Failure of hemoconcentration during standing to reveal plasma volume decline induced in the erect posture

The hypothesis was tested that the hemoconcentration observed during standing provides erroneous information about the induced plasma volume (PV) decline. Male volunteers (n = 10) stood quietly for 15 min after supine rest. On standing arterial hemoglobin (Hb) rose slowly to reach an increase of 5.9 +/- 0.3% (SE) after 15 min. Early after resuming the supine position, Hb increased further to 9.2 +/- 0.5% above control level and then declined gradually. Venous antecubital blood from the left arm supported horizontally at heart level in both the supine and standing positions (no hydrostatic load) showed very similar changes. However, Hb in venous blood collected during standing from the right arm held in the natural dependent position rose much more markedly than that in arterial blood and in venous blood from the horizontal arm (470 +/- 122, 105 +/- 24, and 55 +/- 7% greater increase at 5, 10, and 15 min, respectively). Taken together, these observations indicated that 1) analyses of arterial blood sampled from the standing subject grossly underestimated the prevailing "overall" hemoconcentration and PV decline, a phenomenon ascribed to incomplete mixing of blood between dependent and nondependent regions; 2) arterial blood sampled from the recumbent subject early (60 s) after completion of standing reflected the "true" overall intravascular hemoconcentration, with a calculated PV decline of no less than 511 +/- 27 ml, because the supine position facilitated proper mixing of blood between circulatory compartments; 3) data from common venous sampling from the dependent arm during standing primarily reflected a regional hemoconcentration (fluid loss) in the arm rather than PV decline; and 4) short-term quiet standing caused a more prominent and hemodynamically important decrease in PV than usually believed.