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Appropriate inappropriate shocks: Inappropriate oversensing during slow ventricular tachycardia in a patient with a subcutaneous implantable cardioverter-defibrillator

Open AccessPublished:January 09, 2023DOI:https://doi.org/10.1016/j.hrcr.2022.12.020

      Keywords

      Key Teaching Points
      • Preimplant screening and high-pass filtering have dramatically reduced the rates of inappropriate shocks with subcutaneous implantable cardioverter-defibrillators (ICDs). However, these inappropriate shocks can still occur.
      • Changes in QRS morphology during ventricular tachycardia can lead to QRS double-counting or T-wave oversensing. In patients with apical or epicardial ventricular tachycardia, these changes in QRS morphology may be more dramatic.
      • QRS double-counting during ventricular tachycardia is difficult to program around in subcutaneous ICDs. In patients with slow ventricular tachycardia where this occurs, transvenous ICD implantation should be considered.

      Introduction

      Inappropriate shocks have plagued subcutaneous implantable cardioverter-defibrillators (S-ICD) since their inception.
      • El-Chami M.F.
      • Harbieh B.
      • Levy M.
      • Leon A.R.
      • Merchant F.M.
      Clinical and electrocardiographic predictors of T wave oversensing in patients with subcutaneous ICD.
      Inappropriate shocks in S-ICD patients are most commonly due to T-wave oversensing (TWOS) or low-amplitude signals.
      • Boersma L.
      • Barr C.
      • Knops R.
      • et al.
      Implant and midterm outcomes of the subcutaneous implantable cardioverter-defibrillator registry: the EFFORTLESS study.
      Advances in S-ICD technology, including preimplant screening, the addition of conditional shock zones, and high-pass filtering (SMART Pass), have helped to dramatically reduce the rate of inappropriate shocks due to TWOS to rates that are comparable to transvenous ICDs.
      • Gold M.R.
      • Weiss R.
      • Theuns D.A.
      • et al.
      Use of a discrimination algorithm to reduce inappropriate shocks with a subcutaneous implantable cardioverter-defibrillator.
      • Groh C.A.
      • Sharma S.
      • Pelchovitz D.J.
      • et al.
      Use of an electrocardiographic screening tool to determine candidacy for a subcutaneous implantable cardioverter-defibrillator.
      • Theuns D.
      • Brouwer T.F.
      • Jones P.W.
      • et al.
      Prospective blinded evaluation of a novel sensing methodology designed to reduce inappropriate shocks by the subcutaneous implantable cardioverter-defibrillator.
      • Gold M.R.
      • Lambiase P.D.
      • El-Chami M.F.
      • et al.
      Primary results from the Understanding Outcomes With the S-ICD in Primary Prevention Patients With Low Ejection Fraction (UNTOUCHED) trial.
      • Knops R.E.
      • Olde Nordkamp L.R.A.
      • Delnoy P.H.M.
      • et al.
      Subcutaneous or transvenous defibrillator therapy.
      Despite these advances, S-ICDs remain susceptible to inappropriate shocks in certain scenarios. In this case, we describe a patient with apical hypertrophic cardiomyopathy and episodes of slow ventricular tachycardia (VT) arising from the apex causing a radically different vector and QRS than in normal sinus rhythm, resulting in inappropriate shocks owing to double-counting of the QRS.

      Case report

      A 50-year-old man with a past medical history significant for apical hypertrophic cardiomyopathy, obstructive sleep apnea, and hypertension was referred for VT ablation for recurrent ICD shocks. The patient was originally diagnosed with apical hypertrophic cardiomyopathy at the age of 35, after incidentally being noted to have an abnormal electrocardiogram (ECG). He had genetic testing done at that time, which did not show any known genetic mutation associated with sarcomeric hypertrophic cardiomyopathy. Cardiac magnetic resonance imaging was subsequently done and showed a preserved ejection fraction, an apical aneurysm, and patchy late gadolinium enhancement in the apex. Outpatient cardiac monitoring showed short episodes of nonsustained VT. Based on these findings, an S-ICD was implanted after appropriate preimplantation screening. S-ICD was chosen over a transvenous ICD in a shared decision-making process, taking into account the patient’s age, preferences, and desire to avoid long-term complications associated with transvenous leads. His device was programmed to a conditional shock zone at 200 beats per minute, with a shock zone at 240 beats per minute.
      The patient did well for 7 years after his initial S-ICD implantation, until he began developing episodes of sustained VT requiring appropriate S-ICD shocks. Figure 1 shows an example of 2 of these episodes of monomorphic VT, with both episodes falling into the shock zone (>240 beats per minute). These episodes resulted in a loss of consciousness, and he was not aware of the resultant shock. Over the course of the next 5 months, he had 2 similar episodes to this despite up-titration of beta-blocker therapy, both resulting in additional appropriate ICD shocks.
      Figure thumbnail gr1
      Figure 1Tracings from 2 episodes of ventricular tachycardia falling into the shock zone requiring defibrillation. Cycle lengths were 250 ms and 240 ms, respectively. The sensing is programmed to the primary vector. “S” stands for sensed QRS, “T” stands for tachycardia detection, and “C” stands for charge start. The lightning bolt symbol represents the defibrillation attempt. The red boxes highlight instances of T-wave oversensing on the left and QRS double-counting on the right (not all instances are highlighted).
      After discussion with the patient regarding further options, including antiarrhythmic therapy, ablation, and device upgrade to a transvenous device to allow for antitachycardia pacing, the patient opted to move forward with a VT ablation. Prior to the ablation, the patient’s S-ICD was deactivated. During the ablation, his clinical VT was induced, but owing to hemodynamic instability, activation mapping could not be performed. Isochronal late activation mapping in normal sinus rhythm and pace mapping were performed to localize areas of interest for ablation. Deceleration zones were noted at the neck of the apical aneurysm, and the best pace maps were also localized to the apical aneurysm. Extensive ablation with scar homogenization was performed in the apical aneurysm and the neck of the aneurysm. No clinical VT was inducible at the end of the case.
      Five months after the ablation, he again had recurrent monomorphic VT of similar morphology and cycle length to his preablation arrhythmia, resulting in an appropriate shock. He was then brought in for a repeat VT ablation, this time with an endocardial and epicardial approach (Figure 2). Catheter manipulation in the inferolateral epicardium triggered his clinical VT, and this was extensively ablated. Additionally, further endocardial ablation was performed in the apical aneurysm targeting areas of electrical capture to further homogenize the scar. At the end of the case, no clinical VT was inducible. Steroids were given in the epicardial space to prevent postablation pericarditis.
      Figure thumbnail gr2
      Figure 2Epicardial and endocardial electroanatomic map from ventricular tachycardia ablation. Red dots represent ablation lesions with >10 ohms impedance drop. Magenta dots represent lesions with <10 ohms impedance drop. Note the narrow, long neck of the apical aneurysm endocardially, as well as the large distance from the epicardial to endocardial lesions, owing to significant apical hypertrophy.
      In the recovery area, the patient was noted to have frequent runs of slow VT at around 150 beats per minute. He had symptoms of palpitations but was otherwise hemodynamically stable and awake during these episodes. However, his device would deliver defibrillation therapy for these episodes despite being below the conditional shock zone. His device was deactivated with a magnet, and interrogation of his device revealed double-counting of the QRS during the VT, resulting in these inappropriate shocks (Figure 3). This occurred despite the SMART Pass filter being programmed on.
      Figure thumbnail gr3
      Figure 3A: Tracings from an episode of ventricular tachycardia with a rate of 150 beats per minute. Note the QRS double-counting resulting in an inappropriate shock. B: Twelve-lead electrocardiogram of monomorphic ventricular tachycardia. Note the radical change in vector owing to the apical origin.
      He was subsequently started on intravenous lidocaine, with cessation of the VT. The lidocaine was weaned off after a day, with no further VT. He was loaded on sotalol for further arrhythmia suppression. His S-ICD was then extracted and replaced with a single-chamber transvenous ICD. Two months after implantation of a transvenous ICD, he had recurrence of a faster VT, which was successfully terminated with antitachycardia pacing.

      Discussion

      S-ICDs are an increasingly used option for the prevention of sudden cardiac death, especially in patients in whom clinicians are hoping to avoid complications that are associated with long-term transvenous leads. However, despite advances in patient selection and ECG filtering, S-ICDs remain susceptible to sensing abnormalities that can lead to inappropriate shocks. This case highlights such an event, where QRS double-counting occurred during a slow VT, leading to an inappropriate shock below the rate cutoff.
      The majority of preimplant screening is now performed with automated screening technology.
      • Bogeholz N.
      • Pauls P.
      • Guner F.
      • et al.
      Direct comparison of the novel automated screening tool (AST) versus the manual screening tool (MST) in patients with already implanted subcutaneous ICD.
      Studies have shown that the screening tool is sensitive but not specific in identifying ineligible candidates.
      • Bogeholz N.
      • Pauls P.
      • Guner F.
      • et al.
      Direct comparison of the novel automated screening tool (AST) versus the manual screening tool (MST) in patients with already implanted subcutaneous ICD.
      ,
      • Zeb M.
      • Curzen N.
      • Allavatam V.
      • et al.
      Sensitivity and specificity of the subcutaneous implantable cardioverter defibrillator pre-implant screening tool.
      Because screening is performed preimplant, it is not possible to account for possible future alterations to the baseline QRS that may affect sensing, such as the development of bundle branch block, progression of ventricular hypertrophy, or the development of ventricular arrhythmias. This case shows such an example, where the baseline sinus QRS is appropriately sensed but the QRS is double-counted during VT owing to the change in morphology. The vector of this QRS in VT was apex-to-base, which is reversed from the normal sinus vector, which is base-to-apex, as well as epicardial-endocardial. This radical change in vector and QRS morphology during epicardial apical VT is even more difficult to account for during preimplant screening. In fact, during prior VT episodes, the patient was noted to have occasional double-counting of his QRS (Figure 1). Given that the rate of VT during those episodes fell into the shock zone, this did not result in any difference in clinical treatment, but could have provided a clue that the patient was at risk of double-counting in the future.
      The patient’s diagnosis of apical hypertrophic cardiomyopathy may have made him more prone to inappropriate shocks. Per the 2017 AHA/ACC/HRS guidelines, while S-ICD implantation can be considered in select patients with hypertrophic cardiomyopathy, there is an increased potential for TWOS and inappropriate shocks.
      • Al-Khatib S.M.
      • Stevenson W.G.
      • Ackerman M.J.
      • et al.
      2017 AHA/ACC/HRS guideline for management of patients with ventricular arrhythmias and the prevention of sudden cardiac death: executive summary: a report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society.
      In contrast, a study by Nazer and colleagues
      • Nazer B.
      • Dale Z.
      • Carrassa G.
      • et al.
      Appropriate and inappropriate shocks in hypertrophic cardiomyopathy patients with subcutaneous implantable cardioverter-defibrillators: an international multicenter study.
      showed that inappropriate shocks in patients with hypertrophic cardiomyopathy with S-ICDs were rare and more likely in patients with secondary prevention devices. However, in that study, only 4 patients had apical hypertrophy, and the overall rate of therapies was very low.
      • Nazer B.
      • Dale Z.
      • Carrassa G.
      • et al.
      Appropriate and inappropriate shocks in hypertrophic cardiomyopathy patients with subcutaneous implantable cardioverter-defibrillators: an international multicenter study.
      We suspect that this patient’s apical substrate may have placed him at a higher risk of inappropriate shocks owing to the radical morphologic changes while in VT (Figure 3B). This, however, is speculative, as no studies to date have examined whether a higher rate of inappropriate shocks is seen in patients with apical and/or epicardial VT.
      In the more recent VT (Figure 3A and 3B), postablation inflammation likely resulted in an automatic VT of a different morphology and slower rate than the patient’s previous clinical VT. Postablation VT such as that seen in this case can be successfully treated with antiarrhythmics for a brief period, usually for a few weeks to a few months. Owing to the QRS double-counting seen in Figure 3A, the device labeled the tachycardia as falling in the shock zone and delivered an inappropriate shock.
      Therefore, one potential treatment option in this patient included leaving the S-ICD in and treating with antiarrhythmic therapy until the “healing phase” was complete. However, given the observed double-counting, it was felt that inappropriate shocks may very well happen again in the future with further ventricular arrhythmias. Adjusting the sensing vector could have been attempted after the first recorded episode of QRS double-counting and TWOS (Figure 1), but double-counting or TWOS during future ventricular arrhythmias may have still occurred. Another potential option would be to attempt to stimulate VT during an electrophysiologic study and by pacing from the apex while recording the subcutaneous electrocardiogram morphology and attempt to adjust the sensing vector, as has been reported previously.
      • Ferrick N.J.
      • Fisher J.D.
      • Gross J.N.
      • Krumerman A.K.
      A case of pseudo-appropriate shock.
      However, given the very recent ablation, this was felt to not be pragmatic. Furthermore, this approach would not account for future possible ventricular arrhythmias as a result of further substrate changes.
      Most importantly, the patient was keen to avoid future shocks while awake. While this shock was “appropriate” in one sense, as the patient was in VT, it was inappropriate because it occurred below the rate cutoff. Most slow VT that is not hemodynamically unstable is not life-threatening and can often be terminated with antitachycardia pacing.
      • Sadoul N.
      • Mletzko R.
      • Anselme F.
      • et al.
      Incidence and clinical relevance of slow ventricular tachycardia in implantable cardioverter-defibrillator recipients: an international multicenter prospective study.
      ,
      • Lusebrink U.
      • Duncker D.
      • Hess M.
      • et al.
      Clinical relevance of slow ventricular tachycardia in heart failure patients with primary prophylactic implantable cardioverter defibrillator indication.
      Furthermore, both inappropriate and appropriate shocks are associated with increased mortality.
      • Proietti R.
      • Labos C.
      • Davis M.
      • et al.
      A systematic review and meta-analysis of the association between implantable cardioverter-defibrillator shocks and long-term mortality.
      Thus, the decision was made to extract the S-ICD and replace it with a more reliable single-chamber transvenous ICD. While QRS double-counting can still occur in transvenous ICDs, the increased programming options available in transvenous ICDs, such as adjustments to the post–ventricular blanking period, sensitivity, decay delay, and true bipolar sensing, can help greatly to mitigate the issue.
      • Home M.
      • Ladia V.
      • Fisher R.
      • Win-Kuang S.
      • Srivathsan K.
      R-wave double counting secondary to antiarrhythmics and ablation leads to unnecessary shock.

      Conclusion

      As S-ICD implantations become increasingly prevalent, it is important for implanters to be aware of their limitations and pitfalls. This case highlights one of the limitations of S-ICDs despite appropriate preimplant screening and the use of modern filtering technology, ultimately necessitating explant of the S-ICD and implantation of a traditional transvenous system. In patients with known slow monomorphic VT, especially with a significant change in QRS vector such as that seen in apical or epicardial VT, traditional transvenous ICDs should be considered over S-ICD. Novel preimplant screening strategies for patients, particularly those with apical hypertrophy, may be needed to reduce the risk of inappropriate shocks with S-ICDs.

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