ENHANCEMENT OF SURFACTANT EFFECT BY A MECHANICAL VOLUME RECRUITMENT MANEUVER DEPENDS ON THE LUNGS PREEXISTING DISTENSION

'Rescue' surfactant treatment of premature infants with respiratory di stress syndrome (RDS) results in two mechanisms: stabilization of alve oli already being ventilated and recruitment of alveoli collapsed befo re surfactant administration. Mechanical recruitment of alveoli from t his collapsed compartment might enhance the immediate effects of surfa ctant treatment, To test this hypothesis 15 mechanically ventilated yo ung rabbits underwent repeated airway lavage with normal saline at a c onstant, positive end-expiratory pressure (PEEP) of 3 cm H2O until bot h 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 v olume recruitment maneuver (VRM) which consisted of an increased PIP b y 33% of baseline values, The effects of surfactant were assessed usin g two indices of gas exchange (a/A ratio and PaCO2), four indices of l ung function (functional residual capacity, FRC, dynamic lung complian ce, C-rs, V-T and the alveolar portion of VT) and postmortem pressure- volume curves and were compared with two groups of rabbits receiving s urfactant 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 aft er 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 Res pir Crit Care Med 1997; 156:862-866], significant differences have bee n disclosed for C-rs, V-T and FRC. In conclusion, the enhancement of s urfactant effect by VRM largely depends on the preexisting distension of the lungs.