An implantable medical device system detects and treats monomorphic ventricular tachycardia (VT). The system includes a sensing module for receiving a cardiac signal and a therapy delivery module including a capacitor and an output circuit. A control module is coupled to the sensing module and the therapy delivery module and is configured to detect a tachycardia from the cardiac signal and initiate charging of the capacitor in response to detecting the tachycardia. The control module performs a method for simultaneously monitoring a voltage on the capacitor and a morphology of the cardiac signal during the charging and controls the therapy delivery circuit to deliver a shock pulse at less than a programmed shock energy in response to both the voltage reaching at least a minimum voltage and the morphology being monomorphic.