| Abstract: | ![]()
Summary. The effects of 28 days continuous 6° head-down tilt bed-rest on heart rate variability and the slope of the spontaneous arterial baroreflex were evaluated during supine rest and the first 10 min of 60° head-up tilt. Twelve healthy men were assigned to either a no counter-measure (No-CM), or a counter-measure (CM) group so that there was no difference in maximal oxygen uptake. Counter-measures consisted of short-term, high resistance exercise for 6 days per week from days 7–28, and lower body negative pressure (-28 mmHg) for 15 min on days 16, 18, 20 and 22–28. In spite of balanced between-group fitness, mean RR-interval was different between the No-CM and the CM group prior to bed-rest, but neither this nor any other variables showed significant counter-measure by bed-rest interaction effects. Therefore, all data presented are from the main effects of bed-rest or tilt from the analysis of variance. RR-interval was reduced significantly by bed-rest and by tilt (P<0–0001). Indicators from spectral analysis of heart-rate variability suggested reduced parasympathetic nervous system activity with bed-rest (P<0–01) and head-up tilt (P<0–05), and increased sympathetic nervous system activity after bed-rest (P<0–01). An indicator of complexity of cardiovascular control mechanisms, taken from the slope (β) of log spectral power vs. log frequency relationship, suggested reduced complexity with bed-rest (P<0–05) and head-up tilt (P<0–01). The spontaneous baroreflex slope was reduced significantly by bed-rest (P<0–03) and by head-up tilt (P<0–04). Taken together, these data support the concept of altered autonomic nervous system function in the aetiology of cardiovascular deconditioning with bed-rest or space travel; and it would appear that no benefit is derived from these specific counter-measures. |
