Validity of exchangeable solute balance as a measure of blood volume in neurologically injured adults

Background: Daily weight (DW) and examination of fluid balances (FB) are commonly used in assessments of extracellular fluid (ECF) and circulating blood volume (BV). We hypothesized that a calculated total body exchangeable solute, the main determinant of the ECF, would have high agreement and correlation with actual BV. Methods: The University of Washington IRB approved the study. We included a sample of consecutive adult patients in whom a BV was measured, while in the neuroscience intensive care units of a large academic medical center. BV was measured as part of routine care using iodinated ¹³¹I albumin injection and the BVA-100 (Daxor Corp, New York, NY, USA). Total body exchangeable solute was estimated at the time of BV measurement by multiplying the calculated total body water by the sum of the sodium plus potassium and chloride measured in plasma. The correlation between the change in DW, FB (adjusted for insensible fluid loss), exchangeable solute, and BV was performed using linear regression with adjustment for number of days between admit and BV measurement, and capillary leak. Errors were computed using robust variance estimation. Results: 55 patients had BV tests available, and 43 of them had subarachnoid hemorrhage. Total body exchangeable solute strongly correlated with BV (r = 0.75, 95 % CI 0.63-0.84, p < 0.01 for Na⁺/K⁺, and r = 0.71, 95 % CI 0.58-0.81, p < 0.01 for Cl^-). DW (r = 0.21) and FB (r = 0.11) were not correlated with BV. Conclusions: Total body exchangeable solute appears to be a valid and reliable measure of BV and can be calculated using information readily available at the bedside. The value of having this information automatically calculated and available at the bedside should be explored.