The pressure at the lower inflexion point has no relation to airway collapse in surfactant-treated premature lambs

Background: The lower inflexion point (LIP) on the inspiratory part of the pressure-volume (PV) loop has been suggested to be related to the pressure at which air spaces collapse. Our hypothesis is that airway collapse might instead be assessed from the upper inflexion point on the expiratory part o f the PV-loop (UTP,,), where lung volume starts to decrease significantly. We therefore examined whether there was a relation between LIP and UIPexp i n premature surfactant-treated lambs. Methods: Ten lambs, at 119-141 days of gestational age, were delivered by c esarean section and given 200 mg/kg modified natural porcine surfactant bef ore the first breath. The lambs were then connected to a ventilator and PV- loops using airway pressures of 0-35-0 (ZEEP-loop) and 5-35-5 cmH(2)O (PEEP -loop) were obtained after lung recruitment at 15, 60 and 120 min after bir th. From the loops, LIP, UIPexp, upper inflexion point of the inspiratory p art of the loop (UIT (insp)), inspiratory capacity (IC) as well as inspirat ory and expiratory maximal compliance of the respiratory system (Crs(insp), and Crs(exp)) were calculated. Results: The ZEEP-loop showed a substantial hysteresis with a distinct LIT at 19 +/-2 cmH(2)O (mean +/- SD), which was different (P <0.001) from UITex p (9 +/-2 cmH(2)O). The pressures at LIP and UIPexp were unrelated (r(2)=0. 06). UIPinsp was located at 28 +/-2 cmH(2)O. Crs(insp) was 2.1 +/-0.6 ml (. ) cmH(2)O(-1) kg(-1), which was lower (P <0.001) than Crs(insp) (2.8 +/-0.6 ml (.) cmH(2)O(-1) kg(-1)). IC was 26 +/-6 ml/kg. The PEEP-loop had a mini mal hysteresis with an expiratory part coinciding with that of the ZEEP-loo p. Conclusion: In surfactant-treated premature lambs the pressures at LIP and UIPexp are not related, showing that LIP does not indicate the pressure at which airways collapse.