Abstract des Autors

Effects of 28 day of continuous 6ø head-down tilt bed rest on spontaneous vagally mediated baroreflex response slope were evaluated from beat-by-beat relationships between R-R interval and systolic arterial blood pressure. Twelve healthy men (age 27-42 yr) were assigned to either countermeasure (CM) or no-countermeasure (no-CM) groups. CM consisted of strenuous short-term exercise once per day 6 days/wk from days 7 to 28 and lower body negative pressure (LBNP) for 15 min on days 16, 18, 20, and 22-28. Spontaneous baroreflex slope was evaluated by application of linear regression to sequences of at least three beats in which systolic blood pressure and R-R interval changed in the same direction. Measurements were made pre, mid- (day 15), and post-bed rest at rest and during progressive LBNP tests (3 min at each of -20, -30, -40, and -50 mmHg). R-R interval decreased progressively and significantly (P<0.0001) over duration of bed rest. Spontaneous baroreflex slope at rest in pre-bed rest was 18.5+/-2.1 ms/mm Hg for CM and 14.9+/-1.6 ms/mmHg for no-CM. There was a significant reduction in baroreflex slope as a function of bed rest, and it was further reduced during LBNP (P<0.0001). Between CM and no-CM groups differences existed, but these were present pre-bed rest and appeared unaffected by countermeasures. A linear relationship existed between baroreflex slope and R-R interval (r=0.61) such that at -50 mmHg of LBNP in post-bed-rest tests baroreflex slopes were 5.4+/-0.8 and 5.2+/-1.0 ms/mmHg for CM and no-CM, respectively. Because we saw a linear reduction in spontaneous baroreflex slope as a function of the reduction in R-R interval, we hypothesize that spontaneous baroreflex slope was in part a function of background vagal activity to the heart. Verf.-Referat