A DEVELOPMENTAL-STUDY OF THE DOSE-RESPONSE CURVE OF THE RESPIRATORY SENSORY REFLEX

We have shown previously that inhalation of high concentration of CO2 (about 8%) inhibits breathing in preterm infants, presumably through a n upper airway sensory reflex. To study the developmental aspects and the dose-response curve of this reflex, we studied eight preterm infan ts (body weight, 1.6 +/- 0.1 kg mean +/- SE; gestational age, 31 +/- 1 wk; postnatal age, 22 +/- 5 days) and eight term infants (body weight , 3.2 +/- 0.1 kg; gestational age, 39 +/- 1 wk; postnatal age, 8 +/- 6 days) using a flow-through system; eight adult subjects (weight, 67 /- 5 kg; age, 30 +/- 4 yr) were studied during quiet sleep using a nas al mask. We gave 2, 4, 6, and 8% CO2 in 21% O2 randomly for 20 to 30 s . A clear inhibition of breathing typically occurred during inhalation of 8% CO2 only in preterm infants, as reflected by the presence of an apnea of 11 +/- 1 s occurring at 7 +/- 2 s after the beginning of CO2 inhalation. Short apneas were occasionally observed with lower concen trations of CO2, but they were significantly fewer and shorter than wi th 8% CO2. No clear inhibition was observed in term infants or adult s ubjects, but pauses of 4 and 6 s were observed in the former group and a pause of 7 s was observed in the latter one. The associated changes in minute ventilation during inhalation of 2, 4, and 6% CO2 were not significantly different between the three groups. During inhalation of 8% CO2, minute ventilation decreased only in preterm infants (-26 +/- 10 compared with +32 +/- 10 in term infants and to +17 +/- 5% in adul t subjects; p = 0.003 between groups). These findings suggest that pre term infants have a unique response to a high concentration of CO2, pr esumably through a strong upper airway sensory reflex. In term infants and adult subjects this reflex may be masked because of the stimulati on of the peripheral chemoreceptor by CO2. The relevance of this refle x response to spontaneous apnea in preterm infants remains speculative .