Reduced baroreflex control of heart period after bed rest is normalized by acute plasma volume restoration

1 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas 75231; and 2 University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235; and 3 Department of Hygiene and Space Medicine, Nihon University School of Medicine, Tokyo 173-8610, Japan Submitted 4 October 2002 ; accepted in final form 1 June 2004 Adaptation to spaceflight or head-down-tilt bed rest leads to hypovolemia and an apparent abnormality of baroreflex regulation of cardiac period. In a previous study, we demonstrated that both chronic (2 wk) head-down-tilt bed rest and acute induced hypovolemia led to similar impairments in spontaneous baroreflex control of cardiac period, suggesting that a reduction in plasma volume may be responsible for this abnormality after bed rest. Therefore we hypothesized that this reduced "baroreflex function" could be restored by intravenous volume infusion equivalent to the reduction in plasma volume after bed rest. Six healthy subjects underwent 2 wk of –6° head-down bed rest. Beat-by-beat arterial blood pressure and ECG were recorded during 6 min of spontaneous respiration and fixed-rate breathing (0.2 Hz), and transfer function analysis between systolic blood pressure and R-R interval was performed. Plasma volume was measured with Evans blue dye, and cardiac filling pressures were directly measured (Swan-Ganz catheter). After bed rest, studies were repeated before and after plasma volume restoration, with which both plasma volume and left ventricular end-diastolic pressure were restored to pre-bed rest levels by intravenous dextran 40 infusion (288 ± 31 ml). Transfer function gain in the high-frequency range, used as an index of vagally mediated arterial-cardiac baroreflex function, decreased significantly (13.4 ± 3.1 to 8.1 ± 2.9 ms/mmHg, P < 0.05) after bed rest. However, reduced transfer function gain was normalized to the pre-bed rest level (12.2 ± 3.6 ms/mmHg) after precise plasma volume restoration. This result confirms that reductions in plasma volume, rather than a unique autonomic nervous system adaptation to bed rest, are largely responsible for the observed changes in spontaneous arterial-cardiac baroreflex function after bed rest. spectral analysis; hypovolemia; microgravity; dextran infusion Address for reprint requests and other correspondence: B. D. Levine, Institute for Exercise and Environmental Medicine, 7232 Greenville Ave, Dallas, TX 75231 (E-mail: benjaminlevine{at}texashealth.org )