BACKGROUND: The revised Starling law suggests that intravenously infused fluid may leak into the interstitium and not remain in the intravascular space. This hypothesis is supported by clinical findings that postoperative weight gain is proportional to the amount of infused fluid. The distribution of intravenously administered fluid between the interstitium and intravascular space deserves evaluation, as postoperative weight gain because of intraoperative infusion is an important risk factor for postoperative adverse events. We quantitatively estimated fluid movement in patients undergoing orthognathic surgery by performing a volume kinetic study using hemoglobin concentration as a marker of dilution.
METHODS: Forty-one patients scheduled to undergo orthognathic surgery were enrolled in this study. The arterial hemoglobin concentration was measured at each procedural step. Acute normovolemic hemodilution was induced by withdrawing 400 mL of blood followed by the infusion of a known amount of hydroxyethyl starch, enabling the initial blood volume to be estimated. The dilution rate of the arterial hemoglobin concentration enabled the volume of fluid in the intravascular space to be quantified. The fluid volume that leaked into the interstitium was then calculated based on the change in the estimated intravascular plasma volume.
RESULTS: The blood volume estimated via this method was close to the value derived from a previously published formula. The mean volume of crystalloid infused as a maintenance fluid was 2062 408 mL, ranging from 1220 to 3050 mL. None of the cases required blood product transfusion. The amount of infused fluid that remained intravascular varied widely from 2.0 to 35.7 mL/kg (mean, 12.0 +/- 8.2 mL) after surgery, corresponding to 5.3% to 95.7% of the infused volume. The change in intravascular fluid volume during surgery was not strongly correlated with the infusion amount (Pearson correlation analysis: r = -0.05, P = .75, -0.44 < rho <= 0.35, confidence intervals; Spearman correlation analysis: r = -0.14, P = .38, -0.51 < rho <= 0.27). However, the amount of fluid that leaked into the interstitium during surgery did correlate with the infusion amount (Pearson correlation analysis: r = 0.42, P = .01, 0.03 < rho <= 0.70; Spearman correlation analysis: r =0.45, P = .003, 0.07 < rho <= 0.72).
CONCLUSIONS: We found that the increase in intravascular fluid volume caused by intravenous fluid administration was not correlated strongly with the volume of infused fluid. Instead, the amount of fluid leakage into the interstitial space depended on the infused fluid volume. This clinical result supports the revised Starling law, which suggests that intravascular fluid may often leak into the interstitium. More work is needed to better understand the factors governing leakage of infused fluid into the interstitial space.