An implantable medical device senses atrial depolarizations having associated refractory periods thereafter and delivers electrical stimulation pulses during the associated refractory periods to stimulate the atrioventricular (AV) node to cause a prolonged conduction time of an atrial depolarization to a ventricular chamber. The device delivers a pacing pulse to a first ventricular chamber during the prolonged conduction time to cause a contraction and associated refractory period of the first ventricular chamber, the first ventricular chamber contraction occurring earlier than a contraction of a second ventricular chamber.