Direct and indirect methods for determining plasma volume during thermoneutral and cold-water immersion

Area of Science:

• Physiology
• Environmental Medicine
• Cardiovascular Research

Background:

• Plasma volume (PV) changes are crucial for understanding physiological responses to thermal stress.
• Indirect methods using haematocrit (Hct) and haemoglobin ([Hb]) are commonly used to estimate PV.
• Previous studies indicated underestimation of PV changes by indirect methods during thermoneutral immersion.

Purpose of the Study:

• To investigate if indirect PV measurement underestimates PV changes during cold-water immersion.
• To compare direct (Evans blue dye) and indirect PV measurements across different thermal conditions.

Main Methods:

• Seven healthy males underwent 60-minute exposures in air (control), thermoneutral water, and cold water.
• Plasma volume was measured directly using Evans blue dye dilution and indirectly via Hct and [Hb].
• Calculated the percentage difference and underestimation between direct and indirect PV measurements.

Main Results:

• During thermoneutral immersion, direct PV increased by 16.2%, while indirect methods underestimated this by 43%.
• During cold-water immersion, direct PV decreased by 17.9%, with indirect methods underestimating this by 52%.
• The indirect method consistently underestimated PV changes in both thermoneutral and cold water immersion.

Conclusions:

• The indirect method significantly underestimates plasma volume changes during both thermoneutral and cold water immersion.
• The assumption of a constant F-cell ratio in the indirect method is likely violated during water immersion.
• Direct measurement methods are recommended for accurate assessment of plasma volume shifts in aquatic environments.