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

We evaluated four ventilator patterns after the administration of 80 mg/kg bovine lipid extract surfactant (LES) to anesthetized, paralyzed, saline-lavaged New Zealand white rabbits. Two ventilator types were compared: high frequency oscillatory ventilation (HFO) versus conventional mechanical ventilation (CMV), each at high (HI) and low (LO) end-expiratory 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-than-or-equal-to 39/9 cm H2O in the CMV-HI group. Five of six CMV-HI-treated animals did not maintain target Pa(O2) levels. Both ventilator type and strategy influenced outcome significantly. Animals managed with HFO had higher mean arterial pressures (p = 0.004), lower mean airway pressures (Paw) (p < 0.00008) and HCO3- requirements (p < 0.02), larger inflation (p = 0.003) and deflation (p < 0.00001) respiratory system 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), lamellar body (p < 0.00001) and lavage fluid (p = 0.0002) phospholipid levels were superior after the high EELV strategy. We conclude that ventilator pattern strongly influences exogenous surfactant efficacy. Benefits 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 recruitment maneuvers to get the lung onto the deflation limb of its pressure-volume relationship followed by small volume cycles at high rates and sufficient mean pressure to maintain alveolar volume above the point of airway/alveolar closure.