Effect of transpulmonary pressure on blood-flow velocity in pulmonary microvessels.

Flow velocity in the microvessels (30 to 90 micron in diameter) on the exposed lung surface of anesthetized bullfrogs was measured during positive lung inflation by means of a laser Doppler microscope. From the pulsatile flow-velocity contour generated by cardiac cycles, mean flow velocity (MV) and pulsatile amplitude (PA) were calculated and their alterations were studied as a function of transpulmonary pressure (TPP). At the lowest TPP of 1.3 +/- 0.8 cm H2O (mean +/- SD), MV and PA were 1.72 +/- 0.33 and 0.45 +/- 0.27 mm/sec, respectively. With moderate increments in TPP up to 2.8 +/- 1.3 cm H2O, both MV and PA increased, attaining maximum values of 2.11 +/- 0.42 and 0.69 +/- 0.28 mm/sec, respectively. But with further inflation they began to decrease, and there was a marked reduction or a complete arrest of blood flow at excessively high TPPs (6 to 9 cm H2O). With reduction of previously increased TPP, the alterations in MV and PA followed a pattern similar to that noted during elevation of TPP. There was an apparent hysteresis in both the MV-TPP and PA-TPP curves, most marked when excessively high TPP was applied.