Water-Deficit Equation: Systematic Analysis and Improvement.

摘要

Background: The water deficit equation (WD1 = 0.6Bm 1 - 140/Na+) is employed in medicine and nutrition to estimate the volume (liters) of water required to correct dehydration during the initial stages of fluid replacement therapy. Several equation assumptions may limit its accuracy, but none have been systematically tested. Objectives: To quantify the potential error in WD1 for estimating free water (FW) and total body water (TBW) losses and to systematically evaluate its assumptions. Design: Thirty-six euhydrated volunteers were dehydrated (2.2 to 5.8% body mass, Bm) via thermoregulatory sweating. Assumptions within WD1 were tested by substituting measured euhydrated values for assumed or unknown values. These included the known (pre- morbid) Bm (WD2), a proposed correction for unknown Bm (WD3), the TBW estimated from body composition (WD4), the actual plasma sodium (WD5), the substitution of plasma osmolality (Posm) for sodium (WD6), and actual Posm (WD7). Results: Dehydration reduced TBW by 3.49 - 0.91 L, 57% of which (2.02 - 0.96 L) was FW loss, and increased plasma sodium from 139 (135 to 143 mmol/L) to 143 (141 to 148 mmol/L). Calculations for WD1-WD7 all underestimated TBW loss by 1.5 to 2.5 L.