OPTIMIZING ALVEOLAR EXPANSION PROLONGS THE EFFECTIVENESS OF EXOGENOUSSURFACTANT THERAPY IN THE ADULT-RABBIT

We evaluated four ventilator patterns after the administration of 80 m g/kg bovine lipid extract surfactant (LES) to anesthetized, paralyzed, saline-lavaged New Zealand white rabbits. Two ventilator types were c ompared: high frequency oscillatory ventilation (HFO) versus conventio nal mechanical ventilation (CMV), each at high (HI) and low (LO) end-e xpiratory lung volumes (EELV); n = 6, each group; treatment duration = 4 h. Target Pa(O2) ranges were > 350 mm Hg for groups with high EELV (i.e., HFO-HI and CMV-HI) and 70 to 100 mm Hg for those with low EELV (i.e., HFO-LO and CMV-LO). Ventilator pressures were limited to less-t han-or-equal-to 39/9 cm H2O in the CMV-HI group. Five of six CMV-HI-tr eated animals did not maintain target Pa(O2) levels. Both ventilator t ype and strategy influenced outcome significantly. Animals managed wit h HFO had higher mean arterial pressures (p = 0.004), lower mean airwa y pressures (Paw) (p < 0.00008) and HCO3- requirements (p < 0.02), lar ger inflation (p = 0.003) and deflation (p < 0.00001) respiratory syst em volumes at 10 cm inflation pressure, and higher lung lamellar body (p = 0.0006) and lavage fluid (p = 0.003) phospholipid quantities than did CMV-treated animals. The deflation P-V curve (p = 0.0004), lamell ar body (p < 0.00001) and lavage fluid (p = 0.0002) phospholipid level s were superior after the high EELV strategy. We conclude that ventila tor pattern strongly influences exogenous surfactant efficacy. Benefit s arise from keeping EELV high enough to prevent atelectasis and using small (approximately 2 ml/kg) tidal volumes to prevent overdistension . Such a pattern was readily achieved with HFO using volume recruitmen t maneuvers to get the lung onto the deflation limb of its pressure-vo lume relationship followed by small volume cycles at high rates and su fficient mean pressure to maintain alveolar volume above the point of airway/alveolar closure.