Effects of regional hemoconcentration during LBNP on plasma volume determinations.

Blood samples were obtained from forearm vein or artery with indwelling cannula I. before, II. during the last min, and III. about 2 min after lower body negative pressure (LBNP) in 16 experiments to determine whether plasma volume (PV) estimates were affected by regional hemoconcentration in the lower body. Total hemoglobin (THb) was estimated with the CO method prior to LBNP. Hemoglobin (Hb) and hematocrit (Hct) values from II gave only a 3% (87 ml) loss in PV due to LBNP, assuming no change in THb. However, Hb and Hct values from III showed an 11% loss in PV (313 ml). This 72% underestimation of PV loss with II must have resulted from the sequestration of blood and subsequent hemoconcentration in the lower body during LBNP. The effects of LBNP on PV should be estimated 1-3 min after exposure, after mixing but before extravascular fluid returns to the circulation.