Physical fitness and cardiopulmonary baroreflex to passive head-up tilt

The relationship between physical fitness and cardiopulmonary (CP) baroreflex induced by the decrease in venous return was investigated. Maximal oxygen uptake/lean body mass (VO2max/LBM) was used as a measure of physical fitness. CP baroreflex was examined in 8 subjects [22.0 +/- 2.0 (SD) yr] ranging from low (47.3 ml.kg-1.min-1) to highly (72.6 ml.kg-1.min-1) fit subjects who were exposed to the orthostatic load using passive head-up tilt. Tilting from 0 degrees to 70 degrees in 10 degrees increments was applied. Each tilting was preceded by measurements at the basal position of 0 degrees. Heart rate, systolic and diastolic pressure, stroke volume, and forearm blood flow were measured before and during tilting; Cardiac output, mean arterial pressure, and forearm vascular resistance (FVR) were calculated. FVR was increased with the raising in tilt angle in each subject, but the onset of increase in FVR tended to be earlier in proportion to the higher level of physical fitness. There is a negative and significant correlation between the onset of increase in FVR and VO2max/LBM. The physically fit subject exhibited a more rapid onset of an increase in FVR to orthostatic stress than the unfit subject. Thus the higher the physical fitness, the lower the threshold in CP baroreflex response.