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.