Interaction of cardiac and muscle mechanical afferents on baroreflex control of the sinus node during dynamic exercise

The effects of cardiopulmonary baroreceptors and muscle mechanoreceptors stimulation on cardiac baroreflex sensitivity (BRS), and heart rate variability (HRV) were evaluated by measuring continuously and non-invasively systolic blood pressure (SBP) and pulse interval (PI) during upright and supine passive cycling. BRS and HRV were evaluated with the cross-correlation method (xBRS) and in the frequency domain, respectively. At rest, the shift from upright to supine posture enhanced xBRS from 16.4+/-12.1 to 23.4+/-12.9 ms/mmHg, and the high frequency (HF, 0.15-0.4 Hz) power of HRV from 48.9+/-18.6 to 55.1+/-14.7 normalized units (NU), while it attenuated the low-frequency (LF, 0.04-0.15 Hz) power from 51.1+/-18.6 to 44.9+/-14.7 NU (P<0.05), respectively. During both upright and supine passive exercise, xBRS and the HF power were attenuated (10.0+/-8.0 and 12.5+/-9.0 ms/mmHg; 41.1+/-21.2 and 41.5+/-12.7 NU, respectively; P<0.05) and the LF power increased (58.8+/-21.2 and 58.5+/-12.7 NU, P<0.05), compared with rest. The effect of mechanoreflex activation overrides that of the cardiopulmonary baroreceptors loading resulting in decreased cardiac vagal outflow and reduced BRS during supine passive exercise.