Aortic-cardiac reflex during dynamic exercise

We investigated the aortic-cardiac reflex during low-intensity cycling in 10 healthy volunteers. Baroreflex function was assessed by the ratio of change in heart rate to mean arterial pressure (ΔHR/ΔMAP) during phenylephrine (PE) infusion. The ratio obtained during PE combined with low- level lower body negative pressure (LBNP) and calculated neck pressure (NP) was assessed as the gain of the aortic-cardiac reflex. Exercise (~25% maximal O₂ uptake or 25 ± 2 W) significantly increased HR from 64 ± 2 to 98 ± 2 beats/min, MAP from 90 ± 3 to 98 ± 3 mmHg, cardiac output from 6.6 ± 0.5 to 12.0 ± 1.4 l/min, and O₂ uptake from 3.8 ± 0.2 to 10.4 ± 0.6 ml · min^-1 · kg^-1. However, ΔMAP (+11.8 ± 0.4 vs. + 11.3 ± 0.8 mmHg), ΔHR (-12.7 ± 2 vs. -12.9 ± 2 beats/min), and ΔHR/ΔMAP (1.10 ± 0.19 vs. 1.15 ± 0.15 beats · min^-1 · mmHg^-1) were not statistically different between rest and exercise during PE. Although PE significantly increased central venous pressure in both supine rest (from 6.7 ± 0.7 to 10.4 ± 0.7 mmHg) and exercise (5.8 ± 0.8 to 8.6 ± 0.9 mmHg) conditions, when LBNP (- 15 ± 2 vs. -16 ± 1 Torr for rest vs. exercise) was applied, both rest and exercise central venous pressures were returned to the preinfusion baseline values, respectively. During PE + LBNP + NP (NP = 15.5 ± 1 vs. 15.1 ± 1 Torr for rest vs. exercise) ΔHR and ΔMAP were not different between rest and exercise (-10.2 ± 2 and -10.5 ± 2 beats/min and 14 ± 1 and 12 ± 1 mmHg, respectively). Therefore, the calculated aortic-cardiac reflex gain was similar during rest (0.74 ± 0.13 beats · min^-1 · mmHg^-1) and exercise (0.86 ± 0.12 beats · min^-1 · mmHg^-1). These data indicate that the aortic-cardiac reflex responsiveness was maintained during low-intensity exercise.