Dynamic cerebral autoregulation after bed rest: effects of volume loading and exercise countermeasures.

This study assessed effects of head-down-tilt (HDT) bed rest on dynamic cerebral autoregulation (CA) in 21 healthy young adults with volume loading and exercise countermeasures. Of these, seven underwent an 18-day bed rest without exercise countermeasures (sedentary group). Volume loading with dextran infusion was performed after bed rest to restore reduced plasma volume to levels before bed rest. In the other 14 subjects, supine cycling during bed rest was performed to preserve cardiac work from before bed rest (exercise group). Volume loading was also performed in a subgroup of these subjects (Ex+Dex, n = 7). Dynamic CA was estimated by transfer function analysis of changes in arterial pressure and cerebral blood flow (CBF) velocity in the very low (VLF, 0.02-0.07 Hz), low (LF, 0.07-0.20 Hz), and high frequency ranges (HF, 0.20-0.35 Hz). After bed rest, transfer function gain was reduced in the sedentary group (VLF, 0.93 ± 0.23 to 0.61 ± 0.23 cm(-1)·s(-1)·mmHg; P = 0.007) and in the exercise group (LF, 1.22 ± 0.43 to 0.94 ± 0.26 cm(-1)·s(-1)·mmHg; P = 0.005, HF, 1.32 ± 0.55 to 1.00 ± 0.32 cm(-1)·s(-1)·mmHg; P = 0.010). After volume loading, transfer function gain increased in the sedentary group but not in the Ex+Dex group. Taken together, these findings suggest that dynamic CA was preserved or improved after HDT bed rest in both sedentary and exercise subjects. Furthermore, increases of transfer function gain with volume loading suggest that changes in plasma volume may play an important role in CBF regulation.