Beat-by-beat modulation of AV conduction. I. Heart rate and respiratory influences.

We examined the integration of heart rate and neural influences at the atrioventricular (AV) node in conscious dogs. Animals were anesthetized and, under sterile conditions, instrumented to chronically record atrial and ventricular electrograms and blood pressure. In the conscious state, electrocardiogram (ECG), respiration, blood pressure, and electrograms were recorded on a beat-by-beat basis, and heart rate and AV interval were plotted graphically as a function of time. Resting animals exhibited both respiratory sinus arrhythmia and marked oscillations in AV conduction time associated with respiration. During inspiration AV interval was shortened, and during expiration AV interval was prolonged. To obviate the effect of cyclic changes in heart rate, atrial pacing was used to increase heart rate over a wide range both abruptly and linearly. Regardless of the pattern of heart rate change, AV interval oscillated at the respiratory frequency at pacing rates 10-100 beats/min above control. Higher levels of atrial pacing resulted in AV conduction patterns that were correlated with changes in blood pressure. Thus in the conscious dog variations in AV conduction time occur on a beat-by-beat basis in conjunction with respiration; oscillatory activity of AV conduction is not dependent on simultaneous changes in heart rate; and during atrial pacing, autonomic neural activity associated with respiration and blood pressure appears to dynamically modulate AV conduction with respiratory effects predominating at low heart rates and blood pressure effects at high heart rates.