Descriptions of the effects of intratracheally applied surfactant on respiratory system compliance (C rs ) have been somewhat controversial because the commonly used methods for assessing pulmonary function were designed for a linear pressure/volume (P/V) relation of the respiratory system. In infants with lung disease a linear P/V relation cannot be expected. Therefore, a new method (APVNL) was employed which enabled us to calculate respiratory system compliance (C rs ) and resistance (R rs ) based on changes in volume (V). This method is independent of the P/V relation, and was used to assess the effects of intratracheal instillation of surfactant. Fourteen infants (gestational age, 24 to 30 weeks) with respiratory distress syndrome were treated with bovine surfactant intratracheally while the fractional inspired oxygen 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 the equation of motion designed for linear P/V 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 in FiO 2 and the change in C rs , using either the APVNL method or the LR method. 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 change in V of 2 mL/kg (P < 0.05) when the APVNL method was used. The LR method could not show a significant change in C rs after surfactant treatment. Further, R rs did not show significant changes 2 hr after surfactant administration.
We conclude that the APVNL method is more appropriate for evaluating changes of C rs elicited by surfactant treatment than the LR method. The APVNL method demonstrated significant initial improvements in compliance as lung volumes were increased; there were no significant further decreases in C rs as peak inspiratory pressures and the upper limits of tidal volume were approached.