Chemoreceptor reflexes in preterm infants: II. The effect of gestational and postnatal age on the ventilatory response to inhaled carbon dioxide.

We studied nine "healthy" preterm infants (birthweight, 1,000 to 2,000 gm) 58 times during postnatal life to define the effects of gestational and postnatal age on the ventilatory response to carbon dioxide. The infants were given air and 2% and 4% carbon dioxide in air to breathe for five minutes each. We determined respiratory minute and tidal volumes, frequency,heart rate, and alveolar PCO2 and PO2. We measured ventilation with a nosepiece and a screen flowmeter, using a constant flow-through to eliminate valves and reduce diad space. Analysis were made during the fifth minute while the baby breathed the various gas mistures. The slope of the carbon dioxide response increased 42% from 32 to 37 weeks gestation (P smaller than .05) and 62% from 2 to 27 days of age (P smaller than. 025). However, the intercept at .3 liter/min/kg was the same at different gestational ages, but significantly greater at 2 compared with 27 days of age (P smaller than.05). We sugest that the unresposiveness primarily dependent on the mechanical abnormalities of the lung.