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Two Undesired Conditions Resulting From T-Wave Oversensing in Two Patients With Hypertrophic Cardiomyopathy: Inappropriate ICD Shocks and Pacemaker Dysfunction


1 Medipol University Faculty of Medicine, Cardiology Department, Istanbul, Turkey
*Corresponding author: Filiz Kizilirmak, Cardiology Department, Medipol University Hospital, TEM Avrupa Otoyolu Goztepe Cikisi, Istanbul, Turkey. Tel: 90-5055861728, Fax: 90-2124607070, E-mail: filizkizilirmak@hotmail.com.
Research in Cardiovascular Medicine. 6(1): e34299 , DOI: 10.5812/cardiovascmed.34299
Article Type: Case Report; Received: Nov 2, 2015; Revised: Dec 2, 2015; Accepted: Jan 16, 2016; epub: Jun 19, 2016; collection: Jan 2017

Abstract


Introduction: T-wave oversensing (TWOS) is generally seen in patients with hypertrophic cardiomyopathy (HCM) and is a rare cause of inappropriate implantable cardioverter defibrillator (ICD) shocks. TWOS rarely causes pacemaker dysfunction.

Case Presentation: In this paper, we present two patients with hypertrophic cardiomyopathy (HCM). One patient had several inappropriate ICD shocks, and the other experienced pacemaker dysfunction due to TWOS. ICD interrogation revealed that TWOS occurred only during high heart rates in the first patient. Attempts to fix TWOS, including a higher beta blocker dose, electrophysiology study, and ICD re-programming, were unsuccessful.

Conclusions: We replaced the previously implanted ICD generator (Medtronic Maximo II DR) with a new one that has a specific diagnostic algorithm to prevent TWOS (Medtronic Protector). After replacement, the patient did not have any inappropriate shocks due to TWOS. The second patient had bradycardic pacemaker rhythm due to TWOS. Although we reposed, the ventricular lead did not stabilize with an adequate threshold, and thus we decided to replace the lead with a new one. The pacemaker dysfunction disappeared after the lead replacement. Patients with HCM must be observed carefully for these undesired conditions.

Keywords: T-Wave Oversensing; Hypertrophic Cardiomyopathy; Implantable Cardioverter Defibrillator

1. Introduction


T-wave oversensing (TWOS) is generally seen in patients with hypertrophic cardiomyopathy (HCM) and is a rare cause of inappropriate implantable cardioverter defibrillator (ICD) shocks (1). TWOS rarely causes pacemaker dysfunction (2). In this paper, we present two patients with HCM. The first patient had several inappropriate ICD shocks, and the second one had pacemaker dysfunction due to TWOS.

2. Case Presentation


2.1. First Case

A 19-year-old female patient was admitted to our department with palpitations and painful ICD shocks. She reported that she had had five shocks in the last two months. She was on 100 mg of metoprolol once daily. Her medical history included a diagnosis of HCM dual chamber ICD, implantation for primary prevention of sudden cardiac death (SCD), and ablation of focal right atrial tachycardia. Physical examination revealed 3/6 pansystolic murmur at the apex of the heart. Resting electrocardiogram (ECG) showed left ventricular hypertrophy.


Echocardiographic examination showed hypertrophy of the left ventricle and moderate mitral insufficiency. Interventricular septum thickness was 22 mm, and posterior wall thickness was 15 mm. Inappropriate ICD shocks resulting from TWOS were detected (Figure 1).


Figure 1.
Initial Interrogation of the Implantable Cardioverter Defibrillator Revealed T-Wave Oversensing Resulting Inappropriate Shock

Interrogation also showed that T-waves were not sensed during the sinus rhythm, and TWOS was present only during the tachycardia periods. The tachycardia periods were consistent either with the sinus tachycardia or with supraventricular tachycardia (SVT) (i.e., atrial tachycardia). The beta blocker dosage was increased to the maximum daily dose but it was not effective. The ventricular electrogram morphology tachycardia discrimination algorithm (Wavelet Dynamic Discrimination, Medtronic, Minneapolis, MN, USA) to distinguish between SVT and VT/VF was switched on.


We decided to conduct a new electrophysiology study (EPS) because the patient had a history of atrial tachycardia. However, no SVT was inducible in the EPS. Amplitudes, thresholds, and impedances were normal for atrial and ventricular leads during the ICD interrogation. Ventricular tachycardia (VT) zone was set to 170 - 189 bpm, and ventricular fibrillation (VF) zone was set to over 190 bpm. Therapies were ATP and shocks for the VT zone and shocks 35 J and ATP during changing for the VF zone. Seven inappropriate shocks were observed, and all shocks were due to TWOS. Note that TWOS was present only during heart rates over 110 bpm. The previous ICD generator (Medtronic Maximo II DR) was replaced with a new one (Medtronic Protecta DR). The R-wave amplitude was 11.1 mV, and the T-wave amplitude was 6.8 mV when connected to the previously implanted ICD lead. Automatic sensitivity control with enhanced T-wave suppression was programmed as follows: upper threshold (UT) for the detection of 75% of peak amplitude, lower threshold (LT) of above 25%, and minimum threshold at 0.8 mV. The UT is active in the T-wave discrimination period of 350 ms (including 100 ms of the refractory period). Atrial pacing up to 160 bpm did not result in TWOS. The patient did not have any shocks during the three-month follow-up period (Figure 2).


Figure 2.
Postprocedural Interrogation of the Implantable Cardioverter Defibrillator Revealed no T-Wave Oversensing and Inappropriate Shock

2.2. Second Case

A 31-year-old male patient was admitted to our department with dizziness, weakness, and dyspnea. He had an ICD (Medtronic, SN: PZM610875S) implanted because of complete atrioventricular block and sick sinus syndrome with HCM two years ago. He reported dizziness, weakness, and dyspnea upon exertion in the last six months.


Physical examination revealed 2/6 systolic murmur at the apex of the heart. Resting ECG showed bradycardic pacemaker rhythm (Figure 3).


Figure 3.
Initial 12-Lead ECG at Our Hospital Revealed a Bradycardic Pacemaker Rhythm

During the echocardiographic examination, the patient had left ventricular hypertrophy and middle mitral valve insufficiency. Interventricular septum thickness was 21 mm, and posterior wall thickness was 1.5 mm. ICD interrogation revealed that the pacemaker dysfunction from oversensing was due to TWOS. TWOS was interpreted by the ICD as a ventricular impulse. The following atrial sensing fell within the postventricular atrial refractory period. The next ventricular paced was withheld and caused decreased ventricular pacing. Manipulations to reduce the ventricular sensitivity and increase the R-wave amplitude failed. We decided to repose the ventricular lead. However, the ventricular lead did not stabilize with an adequate threshold, and thus we decided to replace the ventricular lead (Medtronic: SPRINT QUATTRO TDC104500V) with a new one (Medtronic: TDG582971M) that had an active fixation system. The new ventricular lead and the old atrial lead were connected with the previously implanted ICD generator (Medtronic, SC:PZM610875S ).


Amplitudes, thresholds, and impedances were normal for atrial and ventricular leads during the ICD interrogation. The R-wave amplitude was 11.1 mV, and the T-wave amplitude of 6.8 mV. The patient did not have any pacemaker dysfunction from oversensing due to TWOS during the two-month follow-up period.

3. Discussion


ICD is suggested to patients with HCM for the primary and secondary prevention of SCD (1). The inappropriate delivery of a shock remains one of the most common adverse events associated with ICDs. TWOS is a rare etiology of inappropriate ICD shocks that occur in up to 14% of patients with ICDs (3). The first step for the treatment of this phenomenon is to determine the underlying pathology if possible and to reprogram the ICD (i.e., decreasing the sensitivity of the device if the R-waves are of adequate amplitude, increasing the postventricular blanking period, and increasing the lower rate of the tachycardia zone). The other treatment options include replacing/repositioning the ventricular lead and changing the pulse generator (4). HCM is associated with a higher risk of TWOS related to inappropriate ICD shocks. The rate of inappropriate ICD shocks in HCM is reported to be 23% - 33% (1). In one study, inappropriate ICD shocks occurred in 30% of HCM patients implanted with ICD over a long follow-up period, and the cause of inappropriate ICD shocks was TWOS in half of the cases (5). Although the study did not specify the pathogenesis of TWOS in hypertrophic cardiomyopathy, Kapa et al. attributed it to the hypertrophy-related prominent T-wave (6). The increase in T-wave amplitude may be due to the tachycardia-induced ischemia in hypertrophic cardiomyopathy. By using digital sensing technology, the device can automatically detect TWOS in patients with a high risk of TWOS related to inappropriate ICD shocks without compromising the VF sensitivity of the device as seen in the first patient. TWOS induced by hyperkalemia has been reported in ICDs but not in pacemakers because of the very high ventricular sensitivity of ICDs (7). In our first patient, TWOS was due to HCM and led to decreased ventricular pacing. A case report indicates a low percentage of biventricular pacing in a patient who had an ICD implanted with cardiac resynchronization therapy (2). Patients with HCM are usually young and have ICDs. When they are pacemaker dependent similar to our second patient, the ventricular pacing rate may be more important. Replacing or repositioning the lead with one of the several methods may prevent TWOS in cases similar to our second patient.


3.1. Conclusion

TWOS is an important problem in HCM. It may cause inappropriate shocks or pacemaker dysfunction. In patients with HCM, this problem can be prevented using different methods. Patients with HCM must be observed carefully for these undesired conditions.

Footnotes

Funding/Support: This research did not receive any grant from any funding agency in the public, commercial, or not-for-profit sectors.

References


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Figure 1.

Initial Interrogation of the Implantable Cardioverter Defibrillator Revealed T-Wave Oversensing Resulting Inappropriate Shock

Figure 2.

Postprocedural Interrogation of the Implantable Cardioverter Defibrillator Revealed no T-Wave Oversensing and Inappropriate Shock

Figure 3.

Initial 12-Lead ECG at Our Hospital Revealed a Bradycardic Pacemaker Rhythm