Pulmonary microvascular pressure profile during development of hydrostatic edema

To measure, in intact closed chest, the pressure in the pulmonary microvasculature during transition to mild interstitial edema.

In anesthetized spontaneously breathing rabbits, the pulmonary artery and left atrium were cannulated. Pleural windows were prepared to view the superficial pulmonary microvascular network through the intact parietal pleura. After intravenous infusion of 96.4 +/- 12.3 ml of saline at a rate of 0.5 ml/kg h, the hydraulic pressure in the pulmonary microvessels (15-240 microns in diameter) were measured using glass pipettes driven through the pleural window and connected to a servonull system.

After saline, plasma protein concentration decreased from 6 +/- 1 to 4.8 +/- 0.5 g/dl; pulmonary arterial and left atrial pressures averaged 22.3 +/- 6.4 and 2.3 +/- 2 cm H2O in control and 23.1 +/- 4.2 and 4.2 +/- 2 cm H2O after infusion. After saline loading, 16.4% of total pressure drop occurred from pulmonary artery to 80-microns arterioles, 60.3% in 30-80 microns arterioles, 6.9% from 30-microns arterioles to 30-microns venules and 16.4% in the downstream segment.

Mild interstitial edema induced, with respect to control, constriction of small arterioles and capillary recruitment to maintain a low capillary pressure. Hence, in initial edema, pulmonary circulation prevents further fluid filtration, acting like an intrinsic safety factor to delay development of severe edema.