Estimations of changes in plasma volume during simulated weightlessness

Area of Science:

• Human physiology
• Space medicine
• Cardiovascular research

Background:

• Simulated microgravity, achieved through head-out water immersion (WI), is known to alter plasma volume (PV).
• Previous studies inferred PV changes using hematocrit (Hct) and hemoglobin (Hgb) levels.
• The accuracy of Hct and Hgb as sole indicators of PV during simulated microgravity requires direct validation.

Purpose of the Study:

• To directly measure plasma volume (PV) changes during simulated microgravity.
• To assess the accuracy of hematocrit (Hct) and hemoglobin (Hgb) as indirect markers of PV.
• To compare direct PV measurements with calculated changes from Hct and Hgb during water immersion and head-down tilt.

Main Methods:

• Direct measurement of plasma volume (PV) using a modified Evans blue dye dilution technique.
• Experiments included 12-hour thermoneutral (34.5°C) head-out water immersion (WI).
• PV was also measured during acute 6° head-down tilt (HDT) to compare with WI effects.

Main Results:

• Direct PV measurements revealed initial increases followed by decreases below pre-immersion levels during WI.
• Calculated PV changes derived from Hct and Hgb did not consistently match direct PV measurements.
• Discrepancies highlight the limitations of using Hct and Hgb alone to track PV dynamics in simulated microgravity.

Conclusions:

• Direct measurement of plasma volume is crucial for accurately assessing fluid shifts during simulated microgravity.
• Hematocrit and hemoglobin are not always reliable indicators of plasma volume changes in these conditions.
• Further research is needed to understand the physiological mechanisms behind these discrepancies.