Dual chamber pacemaker

A dual chamber cardiac pacemaker, having means for sensing atrial heartbeats and means for delivering ventricular stimulus pulses, comprising

• monitoring means for monitoring the time relationships between said ventricular stimulus pulses and the respective succeeding atrial heartbeats of the patient, means for storing data reflective of a plurality of said monitored time relationships, and means for determining the occurrence of pacemaker mediated tachycardia in accordance with said data, and
• means for altering pacer operation in response to the determining of a said occurrence.

A cardiac pacemaker having cyclical means for cyclically sensing P signals representative of a patient's atrial heartbeats and generating stimulus pulses for delivery to a patient's ventricle, further comprising

• means for measuring the stability of the V-A intervalbetween a delivered ventricular stimulus pulse and the next succeeding atrial heartbeat and for determining the existence of tachycardia as a predetermined criteria of said measured V-A internal stability, said means for measuring including means for measuring the occurrence of a plurality of P signals each occurring within a predetermined time window following a respective prior ventricular stimulus pulse, and
• means for changing the operation of said cyclical means over at least a portion of a cycle following determination of the existence of tachycardia.

A dual chamber cardiac pacemaker for pacing a patient in at least an A-V synchronous mode, having means for sensing P signals, A-V timeout means for timing out an A-V delay following a sensed P signal, said A-V delay normally being a predetermined time T.sub.AV, and means for generating a ventricular stimulus pulse for delivery to the patient at the time out of a said A-V delay,

• further comprising means for timing out an atrial refractory period following each sensed P signal, which atrial refractory period corresponds to a predetermined maximum heartbeat rate;
• and
• control means for controlling said A-V timeout means (i) to delay initiation of timing out said A-V delay to the end of said atrial refractory period whenever a P signal is sensed before the end of said atrial refractory period, and (ii) to reduce the length of said A-V delay to a time less than T.sub.AV following each said delay of initiation.