Subsequent siblings of infants who died of the Sudden Infant Death Syndrome are at a four- to six-times increased risk to die of this syndrome. This study compares the respiratory development during sleep state of this epidemiologic high risk group with that of normal infants during the first six months of life. Subsequent siblings exhibited higher respiratory rates in all states at 3 months of age. Quiet sleep and indeterminate respiratory rates were elevated at 1 week of age compared to control infants. Indeterminate respiratory rates remained higher at 6 months of age. These differences were accompanied by a reduced incidence of total breathing pauses of two to five seconds and six to nine seconds duration in siblings. Study groups could not be differentiated on the basis of either breathing pauses of more than ten seconds or central apnea of six seconds or more. Obstructive and mixed apnea (6 seconds or more) were infrequently observed in these study groups. A high degree of intersubject variability characterized all data on breathing pauses.

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