Effect Of A Single Bout Of Exhaustive Exercise On Baroreflex Function After 16Days Of Head-Down Tilt

摘要

We tested the hypothesis that one bout of maximal exercise performed 24 h priorto re-ambulation from 16 days of 6 degrees head-down tilt (HDT) could increase integrated baroreflex sensitivity. Isolated carotid-cardiac and integrated baroreflex function was assessed in seven subjects before and after two periods of HDT separated by 11 months. On the last day of one HDT period, subjects performed a single bout of maximal cycle ergometry (exercise). Subjects did not exercise after the other HDT period (control). Carotid-cardiac baroreflex sensitivity was evaluated using a neck collar device. Integrated baroreflex function was assessed by recording heart rate (HR) and blood pressure (MAP) during a 15-sec Valsalva maneuver (VM) at a controlled expiratory pressure of 30 mmHg. The ratio of change in HRto change in MAP (delta HR/delta MAP) during phases II and IV of the VM was used as an index of cardiac baroreflex sensitivity. Baroreflex-mediated vasoconstruction was assessed by measuring the late phase II rise in MAP. Following HDT, carotid-cardiac baroreflex sensitivity was reduced (2.8 to 2.0 msec/mmhg; P = 0.05) as was HR/ MAP during phase II (-1.5 to -0.8 beats/mmHg; P = 0.002). After exercise, isolated carotid baroreflex activity and phase I IAHR/AMAP returned to pre-HDT levels but remained attenuated in the control condition. Phase IVdeltaHR/deltaMAP was not altered by HDT or exercise. The late phase II increase of MAP was 71% greater following exercise compared to control (7 mmHg vs. 2 mmHg; P=0.04l). Heightened baroreflex function was associated with maintenance of orthostatic tolerance following HDT in the exercise condition.