Enhancement of surfactant effect by a mechanical volume recruitment maneuver depends on the lungs preexisting distension

'Rescue' surfactant treatment of premature infants with respiratory distress syndrome (RDS) results in two mechanisms: stabilization of alveoli already being ventilated and recruitment of alveoli collapsed before surfactant administration. Mechanical recruitment of alveoli from this collapsed compartment might enhance the immediate effects of surfactant treatment, To test this hypothesis 15 mechanically ventilated young rabbits underwent repeated airway lavage with normal saline at a constant, positive end-expiratory pressure (PEEP) of 3 cm H2O until both the a/A ratio was < 0.12 and a peak inspiratory pressure (PIP) > 20 mbar was needed to keep the tidal volume (V-T) at 10 ml/kg. Surfactant (Survanta(R)) was given over 4 min in conjunction with a mechanical volume recruitment maneuver (VRM) which consisted of an increased PIP by 33% of baseline values, The effects of surfactant were assessed using two indices of gas exchange (a/A ratio and PaCO2), four indices of lung function (functional residual capacity, FRC, dynamic lung compliance, C-rs, V-T and the alveolar portion of VT) and postmortem pressure-volume curves and were compared with two groups of rabbits receiving surfactant alone or no surfactant at all. VRM yielded an increase in C-rs by +22% whereas V-T increased by +58% suggesting overdistension of terminal airways to some extent. There were no differences between the surfactant and the VRM+surfactant groups for all indices assessed after an observational period of 2 h following surfactant administration. In a previous model using a PEEP of 1 cm H2O [Krause et al., Am J Respir Crit Care Med 1997; 156:862-866], significant differences have been disclosed for C-rs, V-T and FRC. In conclusion, the enhancement of surfactant effect by VRM largely depends on the preexisting distension of the lungs.