Transcutaneous Fluorescence Dilution Cardiac Output and Circulating Blood Volume during Hemorrhagic Hypovolemia

BACKGROUND: Cardiac output and circulating blood volume are important parameters for assessing cardiac function in the intensive care setting and during major surgeries. The authors tested in an animal model of hemorrhagic hypovolemia the feasibility of measuring these parameters simultaneously by transcutaneous fluorescence monitoring of an intravenous bolus injection of indocyanine green. METHODS: Fluorescence dilution cardiac output was measured in seven anesthetized rabbits and compared to thermodilution cardiac output. The optical probe used to excite the indocyanine green fluorescence was in contact with the skin above the ear artery. Local heating enhanced blood perfusion of the measurement site. Cardiac output was measured during baseline conditions, during hemorrhagic hypovolemia, and after partial restoration of the blood volume with reinfused blood. Estimates of the circulating blood volume were simultaneously obtained from the analysis of the fluorescence dilution traces. RESULTS: Cardiac output measured by fluorescence dilution (thermodilution) averaged 455 +/- 16 (450 +/- 13) ml/min in baseline conditions and 323 +/- 15 (330 +/- 13) ml/min during hypovolemia. Fluorescence dilution cardiac output was linearly related to thermodilution cardiac output (slope = 1.13 +/- 0.05, ordinate = -50 +/- 19 ml/min, R = 0.92). Interanimal differences explained most of the variance between cardiac output estimates obtained with the two techniques. Circulating blood volume decreased from 204 +/- 5 ml in baseline conditions to 174 +/- 8 ml after bleeding and reflected blood volume changes in this acute bleeding-reinfusion model. CONCLUSIONS: The study extends the applicability of the fluorescence dilution technique for cardiac output measurement to hypovolemic conditions and demonstrates its ability to produce accurate estimates of the circulating blood volume in experimental animals.