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Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, and Space Medicine Laboratory, Department of Internal Medicine, The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235
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° 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 × 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.
plasma volume; baroreflex; spectral analysis; men; furosemide; microgravity
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