We made 84 studies on 20 "healthy" preterm infants during the first 34 days of life to discover whether infants breathing periodically hypoventilate or hyperventilate and whether the major defect is at the central or peripheral chemoreceptors. Six infants breathed periodically (n = 26) and seven regularly (n = 27) at all times; seven infants breathed periodically (n = 15) or regularly (n = 16) in different studies. In these three groups (periodic, regular, and intermediate), we compared respiratory minute volume and frequency, heart rate, alveolar oxygen tension (PAO2) and alveolar carbon dioxide tension (PACO2) and PO2, PCO2 and pH of arterialized capillary blood, alveolar-capillary differences for PO2 and PCO2, peripheral chemoreceptor sensitivity and CO2 responses. We measured ventilation with a nosepiece and a screen flowmeter. The mean values for the intermediate and periodic groups were similar. There were major differences between the periodic and regular groups. The infants breathing periodically (1) hypoventilated, (2) showed a significant shift of the CO2 response curve to the right with a 22% decrease in slope, and (3) had an increased response to O2. However, the two groups had similar alveolar-capillary PO2 and PCO2 differences. These findings suggest that the major defect is not in the lungs or at the peripheral chemoreceptors but at the respiratory center (or central receptors).

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