 Abstract |
Previous data have shown that the most prevalent, indirect
plasma volume (PV) measurement technique, which utilises changes in haematocrit
(Hct) and haemoglobin concentration ([Hb]), underestimates actual PV changes
during immersion, when compared to a direct tracer-dilution method. An increase
in the F-cell ratio (whole-body haematocrit (Hctw) to large-vessel haematocrit (Hctv)
ratio) has been purported as a possible explanation, probably due to hydrostatic
and thermally-mediated changes during water immersion. Previous investigators
have not quantified the F-cell ratio during immersion. Therefore, this study
sought to determine the effect of the F-cell ratio on the indirect method during
both, thermoneutral and cold-water immersions.
Seven healthy males were tested three times, seated upright in air (control:
21.2°C SD ±1.1), and during thermoneutral (34.5oC SD ±0.2) and cold-water
immersion (18.6oC SD ±0.2), immersed to the third intercostal space for 60 min.
Measurements during the immersion tests included PV (Evans blue dye column
elution, Evans blue dye computer programme, and Hct [Hb]), red cell volume (RCV;
sodium radiochromate), cardiac frequency (fc) and rectal temperature (Tre).
Plasma volume during the control trial remained stable, and equivalent across
the three tests. There was a hydrostatically-induced increase in PV during
thermoneutral immersion, when determined by the Evans blue dye method (16.2%).
However, the Hct/[Hb] calculation did not adequately reflect this change, and
underestimated the relative PV change by 43%. In contrast, PV decreased during
cold immersion when determined using the Evans blue dye method by 17.9% and the
Hct/[Hb] calculation by 8.0%, respectively, representing a 52% underestimation
by the latter method. There was a non-significant decline in RCV during both
immersions. Furthermore, an increase (8.6%) and decrease (-14.4%) in blood
volume (BV) was observed during thermoneutral and cold-water immersions,
respectively. The decline in RCV during thermoneutral immersion attenuated the
BV expansion. Despite the disparity between the PV methods, there was no
increase in the F-cell ratio during either immersion. In contrast, there was a
significant decline in the F-cell ratio during the control: air and
thermoneutral immersion, which may indicate that other, undefined variables may
impact on the stability of the red cell compartment.
The current study is the first to show that the Hct/[Hb] method clearly
underestimates PV changes during both thermoneutral and cold-water immersion.
Furthermore, RCV was shown, for the first time, to decline during both
immersions. However, the changes in the F-cell ratio during this study, did not
account for the underestimation of PV change using the Hct/[Hb] method. |
