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  • As for the detection rate we should select a different


    As for the detection rate, we should select a different setting in patients with primary prevention and secondary prevention because ICD patients with primary prevention experience the faster lethal arrhythmia with an average rate of 200 beats per minute [27]. On the other hand, ICD recipients with secondary prevention indication had the appropriate ICD therapy with an average of 160 beats per minute [28]. In addition, antiarrhythmic drugs such as amiodarone are often used in these patients, which affect the VTCL and induce underdetections (Fig. 3). In fact, the risk of a slow VT above the tachycardia detection interval was about 3% (relative risk of 5%) per year during the first 4 years after ICD implantation [29]. HF prior to ICD implantation, spontaneous monomorphic VTs, and amiodarone use at discharge, significantly increased the risk of a VT above tachycardia detection interval. In general, cutoff intervals are often set at the appropriate values of documented or induced VT intervals with a safety margin of 30–60ms in ICD patients with secondary prevention indication; we should therefore take into account of the risk of a slow VT above the tachycardia detection interval, particularly in ICD patients taking amiodarone. At this point, we have insufficient data regarding the relative benefits of a high cutoff rate or a longer detection interval; however, the MADIT-RIT study is now ongoing to address this issue, and this may change the settings for detection in patients with primary prevention indication.
    Discriminators There are several algorithms for discrimination of supraventricular tachycardia (SVT) and this depends on each company\'s product. We usually program the discriminators up to a rate of 200 beats per minute based on its efficacy, particularly for the dual chamber system. Friedman et al. reported that more than 10% of inappropriate detection was associated with a GW788388 rate of over 200 beats per minute [30], and based on this report, the SVT limit should nominally be set at 230 beats per minute in the most current product made by Medtronic (ProtectaTM). However, there is no clear evidence regarding the safety of this high rate setting, although underdetection of VF using an SVT limit of 200 beats per minute has not been reported, and we should consider the risk of overdiscrimination when withdrawing therapies for lethal arrhythmia events. On the other hand, we must use SVT discriminators in the VT zone because overlapping rates between VT and SVT were reported in ICD patients [31] and the use of SVT discriminators reduced inappropriate shocks at 1 year by 20%, without underdetection [32]. In the VT zone (relatively lower heart rate), underdetection occurs in less than 1% of true VT episodes with single or dual chamber discriminators [33–35].
    Recent advances There are several algorithms developed by suppliers to reduce mainly inappropriate shocks. Medtronic is one of leading manufacturers to address how to reduce inappropriate shocks by developing new algorithms and to fund all major shock reduction trials. A new algorithm by Medtronic, “SmartShock™ Technology” includes 4 exclusive algorithms that discriminate true lethal arrhythmias from other arrhythmic and nonarrhythmic events. Adaptive PR Logic plus Wavelet is a famous algorithm that combines morphology and A-V pattern recognition to better discriminate against all types of SVT. Initially, the use of Adaptive PR Logic would occasionally misclassify SVT, such as in the case of sudden onset atrial tachycardia. The addition of Wavelet to the SVT discrimination logic will allow rhythms that are misclassified by Adaptive PR Logic to be reclassified by Wavelet so that a shock is appropriately withheld. A new algorithm for T-wave discrimination has been developed to identify T-wave oversensing and provide the opportunity to withhold therapy delivery without compromising VT/VF detection sensitivity by discrimination of R-waves and T-waves through differential filtering of the sensing signal (Fig. 4). Lead Integrity Suite is also important, which consists of RV Lead Noise Discrimination and RV Lead Integrity Alert [36]. The RV Lead Noise Discrimination algorithm analyzes a far-field electrocardiogram signal to differentiate RV lead noise from VT/VF. Confirmation+ identifies whether a tachycardia has been terminated with ATP or spontaneously during the charge and aborts the shock. Application of “SmartShock™ Technology” to the episode data from the SCD-HeFT study reduced inappropriate ICD shocks by an estimated 59%, which was confirmed by simulation [37]. Boston adopts further enhanced filtering and sensing systems with a unique auto gain control and noise window, which reduced T-wave oversensing in the real world. Moreover, we can choose between the discriminator algorithm from Rhythm ID and a classical onset/stability algorithm. Rhythm ID includes morphology defined as vector timing correlation. If patients show atrial tachycardia or atrial flutter, we should choose Rhythm ID because these arrhythmias are usually stable and have a sudden onset, which suggests difficulty in discriminating SVT from VT by using a classical onset/stability algorithm. St. Jude Medical has developed “ShockGuard™ Technology”, which includes Decision TX and a new sensing filter. Decision TX relies on new nominal settings for detection intervals, detection rate, and discriminators such as interval stability, AV association, and sudden onset, etc. The sensing filter was also changed to reduce undersensing of the R-wave and oversensing of the T-wave. “ShockGuard™ Technology” has shown a reduction of inappropriate ICD shock as well as unnecessary shocks, and the Food and Drug Administration in the USA has therefore approved this algorithm. Sorin group and Biotronik have also developed new algorithms to reduce inappropriate ICD shock as well as unnecessary shocks.