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 (Delta HR/Delta MAP) 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 (similar to 25% maximal O-2 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 O-2 uptake from 3.8 +/- 0.2 to 10.4 +/- 0.6 ml . min(-1). kg(-1). However, Delta MAP (+11.8 +/- 0.4 vs. + 11.3 +/- 0.8 mmHg), Delta HR (-12.7 +/- 2 vs. -12.9 +/- 2 beats/min), and Delta HR/Delta MAP (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) Delta HR and Delta MAP 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.