A problem intrinsic to the measurements of the pulmonary extravascular water volume by the thermal-dye technic with the sampling site in the bifurcation of the aorta. Thermal equilibrium with the aortic wall.

When estimating the pulmonary extravascular water volume (PEWV) as the lung thermal volume (LTV), by the double indicator dilution technic using heat and indocyanine green (ICG), a part of the left ventricular wall comes to the thermal equilibrium, and this leads to an overestimation of PEWV, when the samplings are made in the aortic root. In a previous study from this laboratory, this overestimation was approximately 10% of the measured LTV in the aortic root. In the present study, we evaluated the extent to which the thermal equilibrium with the aortic wall would cause LTV to slightly overestimate PEWV. For this purpose, we injected a mixture of the indicators into the right atrium, and recorded the indicator dilution curves at the bifurcation of the aorta (AB). We then compared this LTV with the one calculated from the indicator dilution curves recorded simultaneously in the aortic root (Ao). We obtained the following results: Firstly, the values of cardiac output (CO) from the dye dilution curve and from the thermodilution curve at two sites, Ao and AB, were all in agreement. Secondly, LTV estimated in Ao (LTVAo) and LTV estimated in AB (LTVAB) were not the same, and their relationships were: LTVAB = 1.21 X LTVAo + 0.44 (ml/kg), n = 32, (r = 0.98, p less than 0.001) A close agreement of CO determined at two sites indicated that there was virtually no loss of heat between the two sampling sites, Ao and AB. An excess of LTVAB over LTVAo came to about 20%, and this excess appeared to be ascribable to the incorporation of the thermally equilibrated "volume" of the aortic wall. This finding appeared to be a challenge to the validity of estimating LTV when sampling the indicators in the distal abdominal aorta.