EFFECT OF 28-DAY HEAD-DOWN BED REST WITH COUNTERMEASURES ON HEART-RATE-VARIABILITY DURING LBNP

The effect of exercise and LBNP countermeasures on the cardiovascular deconditioning response have been evaluated by the study of heart rate variability during progressive LBNP tests before, during (day 15), an d after 28 d continuous 6 degrees head-down tilt bed rest. Twelve heal thy men (age 27-42 years) were studied in two groups. Six were assigne d to a countermeasure regime (CM) consisting of strenuous short-term r esistance exercise once per day, 6 days per week from day 7 to day 28, combined with lower body negative pressure (LBNP, -28 mm Hg) for 15 m in on days 16, 18, 20, and 22-28. Results were compared with data from six subjects, matched on the basis of maximum oxygen uptake, who rece ived no countermeasures (No-CM). The main effects of bed rest were see n in reductions in RR-interval, and in total spectral power. The resul ts of spectral analysis showed a significant reduction in an index of parasympathetic activity with a nonsignificant increase in a sympathet ic indicator. The fractal component of heart rate variability was redu ced also, suggesting a change in cardiovascular control mechanisms. Al though there were significant differences between CM and No-CM for a n umber of variables, there were no bed rest by countermeasure interacti on effects in the statistical analysis. These results suggested that t his particular series of countermeasures was ineffective in preventing at least certain changes indicative of cardiovascular deconditioning with 28 d head-down tilt bed rest.