Inferring vagal effects on the heart from changes in cardiac cycle length: implications for cycle time-dependency.

Since the now classical experiments by Brown and Eccles in 1934, the effect of a stimulation of the vagus nerve on the heart has been derived from changes in the length of cardiac periods. Based on a simple model for the genesis of heartbeats, it is shown that the procedure employed by Brown and Eccles gives a distorted picture of the actual vagal effect. A corrected procedure for inferring the vagal effect from changes in heart period length is proposed. This new procedure is applied to empirical data from animal experiments with direct stimulation of the vagus nerve. It is shown that, if the vagal effect depends on time of stimulation within the cardiac cycle (cycle time-dependency), single vagal effect curves for each time of stimulation within the cycle have to be constructed. Other data reduction procedures are reviewed with respect to their appropriateness for demonstrating cycle time-dependency.