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The differences in the immediate (30 sec or l min) and late (5 min) ventilatory response to high and low O2have 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 CO2at various O2concentrations in preterm infants (12). Thus, 9 preterm infants and 10 adult subjects were given 21 O2to breathe and then 100 or 15 O2for 5 min each. Adults also breathed 15 O2before 100 O2or 12 O2in order to make their resting arterial PO2more comparable to those of infants breathing 21 O2. The ventilatory response to 100 O2was the same in preterm infants and adult subjects, but the late response to 15 O2remained 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 O2being different in infants and adult subjects is false; 2) the notion that the response to low O2is paradoxical in infants is correct; and 3) the data do not explain why the response to CO2under various background concentrations of O2in 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 CO2during low O2breathing.

Summary: The authors studied 10 preterm infants (birth weight, 1840 ± 270 g; gestational age, 31 ± 3 wk) and 10 term infants (birth weight, 3700 ± 320 g; gestational age, 40 ± 1 wk) to evaluate the effect of sleep state on chest distortion and on the ventilatory response to CO2. Sleep state was defined on the basis of body movements, eye movements, and electroencephalogram. Chest distortion was assessed using micromagnetometers applied to the rib cage and abdomen. After a control period breathing 21% O2in each sleep state, infants were given 3% CO2to breathe. Respiratory minute volume and frequency, tidal volume, alveolar PCO2and PO2, CO2response curves, and chest distortion were measured. It was found that: 1) respiratory minute volume increased and PaCO2decreased during REM as compared to non-REM sleep in preterm and term infants (P < 0.05); 2) chest distortion was not affected by sleep state, but was more frequent in preterm than in term infants (P < 0.02); 3) the ventilatory response to CO2was not affected by sleep state (P > 0.4); and 4) CO2did not affect chest distortion (P > 0.1). These findings indicate: 1) contrary to previous observations, chest distortion is independent of sleep state; and 2) the ventilatory response to CO2was not affected by sleep state. The authors suggest that the higher prevalence of chest distortion in preterm infants is related to their highly compliant chest wall rather than to differences in sleep state.Speculation: Teleologically, almost anything increases with gestational age. Chest stability, non-REM sleep, ventilatory response to CO2, and prevalence of regular breathing, all increase with maturation. The authors would like to speculate, therefore, that the differences in chest distortion are immaturity rather than sleep state dependent. The authors believe that distortion is present not only because the rib cage muscles are weak or chest wall reflexes inefficient, but also because the bone structure of the rib cage is highly cartilagenous and cannot afford stability.

We measured the frequency distribution and the ventilatory correlates of the various types of apneas 3 to 15 s long during sleep in eight term infants (birth weight 3.65 ± 0.16 kg; gestational age 39.5 ± 0.3 wk) and eight preterm infants (birth weight 2.07 ± 0.18 kg; gestational age 34.3 ± 0.4 wk). Each infant was studied on five to seven occasions from birth to 56 wk of postconceptual age using a modified flow-through system. Sixty-six paired epochs of quiet sleep (1163 min) and rapid eye movement sleep (829 min) were analyzed in term infants and 85 paired epochs of quiet sleep (1553 min) and rapid eye movement sleep (1328 min) in preterm infants. Of the 783 apneas recorded in term infants 82 were central, 1.5 obstructive, 0.5 mixed, and 16 were of the breath-holding type; the corresponding figures for the 4086 apneas recorded in preterm infants were 93, 0.5, 1.0, and 5.5. This distribution was similar in the two sleep states but term infants had a higher percentage of breath-holding apneas than preterm infants (p< 0.01). In preterm infants the rate of central apneas decreased with postnatal age (p< 0.01); in term infants the rate did not change significantly. The duration of apneas showed a modal distribution for central apneas at about 8 s for both groups during the 1st month of life (p< 0.05). The findings suggest 1) apneas in the newborn and early infancy are primarily central and are more frequent in preterm than in term infants; 2) the higher rate of apnea in healthy preterm infants is accounted for almost entirely by the higher rate of central apneas; 3) a significant decrease in the rate of apnea occurs during the first 4 months after birth; and 4) preterm infants show longer respiratory pauses in both quiet sleep and rapid eye movement sleep when compared to term infants, and a maturation pattern can be discerned by 3 months of age.