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Reduced orthostatic tolerance following 4 h head-down tilt
G C Butler1, H C Xing, D R Northey
1Department of Kinesiology, University of Waterloo, Ontario, Canada.
Summary
Four hours of head-down tilt (HDT) impaired orthostatic tolerance in healthy men, causing presyncopal symptoms. This suggests rapid cardiovascular adaptation to HDT may be maladaptive when returning to upright posture.
Area of Science:
- Cardiovascular Physiology
- Human Physiology
- Orthostatic Tolerance
Background:
- Head-down tilt (HDT) and head-up tilt (HUT) alter cardiovascular function.
- Understanding adaptations to altered gravity is crucial for spaceflight and clinical settings.
Purpose of the Study:
- To investigate the cardiovascular responses to orthostatic tolerance testing after supine, HDT, and HUT.
- To determine if 4-hour HDT or HUT affects orthostatic tolerance.
Main Methods:
- Eight healthy men underwent a 10-minute head-up tilt orthostatic tolerance test (OST).
- Tests were performed after a supine baseline, 4 hours of HDT, or 4 hours of HUT.
- Cardiovascular parameters including heart rate, stroke volume, and mean arterial pressure were monitored.
Main Results:
- Subjects experienced presyncopal symptoms after HDT but not HUT.
- Stroke volume and cardiac output were higher in the control group compared to HDT and HUT.
- Total peripheral resistance was higher in the HDT group, indicating impaired vascular tone.
Conclusions:
- Rapid cardiovascular adaptation to 4-hour HDT appears maladaptive upon reintroducing an upright posture.
- Impaired orthostatic tolerance after HDT may be due to decreased venous capacitance vessel tone and inadequate venous return.