This document describes systems, devices, and methods that use multiple
morphology templates for discriminating between rhythms, such as
supraventricular tachyarrhythmias (SVTs) and ventricular tachyarrhythmias
(VTs), for delivering a countershock in response to a VT episode, but
withholding delivery of such a countershock in response to an SVT
episode. In certain examples, the particular morphology used for storing
morphological features is selected at least in part using a
sensor-indicated activity level of a subject, or a metabolic need of the
subject.