Underestimation of plasma volume changes in humans by hematocrit/hemoglobin method

Area of Science:

• Physiology
• Cardiovascular Research

Background:

• Hematocrit (Hct) and hemoglobin (Hb) measurements are commonly used to estimate plasma volume (PV) changes.
• Previous studies suggest this method underestimates PV changes during water immersion.
• The accuracy of Hct/Hb for assessing PV shifts during postural changes remains unclear.

Purpose of the Study:

• To compare plasma volume changes determined by Evans blue (EB) dilution with those calculated using Hct/Hb during an acute head-down tilt (HDT).
• To evaluate the accuracy of the Hct/Hb method in estimating PV shifts during postural alterations.

Main Methods:

• Ten males underwent a 6-degree head-down tilt (HDT) posture change.
• Plasma volume was measured using an improved Evans blue (EB) tracer-dilution technique.
• Simultaneously, Hct and Hb concentrations were measured from central venous blood samples.

Main Results:

• Plasma volume increased by 9.3% +/- 2.0% when measured with EB during HDT.
• The Hct/Hb method indicated a smaller PV increase of 4.5% +/- 0.9%.
• The Hct/Hb method significantly underestimated PV changes by up to 50% compared to EB measurements (P < 0.05).

Conclusions:

• The Hct/Hb method significantly underestimates plasma volume changes during acute postural changes like head-down tilt.
• Direct tracer-dilution methods, such as Evans blue, are necessary for accurate PV estimations during antiorthostatic maneuvers.
• Relying on Hct/Hb can lead to substantial underestimation of fluid shifts in response to posture changes.