IN HUMAN PATIENTS, VASCULAR WATER-RETENTION DURING DDAVP-RELATED HYPONATREMIA OCCURS MAINLY IN THE PLASMA-VOLUME AND NOT IN THE ERYTHROCYTE

DDAVP-related hyponatremia induces a blood volume expansion, but the a nalysis of fluid distribution in the vascular compartment has given co ntroversial results in previous animal and human studies. In 5 healthy males, hyponatremia was induced by DDAVP and a free water intake duri ng 3 days. Serum sodium concentration decreased from 138 +/- 0.8 mEq/L to 123 +/- 2.7 mEq/L on day 3. The plasma volume measured by dilution of marked albumin rose from 3033 +/- 230 ml to 3320 +/- 295 ml (p < 0 .01). The mean corpuscular volume measured by microhematocrit increase d slightly from 91.5 +/- 3.8 pl to 92.6 +/- 3.7 pl (p < 0.02). The red blood cell volume calculated with hematocrit and plasma volume did no t change significantly (2565 ml to 2567 ml; not significant). In the p resent work, we demonstrated that in males the expansion of the plasma compartment almost completely amounted for the water retention in the intravascular volume. The erythrocyte volume increased only slightly, a finding that is consistent with an almost perfect adaptation of the erythrocyte cells to the hypoosmolality.