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 pressures in immature lungs, we studied the lungs of 125-day gestation immature lambs (term 150 days). Lambs were delivered and ventilated for 30 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 cm H2O and perfused with autologous blood, using a steady flow pump. Papaverine was added to the blood to eliminate vasomotion. We measured pulmonary artery and arterial, double and venous occlusion pressures, with flow at 100, 200 and 260 ml . kg(-1). min(-1) and left atrial pressure (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 similar conditions of flow and P-1a. Our data show that elevation of P-1a results in significant increases in capillary (double occlusion) pressure; however, the preterm lung accommodates increases in flow with little change in capillary pressure. An increase in flow rate in the presence 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 capillaries and hemorrhage will occur when there is marked elevation of capillary pressures resulting from increased blood flow in the presence of pulmonary venous constriction or left ventricular dysfunction.