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Vagal effects on heart rate: Different between up and down

The vagus nerves of rabbits under anesthesia were stimulated to test the effects on heart rate. Repeated stimuli were delivered either at a constant frequency (`tetanic') or in short bursts, one burst per heartbeat (`pulsatile stimulation') at variable moments in the cardiac cycle. The heart rate dynamics in response to short-lasting increases or decreases of ongoing stimulation were studied. Strong stimuli had a short-delay effect on heart rate: 100-120 ms; if stimulation was pulsatile the effects of decreased vagal stimulation could influence the ongoing cycle. However, in the case of tetanic stimulation the delay was much longer, up to 300 ms. It is concluded that observed changes in heart rate from one beat to the next are not always due to the same changes in underlying vagal activity. When using heart rate variability to probe the autonomic nervous system one should realize that the probe itself is highly non-linear.