Respiratory effects of a patent ductus arteriosus in premature newborn lambs

We examined the respiratory effects of a patent ductus arteriosus in 29 premature lambs (131-135 days gestational age) after infiltrating the ductal wall with formaldehyde solution (Formalin) and placing a snare around the ductus to regulate its patency. The lambs were given sheep surfactant, paralyzed, and mechanically ventilated at birth. We first compared 8 lambs with open ductus and 13 lambs with closed ductus during the 12 h after birth. Although lambs with open ductus had greater pulmonary blood flow (301 ± 36 vs. 188 ± ml·min^-1·kg^-1, mean ± SE, at 12 h of age) and mean pulmonary arterial (44 ± 3 vs. 33 ± 2 mmHg) and left ventricular end-diastolic (6 ± 0.6 vs. 4 ± 0.7 mmHg) pressures, we found no differences in dynamic respiratory compliance (Cdyn = 0.55 ± 0.07 vs. 0.55 ± 0.03 ml·cmH₂O^-1·kg^-1), midtidal volume resistance (62 ± 5 x 10^-3 vs. 62 ± 7 x 10^-3 cmH₂O·ml^-1·s), or functional residual capacity (FRC = 27 ± 3 vs. 26 ± 2 ml·kg^-1). Alveolar-arterial PO₂ difference was lower in the lambs with open ductus (238 ± 65 vs. 362 ± 37 Torr). Next, we challenged eight lambs with two separate saline infusions (50 ml·kg^-1 over 3 min), each given with the ductus alternately closed or open. When the ductus was closed, FRC was unchanged, but Cdyn increased by 18% immediately after the infusion. When the ductus was open, FRC decreased by 16% and Cdyn decreased by 12%. We conclude that the premature lamb is surprisingly resistant to changes in respiratory function from ductal patency during the immediate neonatal period.