Increased orthostatic blood pressure variability after prolonged head-down tilt.

Area of Science:

• Cardiovascular Physiology
• Space Medicine
• Human Physiology

Background:

• Orthostatic intolerance is a significant concern for astronauts upon returning to Earth.
• Simulated weightlessness using head-down tilt (HDT) is a common method to study these effects.
• Understanding blood pressure regulation during orthostatic stress is crucial for mitigating risks.

Purpose of the Study:

• To evaluate the effect of 10 days of 6 degrees head-down tilt (HDT) on orthostatic blood pressure (BP) regulation.
• To assess changes in heart rate (HR) and BP variability during passive 70 degrees head-up tilt (HUT) after HDT.
• To investigate the underlying mechanisms of BP and HR responses to orthostatic stress post-HDT.

Main Methods:

• Six healthy male volunteers underwent 10 days of 6 degrees HDT.
• Continuous, non-invasive finger BP and ECG-derived HR were recorded.
• Fast Fourier Transform (FFT) analysis was used to quantify BP and HR variability, specifically oscillations around 0.1 Hz.

Main Results:

• After HDT, HR response to HUT significantly increased (24 to 41 beats/min).
• Total BP variance during HUT nearly doubled, primarily due to oscillations around 0.1 Hz.
• Transient HR decelerations were observed, linked to vagal baroreflex activity rather than impending syncope.

Conclusions:

• Ten days of HDT did not alter absolute BP responses to HUT but increased HR response, indicating heightened sympathetic activity.
• Augmented BP variability around 0.1 Hz post-HDT suggests altered BP regulation mechanisms.
• The findings highlight significant cardiovascular adaptations to simulated weightlessness impacting orthostatic tolerance.