Maturational Effects on Respiratory Responses to Carbon Dioxide in Premature Infants

The respiratory responses of 18 premature infants to breathing steady-state levels of carbon dioxide were quantified by conventional minute ventilation measurements and by measuring the pressure generated after end-expiratory airway occlusions. Since no flow or volume changes occur following airway occlusion, the CO2 response obtained by this measurement is independent of vagal stretch receptor influence and mechanical changes in the lungs. When studied at 3-4 days of postnatal age, a group of 29-32-wk gestation infants had a mean slope of ventilatory response to CO2 of 10.8 +/- 5.6 ml-min-1-kg-1-mmHg-1 as compared to 53.5 +/- 10.8 ml-min-1-kg-1-mmHg-1 (P less than 0.005) for a group of 33-36 wk. When these infants were restudied at 7-10 days of age, the slopes were 21.6+/- 6.6 and 58.4 +/- 10.6 ml-min-1-kg-1-mmHg-1 (P less than 0.01), respectively. The mask pressure response paralleled minute ventilation. We conclude that the increase in ventilatory response to carbon dioxide seen with maturation is not due only to changing mechanical features of the lungs, but to increased sensitivity of respiratory centers to carbon dioxide.