Effects of lung inflation and blood flow on capillary transit time in isolated rabbit lungs

In a previous study, direct measurements of pulmonary capillary transit time by fluorescence video microscopy in anesthetized rabbits showed that chest inflation increased capillary transit time and decreased cardiac output. In isolated perfused rabbit lungs we measured the effect of lung volume, left atrial pressure (Pla), and blood flow on capillary transit time. At constant blood flow and constant transpulmonary pressure, a bolus of fluorescent dye was injected into the pulmonary artery and the passage of the dye through the subpleural microcirculation was recorded via the video microscope on videotape. During playback of the video signals, the light emitted from an arteriole and adjacent venule was measured using a video photoanalyzer. Capillary transit time was the difference between the mean time values of the arteriolar and venular dye dilution curves. We measured capillary transit time in three groups of lungs. In group 1, with airway pressure (Paw) at 5 cmH₂O, transit time was measured at blood flow of ~80, ~40, and ~20 ml · min^-1 · kg^-1. At each blood flow level, Pla was varied from 0 (Pla < Paw, zone 2) to 11 cmH₂O (Pla > Paw, zone 3). In group 2, at constant Paw of 15 cmH₂O, Pla was varied from 0 (zone 2) to 22 cmH₂O (zone 3) at the same three blood flow levels. In group 3, at each of the three blood flow levels, Paw was varied from 5 to 15 cmH₂O while Pla was maintained at 0 cmH₂O (zone 2). Capillary transit time averaged 0.71 s at a Paw of 5 cmH₂O and a blood flow of ~80 ml · min^-1 · kg^-1 in zone 3, comparable to values (0.60 s) measured previously in vivo at functional residual capacity. Capillary transit time was reduced with an increase in both blood flow and Paw but was increased with an increase in Pla from zone 2 to zone 3 conditions.