Introduction:

SIDS has the highest incidence between 1- 4 months and occurs during sleep in both the healthy term and in the preterm infant at similar corrected ages. A leading hypothesis for SIDS, which we support, is that it is due to a deficiency in the arousal response. Arousal is an important protective response to lower the risk of SIDS in the term and preterm infant. Prone {"stomach") sleeping position is the most important risk factor known for Sudden Infant Death Syndrome {SIDS). Preterm infants are at much higher risk (almost 5 times higher) for SIDS than term infants.

Our earlier sleep studies in preterm infants did not show significant effect of sleep position on the total amount of sleep, Quiet Sleep (OS) and Active Sleep (AS). The number of sleep transitions {alternate way to identify arousal by noting frequency of changes in sleep state) was more frequent at 36 wk and 1 mo during supine sleep. Recent literature reports that the response to an environmental stimulus (provoked arousal) is greater during supine compared with prone sleeping position in preterm and term infants. The effect of sleep state and sleep position on spontaneous arousals {i.e., arousals from sleep which happen without an environmental stimulus) in preterm infants has not been previously studied. In the current study a new method for scoring arousals, which measured the level of the arousal, was used to investigate the development of spontaneous arousals patterns during sleep in preterm infants. Comparisons of the frequency of the spontaneous sleep arousal patterns seen during stomach and back sleeping was done to see if fewer arousals occur with stomach sleeping. Sleep studies measuring ECG, brain wave, breathing, and a continuous video were done during a nap at 1 month and 3 months of age in the infant's home. This new method for scoring arousals categorized the level of the arousal into spinal type {involving only body movement), brainstem {body movement with heart rate and breathing changes), and cortical {which includes the above plus a change in brain wave activity). The researchers hypothesized that preterm infants will arouse less often in the stomach position. The study represents an innovative approach to measuring arousal since it is the first to apply this method to score spontaneous arousals. The findings generated by this study could lay the groundwork for novel investigations of how infants make minor and major adjustments in brain function to facilitate protective respiratory responses, which may avert a life-threatening situation and SIDS.

Method:

Fourteen premature infants GA 34 :t 2 wk and B.Wt. 2325 :t 493 g were studied in their home: 4 at corrected age of 1 m, 9 at both 1 and 3 m, and one baby at 3 m. These were "healthy" preterm infants, who had no medical problems at the time of discharge from the hospital. Daytime nap sleep recordings in stomach and back sleeping positions were visually scored for active sleep (AS) and quiet sleep (OS). Gross (movement of body and/or limbs) body movements (GBM) with durations of 3 to 60 s during sleep were scored as

spontaneous arousals. Ih addition heart rate, respiratory, and EEG changes were measured during each GBM.

Start and end of each gross body movement observed on video was used to define begin and end of arousal. Body movement episodes that lasted for more than 60 seconds were considered awakenings, and were not further analyzed.

A minimum of 10 seconds of undisturbed sleep was required before a next arousal to be scored. By a software program (Somnologica) beat to beat heart rate variation (HRV) was measured from the EGG channel. Heart rate was considered changed, if the HRV during arousal was much greater than the period before the change. The size and the strength of the breath were used to measure breathing. The number of breaths per minute and the size of the breath from moment to moment varied. It was considered significant when both breathing channels showed either lower or higher breathing rate; or altered size of the breath compared with the baseline. During an arousal, periods of lower brain wave size or faster brain waves for at least one second were considered significant. Artifact was scored if it was not possible to see the brain waves due to movement.

Results:

A total of 544 spontaneous arousals were identified. In 524 of the 544 (96%) arousals, the heart rate was increased above baseline. In 6 of the arousals, it was not possible to calculate heart rate change, due to artifact, or missing tracings (1%). In 14 arousals (3%), the heart rate was not changed. Heart rate changes were not dependent on age (p=0.3991), nor on sleeping position (p=0.8990). The breathing channels showed changes in 492 of the 544 arousals (90%). Breathing changes were not dependent on age (p=0.8847), noron sleep position (p=0.1481). The brain wave signal could not be interpreted due to artifacts in 129 arousals (24%). In 39 arousals (7%)brain wave changes were seen. No brain wave changes were seen in 137 arousals (25%). Overall, the length of arousals was shorter at 3 m compared with 1 mo. (AS: 11.5 :t 9 s vs. 17.7 :t 10 s, respectively, p=<0.0001; as: 10.5 :t 6 s vs. 14.6 :t 17 s, respectively, ns). There was no effect of position on mean arousal duration in AS at 1 or 3 m but during as at 1 m arousal was longer in supine 19.7 :t 20 s compared with prone 8.1 :t 6 s, p=0.02. There was no effect of age on arousal interval, i.e., the time between arousals. The mean duration of the time interval between successive arousals in AS was longer on stomach position (204 :t 184 s) than in back (144:t 134 s) position (p=0.05). There were no differences in as (p=0.242). There was no effect of pacifier use on duration or frequency of arousal.

Summary and Conclusion:

More frequent and longer duration of spontaneous arousals found in this study confirms earlier findings of greater arousal response in the back sleep position. These results support that back to sleep provides a protective mechanism against life threatening stimuli during sleep and could lower the risk for SIDS associated with arousal deficiency during prone sleep.

Publications:

Ariagno RL, Mirmiran M, van Leimpt S, Adams MM. The influence of sleep state and sleep position on spontaneous arousal in healthy preterm infants at 1 and 3 months corrected age. Pediatric Research, v. 51 (#4/pt.2/SS) pp. 377 A-378A APR 2002. A full-Iength paper of this study is under preparation.