Dual chamber activity responsive pacer

Claim 1

A cardiac pacemaker for implantation in a patient, said pacemaker comprising: ventricular output means for generating ventricular pacing stimuli; atrial output means for generating atrial pacing stimuli; atrial sensing means for sensing natural heart activity in the atrium; first timer means responsively coupled to said atrial sensing means and said ventricular output means for determining a first A-V interval following sensing of natural heart activity in the atrium and for triggering generation of a ventricular pacing stimulus by said ventricular output means at the expiration of said first A-V interval; second timer means responsively coupled to said atrial and ventricular output means for determining a V-A interval following generation of a pacing stimulus by said ventricular output means and for triggering generation of a pacing stimulus by said atrial output means at the expiration of said V-A interval; sensor means responsive to the physical activity of the patient in which said pacemaker is implanted for regulating the duration of said V-A interval; third timer means for determining an atrial refractory period following generation of a ventricular stimulus by said ventricular output means and for preventing initiation of said first A-V interval by said first timer means in response to the sensing of natural heart activity in the atrium during said atrial refractory period; and means for delaying generation of an atrial pacing stimulus by said atrial output means beyond the expiration of said V-A interval in response to the sensing of natural heart activity in the atrium during said atrial refractory period.