Flow-dependence of extravascular thermal volume as an index of pulmonary edema

Using a double indicator (dye and heat) dilution technique of extravascular lung water measurement, we examined the effect of a reduction in cardiac output and positive pressures on the extravascular thermal volume (EVTV) in dogs. Following baseline EVTV measurements, cardiac output was lowered by inflation of balloons in the superior and inferior vena cavas, as well as by bleeding, and positive pressures were applied to the airways. There was good agreement between the baseline EVTV and post-mortem lung water; however, as the cardiac output was lowered there was a reduction in the measured EVTV. In other animals following application of positive airway pressure there was a decrease in the EVTV which appeared to be related to the reduction in cardiac output caused by positive airway pressure. At least in part, loss of thermal indicator appeared to explain the reduction in EVTV. Measurement of EVTV as an index of pulmonary edema may not be accurate in the face of a changing cardiac output.