Arterial Pressure and the Rate of Elimination of Crystalloid Fluid

Excretion of crystalloid fluid is slow during general anesthesia. The distribution and elimination of buffered Ringer's solution were analyzed to determine whether the rate of elimination correlates with a hemodynamic factor, consciousness, patient posture, or the type of general anesthesia. Data were derived from 4 separately published studies in which 30 volunteers and 48 anesthetized patients had received 0.833 (1 series 0.667) mL/kg/min of lactated or acetated Ringer's solution over 30 minutes. Frequent measurements of the blood hemoglobin and mean urinary excretion were used as input in a kinetic analysis according to a 2-volume model and covariates, using microconstants and mixed-effects modeling software. The results show that rate of elimination of crystalloid fluid decreased with the mean arterial pressure (MAP) and patient age, but was unaffected by consciousness and inhalational or intravenous anesthesia. The elimination rate constant was 6.5 (95% confidence interval, 5.2-7.9) x 10(-3) x (MAP/mean MAP)(5.2) x (Age/mean Age)(-1.5). The mean MAP for the 2108 data points was 81.3 mm Hg and the mean age was 40 years. The central fluid space that was expanded by infused fluid (V-c, plasma volume) increased with body weight but decreased with general anesthesia and with reductions of MAP. Simulations revealed a more than 10-fold difference in the excreted fluid volume after a theoretical 30-minute infusion, depending on whether the MAP was 50 or 100 mm Hg. In conclusion, the rate of elimination of crystalloid fluid decreased in proportion to MAP but was independent of general anesthesia and moderate-sized surgery.