An implantable medical device includes a mechanical activity sensor configured to sense movements produced by contractions of a ventricular cavity and output a mechanical activity signal representative of the contractions. The implantable medical device also includes one or more circuits configured to detect a plurality of spontaneous ventricular depolarizations based on electrical potentials representative of the spontaneous ventricular depolarizations, calculate an escape interval, and provide an antibradycardia ventricular pacing therapy in an absence of a detected spontaneous ventricular event after the escape interval. The circuits calculate the escape interval by calculating a first escape interval based on successive detected spontaneous ventricular depolarizations and a second escape interval based on the mechanical activity signal from the mechanical activity sensor, and selecting one of the first escape interval and the second escape interval, wherein the second escape interval is selected in response to a delivery of an antitachycardia atrial pacing therapy.