Single versus double indicator dilution measurements of extravascular lung water. OBJECTIVE: To compare a simplified method of measuring extravascular lung water (EVLW) using only a single indicator (EVLW-SI) with the standard double indicator dilution technique (EVLW-DI). DESIGN: Direct comparison of EVLW-SI with EVLW-DI in 18 critically ill patients. SETTING: A general medical ICU in a university-affiliated hospital. PATIENTS: Nine men and nine women, ages 19 to 80 yr. Six patients were in shock (four from septic shock), and 11 were in respiratory failure. INTERVENTIONS: EVLW-DI and cardiac output were measured in triplicate during injection of cold indocyanine green dye. Cardiac output was calculated both from pulmonary artery (COpa) and femoral artery (COfem-di) thermal dilution signals. EVLW-SI and COfem-si were also measured during three additional injections of cold saline using only thermal signals from the pulmonary and femoral arteries. Order of measurements was random. EVLW-SI was measured in ten patients while blood was withdrawn through the femoral catheter and in ten patients without blood drawn through the femoral catheter. MEASUREMENTS AND MAIN RESULTS: A total of 84 comparisons were made. Although the overall correlations were good (r2 = .86), EVLW-SI systematically overestimated EVLW-DI (p less than .05). This difference was greater when EVLW-SI was measured without blood withdrawal through the femoral catheter. In this subgroup, mean values for EVLW by the two methods were within 20% of one another in only two of ten patients, in contrast to the results in six of eight patients in which blood was withdrawn through the catheter. COfem-si and COfem-di also overestimated COpa. CONCLUSIONS: Theoretically, neither injection of green dye nor blood withdrawal should be necessary during measurements of EVLW-SI, making it a simpler technique for bedside use than EVLW-DI. However, significant discrepancies exist between the two techniques. Some of this difference is apparently due to technical factors related to catheter design. In any case, we cannot recommend use of the single indicator dilution technique at present to estimate EVLW. 