Chemoreceptor reflexes in the new-born infant: effects of varying degrees of hypoxia on heart rate and ventilation in a warm environment.

1. We studied the effects of varying degrees of hypoxia for 3 min periods on the heart rate and respiration of thirty-three healthy full-term infants in a warm environment.2. During the first 5 days of life a decrease in alveolar oxygen tension (P(A, CO2)) below 80 mm Hg induced hyperventilation, a decreased alveolar carbon dioxide tension (P(A, CO2)), and tachycardia during the first minute of hypoxia. During the second and third minute, while the decreased P(A, CO2) and tachycardia persisted, ventilation fell. There was a further fall in ventilation when the baby breathed 21% O(2) again. This response was also observed when the inspired gas was heated to 35 degrees C.3. During the first 5 days of life a decrease in P(A, CO2) between 81 and 100 mm Hg did not affect ventilation or P(A, CO2) during the first minute of hypoxia, but still induced a tachycardia and a fall in minute volume during the second and third minute.4. When the P(A, CO2) was elevated and maintained constant during hypoxia, ventilation increased during the first minute and fell during the second and third minutes, suggesting that hypocapnia did not explain the transient ventilatory response to hypoxia.5. After the first week of life a greater and maintained increase in ventilation was seen during hypoxia. This response was potentiated by the addition of CO(2).6. The possibility that changes in the pulmonary circulation, associated with a functionally patent ductus arteriosus, may explain these differences, is discussed.