Key result
A 21-day -6° head-down bed rest significantly reduced orthostatic tolerance time to pre-syncope from 21.5 minutes to 12.5 minutes in healthy men.
Why the study?
Orthostatic intolerance commonly occurs after prolonged bed rest, and whether prolonged bed rest deconditioning alters overall baroreceptor functioning to reduce orthostatic tolerance in healthy individuals was evaluated.
Does 21 days of head-down bed rest reduce orthostatic tolerance and alter baroreceptor functioning in healthy men?
Does 21 days of head-down bed rest reduce orthostatic tolerance and alter baroreceptor functioning in healthy men?
Prolonged bed rest significantly impairs orthostatic tolerance and depresses sympathetic baroreflex control of vascular resistance in healthy individuals.
Prolonged bed rest may impair orthostatic tolerance; leaves open baroreflex mechanisms and countermeasures in clinical deconditioning.
Orthostatic intolerance commonly occurs after prolonged bed rest, thus, increasing the risk of syncope and falls. Baroreflex-mediated adjustments of heart rate and sympathetic vasomotor activity (muscle sympathetic nerve activity, MSNA) are crucial for orthostatic tolerance. We hypothesized that prolonged bed rest deconditioning alters overall baroreceptor functioning thereby reducing orthostatic tolerance in healthy volunteers. As part of the European Space Agency Medium-term Bed Rest protocol, 10 volunteers were studied before and after 21-days of -6º head down bed rest (HDBR). In both conditions, subjects underwent ECG, beat-by-beat blood pressure, respiratory activity and MSNA recordings while supine (REST) and during a 15-minute 80 head-up tilt (TILT) followed by a 3-minute –10 mmHg stepwise increase of lower body negative pressure to pre-syncope. Cardiac baroreflex sensitivity (cBRS) was obtained in the time (sequence method) and frequency domain (spectrum and cross-spectrum analyses of RR interval and systolic arterial pressure, SAP, variability). Baroreceptor modulation of sympathetic discharge activity to the vessels (sBRS) was estimated by the slope of the regression line between percentage of MSNA burst occurrence and diastolic pressure. Orthostatic tolerance significantly decreased after HDBR (12±0.6min) compared to before (21±0.6min). While supine, heart rate, SAP and cBRS were unchanged before and after HDBR, sBRS gain was slightly depressed after than before HDBR (sBRS: -6.0±1.1 versus -2.9±1.5 burst%*mmHg-1 respectively). During TILT, HR was higher after than before HDBR (116±4 b/min versus 100±4 b/min respectively), SAP was unmodified in both conditions, cBRS indexes were lower after HDBR (α index: 3.4±0.7 ms/mmHg; BRSSEQ 4.1±3.3) compared to before (α index: 6.4±1.0 ms/mmHg; BRSSEQ 6.8±3.7). sBRS gain was significantly more depressed after HDBR than before (sBRS: -4.4±0.4 versus -2.2±0.6 burst%*mmHg-1 respectively). Our findings suggest that baroreflex-mediated adjustments in heart rate and MSNA are impaired after prolonged bed rest. The mechanism likely contributes to the decrease in orthostatic tolerance.
No takes yet. Share an insight, caveat, or question.
Barbic et al. (2019) studied Healthy volunteers (n=10). Head-down bed rest (HDBR) vs. Baseline (before HDBR) was evaluated on Time to pre-syncope during orthostatic testing (minutes) (p=<0.05). A 21-day -6° head-down bed rest significantly reduced orthostatic tolerance time to pre-syncope from 21.5 minutes to 12.5 minutes in healthy men.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: