IN-VIVO EVALUATION OF THE INHIBITORY CAPACITY OF HUMAN PLASMA ON EXOGENOUS SURFACTANT FUNCTION

Objective: The adult respiratory distress syndrome (ARDS) and neonatal respiratory distress syndrome (RDS) are characterized by high permeab ility pulmonary edema which contains plasma-derived proteins inhibitin g pulmonary surfactant function. Currently, discussion continues as to what dose of surfactant is required for treatment of these syndromes. Design: The purpose of this study was to investigate the amount of ex ogenous surfactant needed to overcome the inhibitory components in hum an plasma. Male adult rats suffering from respiratory failure due to s urfactant depletion after whole-lung lavage received human plasma (4 m l/kg body weight) mixed with surfactant at different concentrations, i ntratracheally. Rats receiving surfactant only at different concentrat ions served as controls. Blood gas analysis was performed. Measurement s and results: It was demonstrated that plasma (4 ml/kg approximate to 273 mg plasma proteins/kg) mixed with surfactant at 300 mg/kg was abl e to increase and maintain PaO2 normal values. Plasma mixed with surfa ctant at 100 mg/kg, after initial restoration of blood gases, showed d eterioration of PaO2 values. Plasma mixed with surfactant at a dose of 50 mg/kg did not improve PaO2 whereas surfactant at 50 mg/kg, without plasma, restored blood gases to pre-lavage values. Conclusion: It is concluded that approximately 1 mg surfactant phospholipids is required to overcome the inhibitory effect of approximately 1 mg plasma protei ns. For clinical practice this means that an excess of surfactant shou ld be given, or repeatedly be substituted (''titrated'') at low concen trations, until blood gases improve.