Improvement in respiratory compliance after surfactant therapy evaluated by a new method

Descriptions of the effects of intratracheally applied surfactant on respir atory system compliance (C-rs) have been somewhat controversial because the commonly used methods for assessing pulmonary function were designed for a linear pressure/volume (PN) relation of the respiratory system. In infants with lung disease a linear P/V relation cannot be expected. Therefore, a n ew method (APVNL) was employed which enabled us to calculate respiratory sy stem compliance (C-rs) and resistance (R-rs) based on changes in volume (V) . This method is independent of the PN relation, and was used to assess the effects of intratracheal instillation of surfactant. Fourteen infants (ges tational age, 24 to 30 weeks) with respiratory distress syndrome were treat ed with bovine surfactant intratracheally while the fractional inspired oxy gen concentration (FiO(2)) exceeded 50%. C-rs was evaluated for the infants using the APVNL method and the method of linear regression (LR) based on t he equation of motion designed for linear PIV relationships. Two hours after surfactant treatment, the median reduction of FiO(2) was 33 % (95% CI: 20-50%; P < 0.01). There was no correlation between the change i n FiO(2) and the change in C-rs, using either the APVNL method or the LR me thod. Two hours after surfactant treatment, the median improvement in C-rs was 0.37 mL/cmH(2)O/kg (95% CI: 0.07-1.16 mL/cmH(2)O) at a change in V of 1 mL/kg (P < 0.02) and 0.23 mL/cmH(2)O/kg (95% CI: 0-0.57 mL/cmH(2)O) at a c hange in V of 2 mL/kg (P < 0.05) when the APVNL method was used. The LR met hod could not show a significant change in C-rs after surfactant treatment. Further, R-rs did not show significant changes 2 hr after surfactant admin istration. We conclude that the APVNL method is more appropriate for evaluating change s of C-rs elicited by surfactant treatment than the LR method. The APVNL me thod demonstrated significant initial improvements in compliance as lung vo lumes were increased; there were no significant further decreases in C-rs a s peak inspiratory pressures and the upper limits of tidal volume were appr oached. (C) 2000 Wiley-Liss, Inc.