The functional residual capacity of infants with respiratory distress syndrome.

Positive end-expiratory pressure (PEEP) is used in the treatment of infants with respiratory distress syndrome (RDS) to prevent atelectasis, recruit alveolar space and return the functional residual capacity (FRC) toward normal volumes. This study determined the FRC range of 15 prematurely born infants with RDS receiving PEEP. Ventilator settings were controlled clinically using predominantly results of arterial blood-gas analyses. Measurements of arterial blood-gases and FRC (N2 washout) were made during the infants' second day of life. The FRC of the infants on a PEEP of 4.5 +/- 1.3 cmH2O ranged widely from 3 to 33 ml/kg with a mean of 14.5 ml/kg; 17 +/- 2 ml/kg was considered normal. The FRC was within one SD of the mean in only three of the 15 infants (20%) and outside of two SD of normal in seven (47%). A linear regression of calculated alveolar-arterial oxygen gradient (AaDo2) with FRC yielded a correlation coefficient r = 0.825. The AaDo2 values could be used to identify six of the seven infants having FRC outside of 2SD from normal. We conclude that convential methods of PEEP selection for infants with RDS seldom result in a normalization of FRC. Calculated AaDo2 values may be used to identify most RDS infants with FRC widely divergent from normal values.