Quasistatic volume-pressure curve to predict the effects of positive end-expiratory pressure on lung mechanics and gas exchange in neonates ventilated for respiratory distress syndrome.

The shape of the volume-pressure (V/P) curve indicates alveolar collapse if it is convex to the pressure axis and indicates overdistension if it is concave. Positive end-expiratory pressure (PEEP) should either improve or decrease compliance and oxygenation in neonates ventilated for respiratory distress syndrome (RDS), depending on predominance of either alveolar collapse or overdistension. To test this hypothesis, we determined quasistatic V/P curves in 13 preterm neonates and characterized their shape by an alveolar distension index (ADI) at PEEP levels of 2, 4, and 6 cm H2O. We calculated the ADI dividing the V/P ratio at a low tidal volume by the V/P ratio at a high tidal volume. This ADI was then related to the effect of PEEP changes on respiratory compliance and alveolar to arterial oxygen tension difference (AaDO2). ADI was assumed to indicate alveolar collapse if less than 1 and overdistension if more than 1. An increased PEEP in neonates with alveolar collapse (ADI less than 1) decreased AaDO2 more (12 vs 10 mm Hg/cm PEEP, not significant) and decreased compliance less (3 vs 17%/cm PEEP; P < 0.05) than in those neonates with alveolar overdistension (ADI more than 1). Conversely, a decreased PEEP in neonates with alveolar overdistension increased compliance more (19 vs 5%; not significant) and increased AaDO2 less (7 vs 26 mm Hg; P < .01) than in those with alveolar collapse. AaDO2 and compliance changes after PEEP alterations were significantly correlated to the ADI before PEEP alterations (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)