Abstract
We have recently reported that the atrioventricular (AV) nodal mechanism functions to cancel fluctuation in atrial excitation interval during stair-stepping exercise. However it remained unknown at which level of heart rate (HR) the AV nodal mechanism started to operate and whether fluctuation in atrial excitation interval might influence AV conduction over following beats. To solve these questions, variability of PP, RR, and PR intervals and their interrelationships were analyzed throughout ergometer exercise in eight subjects. The variability of RR interval much more decreased to 0.7% of the control at 160 beats/min during exercise than PP interval variability decreased to 10%, despite the same shortened average interval. In contrast, PR interval variability tended to increase by 87% during exercise, while mean PR interval decreased. A strong inverse relationship between PP and the subsequent change in PR [ΔPR] intervals became evident during exercise, implying that ΔPR interval canceled fluctuation in PP interval. However, there was little correlation between RR and ΔPR intervals and between PP and the next ΔPR intervals in the forthcoming beat. When the slope of the PP–ΔPR relationship considered as sensitivity of the AV nodal function opposing to an alteration in PP interval was plotted against PP interval, the AV nodal function curve was approximated to a sigmoidal curve having the threshold of PP interval near 650 ms and the maximum plateau level of the slope near 1.0. We conclude that when HR exceeds 90–100 beats/min during dynamic exercise, the AV nodal mechanism will function to cancel fluctuation in PP interval within one beat and keep RR interval constant.
