A medical device system and method for monitoring a cardiac signal in a patient senses ventricular R-waves and computes a morphology metric of an R-wave and a corresponding preceding morphology metric of a preceding R-wave. One of a difference and a ratio of the R-wave morphology metric and the preceding R-wave morphology metric is compared to an established detection threshold for discriminating premature ventricular contractions from premature atrial contractions. A cardiac signal measurement is computed from the sensed R-waves in response to detecting a premature ventricular comparison based on the comparison.