Measurement of lung water in dog lobes using inhaled C15O2 and injected H215O

Indicator-dilution analysis was used in a recirculation-free isolated dog lobe preparation to compare an inhaled water tracer (C15O2) and an injected water tracer (H215O) with direct weighing as a measure of total lung water. Residue detection (counting over the lung) was compared with outflow detection (counting over the venous effluent). With outflow detection, inhaled C15O2 measured 74% and injected H215O 90% of the gravimetric lung water. In hemodynamic edema, the increase in lung water measured by residue detection of both tracers correlated well with increases in weight (r = 0.92, slope = 1.03). However, outflow detection of both tracers underestimated the lung water increase by 53% in edema (r = 0.88, slope = 0.47). Thus, in edema, equilibration of both tracers within the lung water volume is rapid, but clearance from the lung is delayed because slowly clearing water pools develop. The errors caused by inhomogeneity of perfusion distribution were investigated after pulmonary arterial injection of 34-, 50-, and 175-micrometers spheres. For the same lung weight, C15O2 transit was delayed and H215O transit accelerated greatly by the 175-micrometers spheres and slightly by the 50-micrometers spheres.