Abstract: A dual chamber pulse generator senses atrial and ventricular activity of a heart and provides pacing pulses to at least a ventricle of the heart. The pulse generator includes a first detector associated with an atrium of the heart for detecting atrial activations of the heart and a second detector associated with a ventricle of the heart for detecting ventricular activations of the heart. An output applies pacing pulses to the ventricle in timed relation to atrial activations of the heart. A timer times an atrial refractory period responsive to each ventricular activation of the heart. The timer normally times an atrial refractory period of a first duration and is responsive to an extension input for timing an atrial refractory period of an extended duration longer than the first duration. The extension input is periodically provided to the timer to cause the atrial refractory period of the extended duration to be periodically timed.