Surfactant metabolism during hyperventilation of newborn lambs with atrial right to left shunts

We studied the effect on surfactant metabolism of 8 h of mechanical ventilation at tidal volumes of 13 ± 0.3 ml/kg and very high tidal volumes of 28 ± 1.5 ml/kg, with and without added CO₂, in the presence of an atrial right to left shunt in 4- to 8-day-old lambs. Similarly aged, spontaneously breathing lambs were used as controls. Right to left atrial shunts were created by inflating a balloon in the right atrium after a Rashkind atrial septostomy, thus creating a stable, easily controlled atrial shunt. Radiolabeled surfactant phospholipid precursors were used to probe incorporation into and secretion of surfactant phosphatidylcholine, whereas intratracheally administered labeled natural surfactant was used to evaluate alveolar clearance. Protein leak from the vascular space to the lungs was measured using radioactive iodine-labeled albumins. At the end of the 8-h study period, tissue association of alveolar surfactant was significantly increased to 63% in the mechanically hyperventilated lambs as compared to 44% in those lambs mechanically ventilated but not hyperventilated (p < 0.05) and to 39% in the spontaneously breathing control animals (p < 0.05). No increased surfactant secretion or decreased compliance was detected with hyperventilation. However, the lambs had very large surfactant-saturated phosphatidylcholine pool sizes, and a large portion (50%) was already in the alveolar pool, even in the spontaneously breathing lambs. Precursor incorporation into saturated phosphatidylcholine was similar in all groups, and very low and comparable protein leaks were seen in the different groups of lambs. These experiments did not detect any adverse effects of 8 h of mechanical ventilation, large atrial shunts or mechanical hyperventilation on the surfactant system of newborn lambs.