USE OF LOWER-BODY NEGATIVE-PRESSURE TO ASSESS CHANGES IN HEART-RATE RESPONSE TO ORTHOSTATIC LIKE STRESS DURING 17 WEEKS OF BED REST

This study examined the heart rate response to lower body negative pressure (LBNP) during 17 weeks of horizontal bed rest to estimate the development and duration of orthostatic instability elicited by this model for space flight. Based on data from Skylab, the authors hypothesized that orthostatic (LBNP) instability would appear during the first 3 to 4 weeks, and would then remain constant for the duration of bed rest. Heart rates of four healthy adult male subjects were monitored at rest and during LBNP for 1 week of ambulatory control, 17 weeks of horizontal bed rest, and 5 weeks of recovery. The LBNP protocol consisted of 10 minutes of control (atmospheric pressure) and 5 minutes each at 5, 10, 20, 30, 40, and 50 mm Hg decompression, followed by a 10-minute recovery period; this protocol was repeated weekly to document the progressive changes in heart rate response to LBNP. Lower body negative pressure was terminated early if symptoms compatible with the onset of syncope occurred. Throughout the study, heart rate was unchanged at 5, 10, and 20 mm Hg, but it increased at 30, 40, and 50 mm Hg LBNP. During the pre-bed rest period, peak heart rate was 97 +/- 10 beats/min (mean +/- SE), occurring at 50 mm Hg for all four subjects. After 3 days of bed rest, all monitored heart rate responses, including values after release of LBNP, were only slightly elevated (NS) above pre-bed rest level. Peak heart rate was 118 +/- 21 beats/min at 50 mm Hg decompression (NS; N = 3). After 3 weeks of bed rest, peak heart rare was 110 +/- 16 beats/min at 50 mm Hg decompression (NS; N = 4). The slight increase (NS) in heart rate persisted throughout the 17 weeks of bed rest. In three subjects, the duration of tolerance to LBNP decreased; in two subjects, decompression was ended before completion of the full 5-minute period at 50 mm Hg; and in one subject, decompression was ended before completion of the full 5-minute period at 40 mm Hg. This indicates a decrease in orthostatic tolerance in three subjects. The heart rate responses after 1 day of reambulation appeared no different than those for weeks 1 through 17 of bed rest. After 3 days of reambulation, the heart rate response during LBNP was similar to that recorded before bed rest (NS). After 32 days of reambulation, the heart rate was slightly lower (NS) than before bed rest at all levels of LBNP and recovery. The data are similar to those obtained during Skylab, including the increase in heart rate response after 1 week of space flight, which was maintained throughout the period of weightlessness. The data indicate that the heart rate changes associated with orthostatic instability develop early (after as few as 3 days of bed rest) and persist throughout the entire 17 weeks of horizontal bed rest.