Measurement of microhemodynamics in the ventilated rabbit lung by intravital fluorescence microscopy.

Pulmonary microhemodynamic parameters were directly measured along with systemic pressures and cardiac output in the ventilated rabbit lung. Subpleural arterioles and venules ranging from 10 to 35 microns luminal diameter were investigated under zone 2 conditions, i.e., during inspiratory plateau at an airway pressure of 8 mmHg. Mean arteriolar and venular diameters (24.6 +/- 3.3 and 21.9 +/- 3.6 microns, respectively), mean red blood cell (RBC) fluxes (1,549 +/- 501 and 1,257 +/- 600 cells/s), and mean RBC velocities (0.79 +/- 0.21 and 0.82 +/- 0.21 mm/s) were measured using a fluorescence video-microscopic technique. Calculated microhematocrit (Hct mu) was below systemic values (Hctsys) (Hct mu/Hctsys: arterioles, 0.75 +/- 0.12; venules, 0.67 +/- 0.08). The mean capillary transit time of RBC was 0.47 +/- 0.16 s over a mean arteriovenous distance of 173 +/- 70 m. Significant correlations were demonstrated between microhemodynamic parameters. A correlation among cardiac output, pulmonary arterial pressure, and RBC velocity demonstrates the connection between macro- and microhemodynamics in the rabbit lung. In conclusion, the present model is the first one enabling the measurements of the principal circulatory determinants for gas exchange, i.e., microvascular blood flow, Hct mu, and capillary transit time of RBCs in the ventilated rabbit lung under simultaneous macrocirculatory control.