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Objectives: To investigate whether a history of maternal tobacco smoking affected the maturation of arousal responses and whether sleeping position and infant age alters these relations. Design: Healthy term infants (13 born to mothers who did not smoke and 11 to mothers who smoked during pregnancy) were studied using daytime polysomnography on three occasions: (a) two to three weeks after birth, (b) two to three months after birth, and (c) five to six months after birth. Multiple measurements of arousal threshold in response to air jet stimulation were made in both active sleep (AS) and quiet sleep (QS) when infants slept both prone and supine. Results: Maternal smoking significantly elevated arousal threshold in QS when infants slept supine at 2-3 months of age (p<0.05). Infants of smoking mothers also had fewer spontaneous arousals from QS at 2-3 months in both prone (p<0.05) and supine (p<0.001) sleeping positions. In infants of non-smoking mothers, arousal thresholds were elevated in the prone position in AS at 2-3 months (p<0.01) and QS at 2-3 weeks (p<0.05) and 2-3 months (p<0.001). Conclusions: Maternal tobacco smoking significantly impairs both stimulus induced and spontaneous arousal from QS when infants sleep in the supine position, at the age when the incidence of sudden infant death syndrome is highest.

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For some time it has been suggested that breathing movements are made "in utero" and recently measurements of tracheal pressure and lung liquid flow in chronic fetal preparations have led to the hypothesis that rapid changes in these parameters are the result of respiratory muscle activity. To test this hypothesis diaphragmatic electrical activity was measured in seven chronic unanesthetized fetal sheep preparations and correlated with lung liquid flow and tracheal pressure. Diaphragmatic activity led to a fall of tracheal pressure and movement of a small volume of lung liquid into the lung. After the activity ceased, tracheal pressure returned to normal and flow diminished to zero or was directed out of the lung. The breathing pattern was unassociated with the net movement of lung liquid out of the lung. A histogram of the interval between breaths revealed a changing pattern of activity throughout gestation. The pattern was significantly altered after premature delivery of one animal with a respiratory problem. These observations provide evidence that respiratory muscles are active "in utero" and that the pattern of activity changes throughout gestation.

The incidence of sudden infant death syndrome has been found to be consistently higher in preterm and low birth weight infants than in infants born at term. Failure to arouse from sleep is one possible mechanism for sudden infant death syndrome. This study compared the arousal responses to nasal air-jet stimulation in a longitudinal study between groups of healthy preterm and term infants. Preterm infants (n = 9) were born at 31-35 wk gestation with normal birth weights for gestational age and studied on three occasions: a preterm study at 36 wk, at 2-3 wk post-term, and at 2-3 mo post-term. Term infants (n = 22) were born at 37-42 wk and were studied at 2-3 wk and 2-3 mo post-term. Arousal thresholds were determined in both active sleep (AS) and quiet sleep (QS). In preterm infants, there was no state-related difference in arousal thresholds at either the 36 wk or 2-3 wk study; however, at 2-3 mo, arousal threshold was significantly greater in QS than AS (p < 0.05). In contrast, in term infants, arousal thresholds were significantly elevated in QS compared with AS at both 2-3 wk and 2-3 mo (p < 0.001). Arousal thresholds in AS were not different between the two groups of infants, with both groups of infants remaining readily arousable. However, in QS at 2-3 mo, arousal thresholds were significantly lower in the preterm infants (p < 0.05). This study has demonstrated that arousability is altered by gestational and postnatal age. The lower arousability that characterizes QS in term infants regardless of age is not evident in preterm infants until 2-3 mo post-term age.

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The progressive decrease in the periodic cycle duration (PCD) of periodic breathing with postnatal age in term infants has been previously reported by a number of authors and is thought to be associated with peripheral chemoreceptor maturation. We hypothesized that a similar decrease should be observed in preterm infants. Therefore, in this study we measured the changes in PCD with postnatal age in a small group of preterm ( n = 4) infants followed longitudinally (36 afternoon nap studies) over the first 6 mo postnatally. PCD declined in these infants from 17.1 ? 3.3 s (mean -t-2 SD) at 9 d to 9.8 +-3.2 s (mean I 2 SD) at 105 d. The regression slope was -0.072 s/d. Beyond 105 d there was no change in PCD up to 6 mo postnatally. We found no significant difference between active and quiet sleep. These results are similar to results previously published in term infants but apparently contradict recent data on a group of preterm infants. Possible reasons for this discrepancy are discussed. By examining long epochs of periodic breathing in these infants we also identified characteristic changes in PCD and VIA ratio, defined as the duration of the ventilatory period divided by the duration of the apneic interval. VIA ratio fell from the start of an epoch from 1.21 2 0.08 (mean + SEM) to a minimum of 0.62 2 0.03 and then increased again to 0.8 -C 0.05 at the end of the epoch. We conclude that important maturational changes occu~ in the neonatal respiratory control system during the first 6 mo postnatally and that these changes are reflected in a fall in PCD of periodic breathing over this period. We also conclude that the characteristic changes which occur in VIA ratio and PCD are consistent with a role for chemical stimuli in the genesis of periodic breathing. (Pediatr Res 38: 533-538, 1995) Abbreviations SIDS, sudden infant death syndrome VIA, ratio of the duration of ventilation to the duration of apnea in one cycle of periodic breathing To, start time of a long epoch of periodic breathing Tmi,, time from To at which VIA ratio reaches its minimum value T,,,, end time of a long epoch of periodic breathing PCD, periodic breathing cycle duration A PCD, change in PCD during long epochs PNA, postnatal age PNA,,, postnatal age at which PCD ceases to decrease Sao,, arterial oxygen saturation ASao,, deviation between the mean Sao, and the peak or trough of the oscillation in Sao, during periodic breathing Periodic breathing has been studied extensively in both preterm and term infants. These studies have been aimed primarily at elucidating the causes of this type of breathing pattern in neonates and establishing the incidence during the first months of life (1,2).Much of the incentive for these studies came from earlier clinical observations that long apneas were often preceded by periodic breathing in preterm infants (3), although this finding has been disputed more recently (4). Also, a high incidence of periodic breathing has been reported in apparent life-threatening events and SIDS sibling infants when compared with ag...