LUNG-FUNCTION, SURFACTANT APOPROTEIN CONTENT, AND LEVEL OF PEEP IN PREMATURELY DELIVERED RABBITS

To study the in vivo activity of the surfactant apoproteins (SP-A, SP- B, SP-C), we administered surfactants with defined apoprotein composit ions to prematurely delivered rabbit pups. Rabbits given simple phosph olipid mixtures containing dipalmitoylphosphatidylcholine and phosphat idylglycerol supplemented with both SP-B and SP-C or either protein al one had significantly greater lung compliance during ventilation and l ung expansion during a quasi-static pressure-volume maneuver than did saline- or lipid-treated controls. The response to the surfactants con taining SP-B/C was markedly dependent on the level of end-expiratory p ressure used during ventilation. When the rabbits were ventilated with a positive end-expiratory pressure (PEEP) of 4 cmH(2)O, lung function in the pups treated with SP-B/C was not significantly different from rabbit surfactant-treated controls. Addition of SP-A to the surfactant s containing SP-B/C did not significantly further improve lung functio n if the pups were ventilated with a PEEP of 4 cmH(2)O. With a lower P EEP of 1 cmH(2)O, lung function in the pups given surfactants containi ng SP-B/C was no longer equivalent to the lung function of the rabbit surfactant-treated controls. At the lower PEEP, SP-A significantly imp roved lung function when it was added to surfactants containing SP-B a nd SP-C. No beneficial effect of SP-A was seen when the surfactant con tained either SP-B or SP-C alone. We conclude that with assisted venti lation that includes a moderate level of PEEP, SP-B and SP-C significa ntly enhance the effect of a simple phospholipid mixture on the lung f unction of prematurely delivered rabbits. At lower levels of PEEP the effects of SP-B and SP-C on lung function are markedly reduced but can be restored by the addition of SP-A.