Alveolar hypoxia increases small pulmonary wedge pressure in awake young lambs

Alveolar hypoxia increases pulmonary artery pressure in lambs and sheep but increases lung lymph flow only in lambs; the reasons for this are unknown. To test the hypothesis that hypoxia-induced pulmonary venous constriction could in part explain the fluid filtration response in young lambs, eight young lambs (13 +/- 3 days old) and four older lambs (60 +/- 4 days old) were prepared for chronic studies of pulmonary hemodynamics, gas exchange, and fluid filtration. In all animals, a 5 Fr catheter was intermittently wedged into a distal pulmonary artery to indirectly estimate pulmonary venous pressure. The pressure measured in this manner was termed the "small wedge pressure." Animals were awake, spontaneously breathing, and unsedated, and were exposed to both acute and chronic isocarbic hypoxia (FIO20.12). We found that hypoxia significantly increased both small pulmonary wedge pressure and lung lymph flow in young lambs but that neither of these effects occurred in the older lambs. In young lambs, the hypoxia-induced increase in small wedge pressure could be lowered by sodium nitroprusside administration, and small wedge pressure could be increased by angiotensin II infusion. We conclude that alveolar hypoxia increases pulmonary venous tone and the critical pressure of pulmonary veins in young lambs, and that these effects disappear with maturation. These data suggest that alterations in pulmonary venous tone can have important effects on lung fluid balance in the newborn.