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

Abstract

We studied the effects of varying degrees of hypoxia for 3 min. periods on the heart rate and respiration of 33 healthy full-term infants in a warm envirnment. During the first 5 days of live a decrease in alveolar O2 tension (PA,CO2) below 8 mm Hg induced hyperventilation, a decreased alveolar CO2 tension (PA, CO2), and tachycardia during the 1st minute of hypoxia. During the 2nd and 3rd minute, while the decreased PA, CO2 and tachycardia persisted, ventilation fell. There was a further fall in ventilation when the baby breathed 21% O2 again. This response was also observed when the inspired gas was heated to 35[degree]C During the first 5 days of life a decrease in PA,CO2 between 81 and 100 mm Hg did not affect ventilation or PA,CO2 during the 1st minute of hypoxia, but still induced a tachycardia and a fall in minute volume during the 2nd and 3rd minute. When the PA,CO2 was elevated and maintained constant during the 1st minute and fell during the 2nd and 3rd minutes, suggesting that hypocapnia did not explain the transient ventilatory response to hypoxia. After the 1st week of life a greater and maintained increase in ventilation was seen during hypoxia. This response was potentiated by the addition of CO2. The possibility that changes in the pulmonary circulation, associated with a functionally patent ductus arteriosus, may explain these differences, is discussed.