Capillary Filtration of Plasma Is Accelerated During General Anesthesia: A Secondary Population Volume Kinetic Analysis

How infusion fluids are distributed and eliminated is of importance to how much and how fast they should be administered. This manuscript applies population pharmacokinetic modeling to intravenous infusions of crystalloid fluid, which is a common therapy in hospital care and mandatory during surgery. The analysis was based on the hemodilution and urine output measured during and after 262 infusions of 1647 ± 461 mL (mean ± SD) of fluid over 30 min in adults. The result shows that distribution of fluid from the plasma to the interstitial fluid space occurred twice as fast during general anesthesia as compared to the conscious state. The increased rate ensures adequate nutritional flow to the cells despite decreased flow in the macrocirculation, which is a characteristic of general anesthesia. This increased capillary leakage of fluid was coupled with an even greater reduction of the urinary output and accumulation of fluid in both the fast-exchange interstitial fluid space and a remote “third fluid space,” the latter of which apparently serves as an overflow reservoir. During the first hour of the experiments, 88% more fluid resided extravascularly in the presence of general anesthesia than in the awake state. General anesthesia increased the half-life from 1.8 to 16.6 h, showing marked impairment in the handling of infused crystalloid fluid.