Variability of ventricular excitation interval does not reflect fluctuation in atrial excitation interval during...

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