The differences in the immediate (30 sec or l min) and late (5 min) ventilatory response to high and low O2 have not been quantitated in preterm infants and adult subjects using the same methods. It was thought that these differences might explain the paradoxical ventilatory response to CO2 at various O2 concentrations in preterm infants (12). Thus, 9 preterm infants and 10 adult subjects were given 21% O2 to breathe and then 100 or 15% O2 for 5 min each. Adults also breathed 15% O2 before 100% O2 or 12% O2 in order to make their resting arterial PO2 more comparable to those of infants breathing 21% O2. The ventilatory response to 100% O2 was the same in preterm infants and adult subjects, but the late response to 15% O2 remained paradoxical, ventilation decreasing at 5 min by 18% in infants and increasing by 19% in adults. The authors conclude: 1) the traditional concept of the ventilatory response to 100% O2 being different in infants and adult subjects is false; 2) the notion that the response to low O2 is paradoxical in infants is correct; and 3) the data do not explain why the response to CO2 under various background concentrations of O2 in infants is the reverse of that in adult subjects, but the depressed ventilatory response to hypoxia in infants may justify, at least in part, their flatter response to CO2 during low O2 breathing.
Speculation The findings suggest that the response of preterm infants to high and low O2 per se is not the cause of the paradoxical response to CO2 under various background concentrations of O2. If it were, it would be expected that the response to low and high O2 would differ in infants and adults. This was true for hypoxia only, the response to hyperoxia being the same in infants and adults. The speculation, therefore, is that differences in cerebral blood flow caused by CO2 and O2 interaction may be responsible for the paradoxical response to CO2.