Effect of head-down-tilt bed rest and hypovolemia on dynamic regulation ofheart rate and blood pressure

Adaptation to head-down-tilt bed rest leads to an apparent abnormality of b aroreflex regulation of cardiac period. We hypothesized that this "decondit ioning response" could primarily be a result of hypovolemia, rather than a unique adaptation of the autonomic nervous system to bed rest. To test this hypothesis, nine healthy subjects underwent 2 wk of -6 degrees head-down b ed rest. One year later, five of these same subjects underwent acute hypovo lemia with furosemide to produce the same reductions in plasma volume obser ved after bed rest. We took advantage of power spectral and transfer functi on analysis to examine the dynamic relationship between blood pressure (BP) and R-R interval. We found that 1) there were no significant differences b etween these two interventions with respect to changes in numerous cardiova scular indices, including cardiac filling pressures, arterial pressure, car diac output, or stroke volume; 2) normalized high-frequency (0.15-0.25 Hz) power of R-R interval variability decreased significantly after both condit ions, consistent with similar degrees of vagal withdrawal; 3) transfer func tion gain (BP to R-R interval), used as an index of arterial-cardiac barore flex sensitivity, decreased significantly to a similar extent after both co nditions in the high-frequency range; the gain also decreased similarly whe n expressed as BP to heart rate 3 stroke volume, which provides an index of the ability of the baroreflex to alter BP by modifying systemic flow; and 4) however, the low-frequency (0.05-0.15 Hz) power of systolic BP variabili ty decreased after bed rest (-22%) compared with an increase (+155%) after acute hypovolemia, suggesting a differential response for the regulation of vascular resistance (interaction, P < 0.05). The similarity of changes in the reflex control of the circulation under both conditions is consistent w ith the hypothesis that reductions in plasma volume may be largely responsi ble for the observed changes in cardiac baroreflex control after bed rest. However, changes in vasomotor function associated with these two conditions may be different and may suggest a cardiovascular remodeling after bed res t.