducting metal cylinders, the inner pin-conductor to the right ventricular coil and outer Can, separated by impermeable insulation. Shock failure depends on 3 conditions: 1) development of a fluid layer in the feedthrough, creating a conduction path in parallel with the shock pathway; 2) the radial gradient of the electric field in the fluid, so resistive heating during a shock vaporizes water to form a high-resistance gas bubble around the pin; and 3) field emission of electrons at the cathode, with rate and energy dependent on the field's strength and the cathode's potential-energy barrier to emission. For cathodal shocks, electrons emitted at the metal pin may initiate an ionization avalanche in the gas until it "breaks down" into a low-resistance plasma, resulting in a short circuit. For anodal shocks, the effective cathode is the liquid-gas interface, where the field is weaker than at the pin. Additionally, solvated electrons in aqueous solution must overcome a higher potential-energy barrier to be emitted. This permits the high-resistance gas bubble to stabilize so that the shock is completed.

Time course of oversensing and impedance changes in developing implantable cardioverter-defibrillator lead fracture.

Background: 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. Objectives: To characterize the relationship between oversensing, impedance, and structural changes in ICD leads developing pace-sense conductor fractures. Methods: 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 (σDCR). Results are reported for 17 other test leads. Results: Initial 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 σDCR (≥0.5 Ω). Impedance threshold alerts occurred at complete pacing coil fracture but only later for ring cable fractures. The oversensing alert triggered before device-detected ventricular fibrillation more frequently than impedance alerts (94% vs 17%; P = .00002). Conclusions: In 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.

Interpreting device diagnostics for lead failure.

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.

Clinical performance of implantable cardioverter-defibrillator lead monitoring diagnostics.

BACKGROUND: Implantable cardioverter-defibrillator (ICD) lead monitoring diagnostic alerts facilitate the diagnosis of structural lead failure. OBJECTIVE: The purpose of this study was to prospectively study the performance of Medtronic ICD lead monitoring alerts. METHODS: A prespecified ancillary substudy, World-Wide Randomized Antibiotic Envelope Infection Prevention Trial, was conducted in patients with an ICD with all available alerts enabled. The investigators reported possible lead system events (LSEs), with or without an alert. An independent committee reviewed all data and classified events as lead failure, other LSE, or nonlead system events (NLEs). RESULTS: In 4942 patients who were followed for 19.4 ± 8.7 months, there were 124 alerts (65 LSEs, 59 NLEs) and 19 LSEs without an alert. Lead monitoring alerts had 100% sensitivity for the 48 adjudicated lead failures (95% confidence interval 92.6%-100%) and for 10 events adjudicated as either lead failure or connection issue. The positive predictive value of alerts for lead failure was 38.7% (48 of 124). For 34 pace-sense lead failures, an alert that incorporated oversensing was more sensitive than the pacing impedance threshold alert (33 patients [97.1%] vs 9 patients [26.5%]; P < .0001). However, the sensitivity was only 13.6% for lead dislodgments or perforations. Inappropriate shocks occurred in 2 patients with pace-sense lead failure (5.9%). No patient had unnecessary lead replacement for any of the NLEs. CONCLUSION: In this first real-world prospective study, lead monitoring alerts had 100% sensitivity for identifying lead failures. Although their positive predictive value was modest, no false-positive alerts resulted in an unnecessary lead replacement. For the diagnosis of pace-sense lead failure, an alert for oversensing was more sensitive than a pacing impedance threshold alert. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT02277990.

Downloadable software algorithm reduces inappropriate shocks caused by implantable cardioverter-defibrillator lead fractures: a prospective study.

BACKGROUND: Downloadable software upgrades are common in consumer electronics but not in implantable medical devices. Fractures of implantable cardioverter-defibrillator (ICD) leads present commonly as inappropriate shocks. A lead-integrity alert (LIA) designed to reduce inappropriate shocks is the first software download approved to enhance nominally functioning, previously implanted ICDs. METHODS AND RESULTS: We performed a prospective study to determine whether an LIA could reduce inappropriate shocks. Patients were included if they had ICD lead fractures confirmed by analysis of explanted leads. The LIA group included the first 213 patients who met the inclusion criteria after the LIA was approved who had the LIA downloaded. The LIA is triggered either by high impedance or rapid oversensing. It responds by delaying detection of ventricular fibrillation and initiating a patient alert every 4 hours. The control group included the first 213 patients who did not have the LIA downloaded. They were monitored by conventional daily impedance measurements that respond with a daily alert. The LIA group had a 46% relative reduction (95% confidence interval 34% to 55%) in the percentage of patients with ≥1 inappropriate shock (LIA 38% versus control 70%, P<0.001) and a 50% relative reduction (95% confidence interval 33% to 61%) in the percentage with ≥5 shocks (25% versus 50%, P<0.001). The LIA group also had a higher percentage of patients who either did not receive a shock or had ≥3 days of warning before the shock (72% versus 50%, P<0.001). CONCLUSIONS: A software download that upgrades previously implanted ICDs without surgical revision reduces inappropriate shocks caused by lead fractures.

Design and Preliminary Results of Sensing and Detection for an Extravascular Implantable Cardioverter-Defibrillator.

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 in the Diagnosis of Lead Malfunction.

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.

Downloadable algorithm to reduce inappropriate shocks caused by fractures of implantable cardioverter-defibrillator leads.

BACKGROUND: The primary method for monitoring implantable cardioverter-defibrillator lead integrity is periodic measurement of impedance. Sprint Fidelis leads are prone to pace-sense lead fractures, which commonly present as inappropriate shocks caused by oversensing. METHODS AND RESULTS: We developed and tested an algorithm to enhance early identification of lead fractures and to reduce inappropriate shocks. This lead-integrity algorithm, which can be downloaded into presently implanted implantable cardioverter-defibrillators, alerts the patient and/or physician when triggered by either oversensing or excessive increases in impedance. To reduce inappropriate shocks, the lead-integrity algorithm increases the number of intervals to detect (NID) ventricular fibrillation when triggered. The lead-integrity algorithm was tested on data from 15 970 patients with Fidelis leads (including 121 with clinically diagnosed fractures) and 95 other fractured leads confirmed by analysis of returned product. The effect of the NID on inappropriate shocks was tested in 92 patients with 927 shocks caused by lead fracture. Increasing the NID reduced inappropriate shocks (P<0.0001). The lead-integrity algorithm provided at least a 3-day warning of inappropriate shocks in 76% (95% CI, 66 to 84) of patients versus 55% (95% CI, 43 to 64) for optimal impedance monitoring (P=0.007). Its positive predictive value was 72% for lead fractures and 81% for lead fractures or header-connector problems requiring surgical intervention. The false-positive rate was 1 per 372 patient-years of monitoring. CONCLUSIONS: A lead-integrity algorithm developed for download into existing implantable cardioverter-defibrillators increases short-term warning of inappropriate shocks in patients with lead fractures and reduces the likelihood of inappropriate shocks. It is the first downloadable RAMware to enhance the performance of nominally functioning implantable cardioverter-defibrillators and the first implantable cardioverter-defibrillator monitoring feature that triggers real-time changes in ventricular fibrillation detection parameters to reduce inappropriate shocks.

Why low-voltage shock impedance measurements fail to reliably detect insulation breaches in transvenous defibrillation leads.

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.