ObjectiveTo evaluate a method of measuring venous blood flowin vitroby using the Fick principle and change in saturation of venous blood and to apply the method to the clinical measurement of hepatic blood flow.DesignExperimental study using anin vitromodel and clinical study for critically ill patients.SettingDepartment of Anesthesiology and Intensive Care Medicine in Osaka City University Medical School.ModelHuman blood deoxygenated by bubbling of nitrogen was circulated in a closed circuit at 10–120 mL/min by a roller pump. A fiber optic sensor was attached to the circuit for continuous monitoring of hemoglobin saturation.PatientsEight critically ill patients, aged 54.3 ± 15.1 yrs, were studied.InterventionsHemoglobin saturation was changed in thein vitrostudy by the injection of 0.2 mL of oxygenated blood. Signals from the optical fiber were analogue-digital converted and recorded in a computer. In the clinical study, an oximetry catheter was inserted into the inferior vena cava (IVC) via the femoral vein. Arterial blood (10 mL) was drawn from the radial artery, and injected into the IVC. The changes in oxygen saturation in the venous blood were recorded.Measurements and Main ResultsBlood flow was calculated using the Fick principle, assuming that all the injected blood passes through the sensor.In vitroestimation of blood flow was well correlated with the actual flow (r2= .94). IVC blood flow was measured above and below the merging point of the hepatic vein. The difference of the two values was assumed to represent hepatic blood flow. IVC blood flow was calculated by the same method as for thein vitrostudy. The blood flows in the IVC above and below the anastomosis with the hepatic vein were 2.82 ± 0.56 (sd) L/min and 1.96 ± 0.61 (sd) L/min. Average estimated hepatic blood flow was 0.86 L/min (range, 0.34–1.75 L/min).ConclusionWe examined the accuracy and reliability of this new method in the presentin vitrostudy. This method may be clinically useful for measuring hepatic blood flow.