A simple technique for measuring alveolar CO2 in infants.

We designed a system consisting of a nostril adapter and a catheter for measurement of end-tidal ("alveolar") CO2 in small infants using the Beckman LB-1 analyzer. Using this system, we obtained a capillary-"alveolar" PCO2 difference (PaCO2-PACO2) of 2.4 Torr in 19 normal term infants, 3.5 Torr in 12 preterm infants who had recovered from respiratory distress syndrome, and 9.0 Torr in 4 preterm infants with bronchopulmonary dysplasia. We also found in 5 infants that systems using faster flow rates (140-400 ml-min-1) or slower response times (greater than 0.3 s) reduced the duration of the alveolar plateaus as well as the calculated end-tidal PCO2. In addition, when term infants were not in a steady state, PACO2 values were unreliable and significantly lower than capillary values. We therefore conclude that measurement of alveolar CO2 using our technique is a simple noninvasive way of evaluating newborn pulmonary function.