Rate-Responsive Cardiac Pacemakers

Pacemaker technology continues to advance in the direction of restoring a normal hemodynamic response under varying physiologic conditions. Rate-responsive pacemakers meet this challenge by adjusting the pacing rate in response to a sensed physiologic variable other than sinus node activity. In an effort to design the ideal rate-responsive system, various physiologic cues have been tested. To translate shifts in the sensed physiologic indicator into an appropriate pacing rate, specialized sensor systems also have been developed and include mechanical, chemical, thermal, and electrical sensors. Although each sensor system offers advantages and disadvantages, continued research and clinical experience will determine the future of this exciting new form of cardiac pacing

THE BEHAVIOR OF ATRIO–VENTRICULAR NODAL RHYTHM FOLLOWING DIRECT PERFUSION OF THE SINUS NODE

Direct perfusion of the canine sinus node with various pharmacological agents having negative chronotropic effects commonly leads either to abrupt sinus arrest or to a gradual transition from sinus to atrio–ventricular (A–V) nodal rhythm with progressive shortening of the P–R interval. The reappearance of sinus rhythm is usually preceded by a change in A–V nodal rate and a progressive lengthening of the P–R interval to a stable value. During A–V nodal rhythm, changes in heart rate are observed following injections into the sinus node artery. As perfusion of the sinus node is selective, these cannot be attributed to a direct pharmacological effect of the perfusates on the A–V node. Deliberate suppression of A–V nodal pacemaking dominance reveals the persistence of slow sinus node activity which is unapparent electrocardiographically during A–V nodal rhythm. It would seem that even in the absence of P waves, the sinus node may still influence the rate of the A–V node. These observations are consistent with the hypothesis that the sinus and A–V nodes behave as a system of coupled relaxation oscillators.

SIMULATION OF SINUS NODE ACTIVITY BY AN ELECTRONIC RELAXATION OSCILLATOR

After the sinus node was destroyed, its rhythmic activity was simulated by an electronic relaxation oscillator coupled to the beating heart. The output of the oscillator was used to stimulate the right atrium, and the ventricular response was returned to the input of the relaxation oscillator. By manually varying the frequency of this artificial pacemaker it was possible to produce changes in the rate of the atrio–ventricular (A–V) node similar to those obtained by perfusion of the intact sinus node with chronotropic agents. Particular attention was paid to the transitions from "oscillator" rhythm to A–V nodal rhythm, and vice versa. The results provide support for the following hypotheses relative to the intact heart: (i) some form of sinus node activity persists during A–V nodal rhythm, and (ii) the principal pacemakers of the heart, the sinus and A–V nodes, behave as a system of coupled relaxation oscillators.