Model-predicted capillary leakage in graded hypotension: Extended analysis of experimental spinal anesthesia.

Abstract

Crystalloid fluid infused during the induction of spinal anesthesia is involved in a complex set of physiological responses, including vasodilatation, reactive vasoconstriction, and changes in mean arterial pressure (MAP). The present evaluation compares the modeled capillary leakage in anesthetized versus nonanesthetized body regions. Ten female volunteers (mean age, 29years) received 25ml/kg of Ringer's acetate over 60min during experimental spinal anesthesia. Blood hemoglobin was measured repeatedly in the radial artery (reference), arm (cubital) vein, and leg (femoral) vein for 240min. Each pattern of data served as a dependent variable in volume kinetic analyses that used mixed models software and MAP as covariate. The capillary leakage of fluid from the plasma to the extravascular space peaked at 17ml/min when MAP was 100mmHg, and the two venous curves were virtually identical. At MAP 60mmHg, the rate was reduced to 10-12ml/min when assessed in arterial blood and leg vein blood, but only 5mmHg in blood collected from the arm vein. The distribution half-life of infused fluid was then 40min in the leg and 80min in the arm. These results suggest that vasoconstriction in nonanesthetized body regions halves the capillary leakage that is observed in vasodilated, anesthetized body regions. Graded hypotension during spinal anesthesia reduced the capillary filtration of fluid as determined by volume kinetic analysis. The effect was twice as great when venous blood was sampled from a nonanesthetized body region than from an anesthetized body region.