A method of estimating the functional residual capacity of infants with respiratory distress syndrome.

This report describes and evaluates a new method of estimating the functional residual capacity (FRC) of newborn infants receiving continuous positive airway pressure (CPAP). Standard nitrogen washout methods of measuring lung volume are time consuming, frequently interfere with patient care, and may be hazardous to prematurely born infants. The authors are using a four-breath nitrogen washout technique to estimate the FRC of infants. The method was evaluated using: (1) a mechanical lung model, and (2) results from 32 newborn infants with RDS. The actual volume of the mechanical lung model was 21.6 ml and the estimated volume was 21.4 +/- 2.3 (SD) n = 13. Using different volumes in the mechanical lung model and comparing with estimated FRCs yielded a correlation coefficient of 0.96 (n = 15). Comparing the FRC of infants determined by standard nitrogen washout with the estimated FRC yielded a correlation coefficient of 0.92, n = 145. Thus, the results of the two methods are in good agreement. The new method reduces the period of breathing pure oxygen from several minutes to just a few seconds, thus, decreasing the dangers of absorption atelectasis and oxygen toxicity. The new system also lends itself well to micro-processor automation.