2 │Discussion:
ICD technology has progressed in recent years with improved battery longevity and additional programming capabilities leading to fewer inappropriate and unnecessary shocks. Inappropriate shock therapy has implications for patients’ quality of life. In addition, randomized trials have demonstrated that smart programming can also decrease mortality showing a link between unwanted ICD therapy and cardiovascular endpoints4.
To reduce inappropriate ICD therapy it is mandatory to discriminate between SVT and VT. In addition to single-chamber ICD discriminators sudden onset, stability of RR intervals and morphology from near-field or far field intracardiac signals, dual chamber ICDs have the possibility to analyse P wave and R wave relations. The presence of more R than P waves has a very high specificity for presence of VT. In patients with a 1:1 relation of P and R waves it is difficult for the ICD (and the cardiologist) to differentiate VT from SVT. Specificity of morphology discrimination is dependent on programmed threshold but also sampling frequency of the intracardiac signals. Timing intervals, however, can be very accurately measured by ICDs and pacemakers. Most VTs show different timing intervals between RV and LV than SVTs. LV sensing has gained interest in the recent years as CRT-Ds from several manufacturer’s including Biotronik (Biotronik SE & Co. KG, Berlin, Germany) can store IEGMs from all three channels during tachycardias demonstrating dissimilarities between RV and LV rhythms ins some cases5. In addition, LV sensing can complicate biventricular pacing because the device has to negotiate three different timing cycles that can lead to unwanted suppression of biventricular pacing6.
We propose that RV-LV timing could be introduced as a novel discriminator between SVT and VT in CRT devices. Pacemakers and ICDs can measure timing intervals very precisely and it is possible that specificity is higher than that of morphology that is currently. In addition, in current Biotronik CRT-Ds it is not possible to use morphology as an VT discriminator.