Vagal blockade suppresses the phase I heart rate response but not the phase I cardiac output response at exercise onset in humans

PURPOSE: We tested the vagal withdrawal concept for heart rate (HR) and cardiac output (CO) kinetics upon moderate exercise onset, by analysing the effects of vagal blockade on cardiovascular kinetics in humans. We hypothesized that, under atropine, the φ(1) amplitude (A(1)) for HR would reduce to nil, whereas the A(1) for CO would still be positive, due to the sudden increase in stroke volume (SV) at exercise onset. METHODS: On nine young non-smoking men, during 0–80 W exercise transients of 5-min duration on the cycle ergometer, preceded by 5-min rest, we continuously recorded HR, CO, SV and oxygen uptake ([Formula: see text] O(2)) upright and supine, in control condition and after full vagal blockade with atropine. Kinetics were analysed with the double exponential model, wherein we computed the amplitudes (A) and time constants (τ) of phase 1 (φ(1)) and phase 2 (φ(2)). RESULTS: In atropine versus control, A(1) for HR was strongly reduced and fell to 0 bpm in seven out of nine subjects for HR was practically suppressed by atropine in them. The A(1) for CO was lower in atropine, but not reduced to nil. Thus, SV only determined A(1) for CO in atropine. A(2) did not differ between control and atropine. No effect on τ(1) and τ(2) was found. These patterns were independent of posture. CONCLUSION: The results are fully compatible with the tested hypothesis. They provide the first direct demonstration that vagal blockade, while suppressing HR φ(1), did not affect φ(1) of CO.