Arterial, capillary, and venous transit times and dispersion measured in isolated rabbit lungs

Transit time and relative dispersion of the arterial, capillary, and venous segments of the pulmonary circulation were measured in isolated perfused rabbit lungs. Fluorescence videomicroscopy was used to record the passage of dye through the main pulmonary artery, subpleural microcirculation, and venous outflow. Dye dilution curves were obtained at the main pulmonary artery, subpleural arterioles and venules, and pulmonary vein. Measurements were made at 5-cmH2O airway pressure, at blood flows of approximately 80, 50, and 25 ml.min-1.kg-1, and at left atrial pressures of approximately 0 cmH2O (zone 2) and approximately 12 cmH2O (zone 3). The dye dilution curves were modeled as lagged normal density curves that were used to calculate transit time and relative dispersion between the pulmonary artery and arteriole (artery), arteriole and venule (capillary), venule and pulmonary vein (vein), and pulmonary artery and pulmonary vein (whole lung). In open-chest anesthetized dogs, the passage of dye was recorded from the subpleural arterioles and venules between the seventh and eighth ribs in the left lateral position. At comparable blood flows, capillary transit time was larger in the dog than in the rabbit lung [3.4 +/- 2.4 (SD) vs. 0.87 +/- 0.47 s]. In the rabbit lung, relative dispersion was greater in pulmonary capillaries (average values 0.83-1.6) and veins (0.91-1.6) than in arteries (0.39-0.50), which was similar to the whole lung dispersion (0.47-0.52). A similarly high dispersion (0.93) was measured in the dog's pulmonary capillaries. Thus high dispersion in pulmonary capillaries and veins cannot be detected by whole lung dispersion measurements.