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

Departments of Pediatrics, Physiology, and Reproductive Medicine, University of Manitoba, Winnipeg, Manitoba, Canada

Submitted 2 June 2003 ; accepted in final form 8 December 2004

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 (PA_CO2) of the last three breaths of regular breathing preceding the first apnea of an epoch of periodic breathing. We also measured the PA_CO2 of the first three breaths of regular breathing after the last apnea of the same periodic breathing epoch. In preterm infants, eupneic PA_CO2 was 38.6 ± 1.4 Torr, the preperiodic PA_CO2 apneic threshold was 37.3 ± 1.4 Torr, and the postperiodic PA_CO2 was 37.2 ± 1.4 Torr. In term infants, the eupneic PA_CO2 was 39.7 ± 1.1 Torr, the preperiodic PA_CO2 apneic threshold was 38.7 ± 1.0 Torr, and the postperiodic value was 37.9 ± 1.2 Torr. This means that the PA_CO2 apneic thresholds were 1.3 ± 0.1 and 1.0 ± 0.2 Torr below eupneic PA_CO2 in preterm and term infants, respectively. The transition from eupneic PA_CO2 to PA_CO2 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.

control of breathing; neonates; carbon dioxide

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