CHEMORECEPTOR REFLEXES IN THE NEWBORN INFANT: EFFECT OF CO2 ON THE VENTILATORY RESPONSE TO HYPOXIA

Abstract

In the newborn infant, hypoxia has an initial stimulant but secondary depressant effect on breathing. To find out whether CO2 would augment this response, eight healthy term infants (6 hr to 11 days) were made hypoxic (Pao2, 60 mm Hg) at three different CO2 levels on 13 occasions. Ventilation was unaffected by 0.2% CO2 in air (Pao2, 110 mm Hg) and increased 6% with 0.2% CO2 plus hypoxia; with 2.3% CO2 there was a 43% increase in breathing at a Pao2 of 110 mm Hg and a 52% increase with hypoxia; with 4.4% CO2 there was a 151% increase at a Pao2 of 110 mm Hg and a 179% increase with hypoxia. This additional hyperventilation was poorly sustained, except in the 11-day-old infant, and breathing tended to decrease during the second and third minutes of hypoxia. In six additional infants, a small but significant decrease in capillary pH occurred with 4.4% CO2 plus hypoxia. Our findings suggest that CO2 does augment the initial stimulant effect of hypoxia in the newborn infant, but this is not sustained. The failure to maintain hyperventilation with hypoxia conveys no apparent benefit to the newborn infant.