Improvement of lung mechanics by exogenous surfactant: effect of prior application of high positive end-expiratory pressure

The use of a ventilation strategy with high positive end-expiratory pressur e (PEEP) that is intended to recruit collapsed alveoli and to prevent recur rent collapse can reduce alveolar protein influx in experimental acute lung injury (ALI). This could affect the pulmonary response to treatment with s urfactant, since plasma proteins inhibit surfactant function. We studied th e effect of exogenous surfactant on lung mechanics after 4 h of mechanical ventilation with high or low PEEP. Twenty-two adult male Sprague-Dawley rat s were anaesthetized, tracheotomized and submitted to pressure-controlled m echanical ventilation with 100% oxygen. One group served as healthy control s (n=6). In the remaining animals acute lung injury was induced by repeated lung lavages to obtain a Pa-O2 < 13 kPa during ventilation with a peak ins piratory pressure (PIP) of 26 cm H2O and a PEEP of 6 cm H2O. These animals were allocated randomly to ventilation with high PEEP (n=8; 100 breaths min (-1), I:E= 1:1, PIP 35 cm H2O, PEEP 18 cm H2O) Or to conventional mechanica l ventilation (PIP 28 cm H2O, PEEP 8 cm H2O; n=8; ventilated control group) . After 4 h of ventilation, all animals were given surfactant (120 mg kg(-1 )) via the trachea and ventilation was continued for 15 min. At the end of the study, pressure-volume curves were constructed to measure total lung ca pacity at 35 cm H2O (TLC35) and maximal compliance (C-max) and bronchoalveo lar lavage was then used to measure alveolar protein influx. After ravage, Pa-O2 remained around 13 kPa in the ventilated control group and was >66 kP a in the high-PEEP group. After surfactant treatment, Pa-O2 increased to >5 3 kPa in both groups. In the ventilated control group alveolar protein infl ux was greater and TLC35 and C-max were lower than in the high-PEEP group. We conclude that the pulmonary response to exogenous surfactant after mecha nical ventilation in experimental ALI is improved when a ventilation strate gy with high PEEP is used.