Thermodilution measurement of lung water

Abstract

The detection and measurement of pulmonary edema by the thermal-dye method appears to be accurate and reproducible under specified laboratory conditions. The ETV, which represents the difference in distribution volumes of the diffusible (thermal) indicator and the intravascular (green dye) indicator, should closely estimate the ELM (ETV = 0.984 ELM). Experimental measurements of ETV have shown a very good correlation with ELM, with a tendency for overestimation in normal lungs and underestimation in severely edematous lungs. In contrast to previous measurements using isotopic water methods, thermal-dye measurements have revealed that the estimation of ELM by ETV in severe edema (alveolar flooding) does not plateau. The limitations of the thermal-dye technique reflect the evenness of lung perfusion. Depending on their size and number, emboli produce perfusion defects and reduce ETV. Airway injury also reduces ETV, apparently by redistribution of blood flow. Alterations of ETV by hemodynamic factors suggest that reduction in perfusion pressure may be more significant than changes in flow, although more data are needed. Atelectasis without a reduction in blood flow does not decrease ETV. PEEP may increase ETV when lung injury is not uniform, perhaps by redistributing blood flow, and this maneuver may be useful in detecting underestimation of ELM. Position of the thermistor produces the greatest degree of variability by distorting the thermodilution curve and prolonging the MTT. This results in an increased ETV and an overestimation of ELM. In laboratory studies, the measurements of ETV can be validated by gravimetric analyses of lung water. Since this method of validation is not possible in clinical studies, measurements of ETV in patients must be interpreted in light of limitations demonstrated in the laboratory. Suggestions for avoiding the most common errors in measuring ETV are listed in Table 3.