Measurement of the CO2 apneic threshold in newborn infants: possible relevance for periodic breathing and apnea.

We measured the PCO2 apneic threshold in preterm and term infants. We hypothesized that, compared with adult subjects, the PCO2 apneic threshold in neonates is very close to the eupneic PCO2, likely facilitating the appearance of periodic breathing and apnea. In contrast with adults, who need to be artificially hyperventilated to switch from regular to periodic breathing, neonates do this spontaneously. We therefore measured the apneic threshold as the average alveolar PCO2 (PaCO2) of the last three breaths of regular breathing preceding the first apnea of an epoch of periodic breathing. We also measured the PaCO2 of the first three breaths of regular breathing after the last apnea of the same periodic breathing epoch. In preterm infants, eupneic PaCO2 was 38.6 +/- 1.4 Torr, the preperiodic PaCO2 apneic threshold was 37.3 +/- 1.4 Torr, and the postperiodic PaCO2 was 37.2 +/- 1.4 Torr. In term infants, the eupneic PaCO2 was 39.7 +/- 1.1 Torr, the preperiodic PaCO2 apneic threshold was 38.7 +/- 1.0 Torr, and the postperiodic value was 37.9 +/- 1.2 Torr. This means that the PaCO2 apneic thresholds were 1.3 +/- 0.1 and 1.0 +/- 0.2 Torr below eupneic PaCO2 in preterm and term infants, respectively. The transition from eupneic PaCO2 to PaCO2 apneic threshold preceding periodic breathing was accompanied by a minor and nonsignificant increase in ventilation, primarily related to a slight increase in frequency. The findings suggest that neonates breathe very close to their PCO2 apneic threshold, the overall average eupneic PCO2 being only 1.15 +/- 0.2 Torr (0.95-1.79, 95% confidence interval) above the apneic threshold. This value is much lower than that reported for adult subjects (3.5 +/- 0.4 Torr). We speculate that this closeness of eupneic and apneic PCO2 thresholds confers great vulnerability to the respiratory control system in neonates, because minor oscillations in breathing may bring eupneic PCO2 below threshold, causing apnea.