MICROVASCULAR PRESSURES IN ISOLATED-PERFUSED IMMATURE LAMB LUNGS - EFFECTS OF FLOW-RATE, LEFT ATRIAL PRESSURE AND SURFACTANT THERAPY

To determine the effect of some physical factors on microvascular pres sures in immature lungs, we studied the lungs of 125-day gestation imm ature lambs (term 150 days). Lambs were delivered and ventilated for 3 0 min prior to isolation of the lungs. Nine had surfactant (Survanta, 100 mg/kg) instilled into the lungs prior to delivery, and 7 did not ( control group). Lungs were kept distended with airway pressure at 20 c m H2O and perfused with autologous blood, using a steady flow pump. Pa paverine was added to the blood to eliminate vasomotion. We measured p ulmonary artery and arterial, double and venous occlusion pressures, w ith flow at 100, 200 and 260 ml . kg(-1). min(-1) and left atrial pres sure (P-1a) at 0, 10 and 15 mm Hg. We found that the vascular pressure profile was similar in control and surfactant-treated lungs under sim ilar conditions of flow and P-1a. Our data show that elevation of P-1a results in significant increases in capillary (double occlusion) pres sure; however, the preterm lung accommodates increases in flow with li ttle change in capillary pressure. An increase in flow rate in the pre sence of elevated P-1a had the greatest effect on capillary pressure. All lungs became hemorrhagic when capillary pressure exceeded 30 mm Hg . We speculate that in preterm infants, stress failure of pulmonary ca pillaries and hemorrhage will occur when there is marked elevation of capillary pressures resulting from increased blood flow in the presenc e of pulmonary venous constriction or left ventricular dysfunction.