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

Abstract

Adaptation to head-down-tilt bed rest leads to an apparent abnormality of baroreflex regulation of cardiac period. We hypothesized that this "deconditioning 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 bed rest. One year later, five of these same subjects underwent acute hypovolemia with furosemide to produce the same reductions in plasma volume observed after bed rest. We took advantage of power spectral and transfer function analysis to examine the dynamic relationship between blood pressure (BP) and R-R interval. We found that 1) there were no significant differences between these two interventions with respect to changes in numerous cardiovascular indices, including cardiac filling pressures, arterial pressure, cardiac output, or stroke volume; 2) normalized high-frequency (0.15-0.25 Hz) power of R-R interval variability decreased significantly after both conditions, consistent with similar degrees of vagal withdrawal; 3) transfer function gain (BP to R-R interval), used as an index of arterial-cardiac baroreflex sensitivity, decreased significantly to a similar extent after both conditions in the high-frequency range; the gain also decreased similarly when expressed as BP to heart rate x 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 variability 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 with the hypothesis that reductions in plasma volume may be largely responsible 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 rest.

Hypovolemia and neurovascular control during orthostatic stress

“…However, despite smaller hematocrit changes in these latter studies, PV reductions measured by direct dilution methods (15% and 17% respectively) were similar compared with the 15.5% reduction in PV induced in the current investigation. Moreover, the reduction in resting supine CVP (1.4 Ϯ 0.2 mmHg) in the present study was within the range reported after head-down tilt bed rest (0.8-2.1 mmHg) (11,23) and during acute hypovolemia (29,51). The IVC diameters (Fig.…”

“…The hypovolemia that occurs during microgravity exposure is well documented, with a range of 6-16% (3,10,12,13,25). Similar levels of hypovolemia have also been reported after headdown tilt bed rest (13,14,23,29) and acute diuretic administration of furosemide (21,29,32,51). PV changes in the current study were calculated based on changes in hematocrit measurements.…”

Arterial baroreflex function and cardiovascular variability: interactions and implications

“…One of the key adaptations to bed rest is a reduction of plasma volume, which may itself alter baroreflex sensitivity (43). The arterial cardiac baroreflex appears to be similarly impaired after head-down bed rest (chronic hypovolemia plus deconditioning) and after acute hypovolemia, suggesting that the reduced cardiac baroreflex gain may be due primarily to a reduction of plasma volume.…”

Vasoconstrictor Reserve and Sympathetic Neural Control of Orthostasis

“…This dosage was chosen because it induced a reduction in plasma volume of 7% to 14% after administration for Ϸ2 hours, equivalent to the loss of plasma volume observed after 2 weeks of head-down bed rest. 17,22 An oral potassium supplement of 20 mEq was given before the injection of furosemide. After injection, urine was collected and cuff BP was measured every 15 minutes.…”