Effect of continuous positive airway pressure on the ventilatory response to CO2 in preterm infants

The effect of continuous positive airway pressure (CPAP) on the ventilatory response to CO2 in newborn infants is unknown. The CO2 response to 4% CO2 in air was studied in nine preterm infants without lung disease before and during administration of CPAP (4 to 5 cm H2O) delivered by face mask. Minute ventilation, tidal volume, respiratory frequency, and end-tidal PCO2 were measured, and the slope and intercept of the CO2 response were calculated. Respiratory pattern and changes in oxygenation were also analyzed by measuring inspiratory and expiratory time, mean inspiratory flow, mean expiratory flow, effective respiratory timing, end-tidal PO2, and transcutaneous PO2. CPAP significantly decreased minute ventilation from 278.7 to 197.6 mL/min/kg (P less than .001). Tidal volume and respiratory frequency were also significantly decreased. The slope of the CO2 response during CPAP was not significantly different from the slope before CPAP (36 v 33 mL/min/kg/mm Hg, P greater than .1), but the intercept was shifted to the right (P less than .001). The decrease in respiratory frequency was primarily due to a prolongation of expiratory time (P less than .05). In addition, transcutaneous PO2 increased during administration of CPAP (P less than .001). These findings indicate that: (1) CPAP significantly decreases ventilation in preterm infants without lung disease, affecting both tidal volume and respiratory frequency; (2) CPAP does not appreciably alter the ventilatory response to CO2; (3) the changes in respiratory frequency are primarily accounted for by a prolongation of expiratory time; (4) CPAP improves oxygenation.