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  • Baseline lead ECGs from immediately prior

    2019-05-24

    Baseline 12-lead ECGs from immediately prior to S-ICD implantation were reviewed for all patients. Baseline intervals (PR, QRS, QT) were obtained from the automated ECG measurements. Leads I, II, and aVF were further used to collect data on QRS and T wave amplitudes, QRS/T amplitude ratio, presence of T wave inversion, and QRS/T discordance. Leads I, II, and aVF were chosen for analysis because they mimic the three sensing vectors of the S-ICD [8]. In each of the three chosen ECG leads, THZ1 QRS and T wave amplitudes were measured using manual calipers, and the presence of T wave inversion and QRS/T discordance was determined by visual inspection by an electrophysiologist blinded to other clinical variables. Patients were stratified based on the presence or absence of TWOS during follow-up. Continuous variables are presented as the mean±standard deviation, and categorical data are summarized as frequencies and percentages. Comparisons across groups were performed using the Student׳s t-test, Fisher׳s exact test, or one-way analysis of variance, as appropriate. For all comparisons, a p value <0.05 was considered to be statistically significant. Analysis was performed using STATISTICA software (Statsoft, Inc., Tulsa, OK).
    Results
    Discussion TWOS remains one of the major drawbacks of the S-ICD [9,10]. Despite the use of a discrimination algorithm (2 zones vs. 1 zone) that reduces the rate of TWOS [6], TWOS is commonly encountered in patients with an S-ICD. In the largest study to date, TWOS occurred in 5.1% of patients with an S-ICD followed for around 3 years [2], accounting for 39% of all inappropriate shocks. An algorithm was developed and tested on stored episodes [11], and this new algorithm reduced the rate of TWOS by around 40% without compromising the detection of ventricular arrhythmias. In this study, we retrospectively reviewed 92 patients who received an S-ICD at our institution and found that the rate of TWOS was 6.5%, compatible with other contemporary reports of TWOS [1–5]. The major predictors of TWOS in our cohort were younger age, higher LVEF, and low QRS amplitude in ECG lead I. There was also a trend toward increased likelihood of TWOS in male patients. It is conceivable that younger patients are more likely to have channelopathies or HCM as an indication for ICD therapy. These medical conditions are associated with ST-T changes that could predispose patients to developing TWOS. In addition, younger age and normal EF might be associated with a higher likelihood of being physically active, which would therefore result in higher sinus rates. The latter in the presence of TWOS and double counting could lead to inappropriate shocks. Young age has previously THZ1 been shown to be a predictor of inappropriate shocks and TWOS in patients with S-ICDs. Jarman et al. reported on their experience with patients implanted with an S-ICD in the United Kingdom [4]. Younger age was a strong predictor of TWOS. In this study, the mean age of patients with TWOS was 24 years, while that in patients without TWOS was 39 years. In addition, most of these patients had an inherited arrhythmia syndrome or HCM, which might explain the higher rate of TWOS in patients with higher LVEF, as also seen in our study. In a more recent study looking at predictors of inappropriate shocks in S-ICD patients enrolled in the EFFORTLESS registry [7], patients with HCM or a history of atrial fibrillation had higher rates of inappropriate shocks, predominantly due to TWOS (73%). In our study, none of the traditional ECG parameters (QT and PR intervals, QRS duration) predicted inappropriate shocks due to TWOS. However, using the ECG leads (I, II, and aVF) that correspond to the S-ICD sensing vectors (primary, secondary, and alternate, respectively), we were able to identify one baseline ECG finding which was associated with an increased risk of TWOS. In patients with TWOS, the mean QRS amplitude in lead I was roughly half the amplitude of that in patients without TWOS (3.7 vs. 7.4mV, p=0.02). It is conceivable that low QRS amplitude in lead I may result in a higher likelihood of TWOS due to an increased likelihood of detecting T waves when the QRS amplitude is low. The fact that most patients in this cohort were programmed with the primary S-ICD sensing vector, which most closely corresponds to ECG lead I, may explain why QRS amplitude in other leads (II and aVF) was not a significant predictor of TWOS in our study.