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

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Abstract
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 of the PV‐loop (UIPexp), where lung volume starts to decrease significantly. We therefore examined whether there was a relation between LIP and UIPexp in premature surfactant‐treated lambs.Methods: Ten lambs, at 119–141 days of gestational age, were delivered by cesarean section and given 200 mg/kg modified natural porcine surfactant before 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 cmH2O (PEEP‐loop) were obtained after lung recruitment at 15, 60 and 120 min after birth. From the loops, LIP, UIPexp, upper inflexion point of the inspiratory part of the loop (UIP insp), inspiratory capacity (IC) as well as inspiratory and expiratory maximal compliance of the respiratory system (Crsinsp and Crsexp) were calculated.Results: The ZEEP‐loop showed a substantial hysteresis with a distinct LIP at 19±2 cmH2O (mean±SD), which was different (Pexp (9±2 cmH2O). The pressures at LIP and UIPexp were unrelated (r2=0.06). UIPinsp was located at 28±2 cmH2O. Crsinsp was 2.1±0.6 ml · cmH2O−1 · kg−1, which was lower (Pexp (2.8±0.6 ml · cmH2O−1 · kg−1). IC was 26±6 ml/kg. The PEEP‐loop had a minimal hysteresis with an expiratory part coinciding with that of the ZEEP‐loop.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.