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% O2 in each sleep state, infants were given 3% CO2 to breathe. Respiratory minute volume and frequency, tidal volume, alveolar PCO2 and PO2, CO2 response curves, and chest distortion were measured. It was found that: 1) respiratory minute volume increased and PaCO2 decreased 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 CO2 was not affected by sleep state (P > 0.4); and 4) CO2 did 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 CO2 was 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.
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.