Quantitative estimation of errors in the indicator dilution measurement of extravascular lung water

Objective: To assess the accuracy of the diffusible indicators heavy water and thermal indicator in the measurement of extravascular lung water (EVLW). Design: Cardiac output (GO), mean transit time and EVLW for the two diffusible indicators were measured. CO for indocyanine green, gravimetric EVLW and the calculated mean transit time for a diffusible indicator were used as independent reference variables. CO, mean transit time and EVLW for the two diffusible indicators were compared to the reference variables and the percentage error for each measured variable was calculated for each bolus injection. Setting: 6 sheep with healthy lungs and 6 with pulmonary oedema in a research laboratory. Interventions: CO was altered with positive end-expiratory pressure and dobutamine. Measurements: All indicators were given together in a bolus through a central venous line. Indicators were detected simultaneously in the aorta, and CO, mean transit time and EVLW were measured. EVLW was measured gravimetrically (EVLWgrav) postmortem. Results: In the combination of a low CO and a large distribution volume, heavy water and thermal indicator produced a large number of slow wash-out curves. These curves were abolished from further analysis. The mean errors in CO and mean transit time for heavy water were close to zero and independent of the distribution volume; the product EVLW was close to EVLWgrav. The mean error in thermodilution CO measured in the aorta was close to zero but dependent on the distribution volume. The mean error in mean transit time for the thermal indicator was 36 % and dependent on the distribution volume. Their product EVLW overestimated EVLWgrav by 70 %. Conclusions: The results obtained for heavy water confirmed the theoretical basis of the indicator dilution method. The mean transit time for the thermal indicator was not proportionate to its distribution volume. The magnitude of this error prevents the calculation of an anatomically defined EVLW using a catheter-mounted thermistor in the aorta.