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There has been a dramatic decrease in SIDS associated with increased supine sleeping in the United States and elsewhere.1 However, the pathophysiologic basis of the detrimental effects of prone sleeping is not well understood. Skadberg and Markestad from Bergen, Norway, enrolled term healthy infants at 2.5 and 5 months in a study of physiologic and behavior patterns during successive six-hour periods of prone and supine (REM active) sleep. Twelve-hour multichannel polysonnographic recordings were used to assess sleep-related behavior. Prone REM sleep was associated with fewer short arousals, body movements, sighs, and shorter duration of apnea than supine sleep at both ages. At 2.5 months, there were fewer episodes of periodic breathing during supine sleep. At both 2.5 and 5 months, higher heart rates and peripheral skin temperatures were documented in the prone position. Young infants may be less able to maintain adequate respiratory and metabolic homeostasis during prone sleep.
The proof that the prone sleeping position contributes to SIDS is suggested by the dramatic fall in deaths from SIDS that has occurred in countries where parents have been advised to put infants to sleep in the supine position. Since the "Back to Sleep" Campaign began in 1994, there has been a further decline in the U.S. SIDS incidence from 1.17/1000 in 1993 to 0.84/1000 in preliminary statistics from 1995.1,2
While there seems to be a causal relationship between the prone sleeping position and SIDS, the pathophysiologic mechanisms are not well understood. The prone sleeping position is thought to increase the risk of SIDS by potentiating airway occlusion of CO2 rebreathing in babies with deficient arousal defense mechanisms. This paper provides additional physiologic data suggesting other mechanisms by which prone sleeping may influence the risk of SIDS. Skadberg and Markestad found that prone sleeping was associated with decreased arousals, body movements, sighs, and shorter apneas during REM vs. supine REM sleep. Furthermore, heart rate and peripheral skin temperature were higher in the prone position during both sleep states. Their findings of a decreased arousability with prone positioning confirm the previous work of Kahn et al.3 However, Skadberg and Markestad don’t discuss differences in their findings about temperature, body movements, and respiratory pattern, compared to the Kahn data.
For each of the cardiorespiratory, thermal, and behavioral findings associated with the prone position reported by Skadberg and Markestad, a similar observation has been about future SIDS victims. Previous studies that infants who later succumb to SIDS have reported higher heart rates, diminished heart rate variability, shorter respiratory pauses, and decreased arousals and movements. The finding of higher peripheral skin temperatures in the prone position is of particular interest. As discussed by the authors, thermal stress from either overwrapping or a warm environment has been identified as a risk factor for SIDS, especially in the infant sleeping prone. Evidence or excessively high temperatures have been found postmortem in SIDS victims, and victims are sometimes found drenched in sweat, indicating intense sympathetic activation. Heat dissipation relies heavily on sympathetic mechanisms by means of sweating and vasodilation. The higher peripheral skin temperatures while prone may be interpreted as an increased risk for overheating as a result of reduced heat loss from two potential mechanisms: 1) increased body surface contact with the mattress, and 2) reduced circulation of fresh air around the body because of decreased arousals and body movements.
Thus, there are a variety of mechanisms by which prone position may increase the risk of SIDS. The important message for the practicing clinician is to advise parents to place their young infants in the supine position for sleep. (Dr. Rosen is Associate Professor of Pediatrics at Yale University School of Medicine and Director of the Sleep Clinic.)
1. Gardner P, et al. Advance report of final mortality statistics, 1993. Monthly Vital Statistics Report. Hyattsville, MD: National center for Health Statistics; 1996.
2. Rosenberg H, et al. Births and deaths: United States, 1995. Monthly Vital Statistics Report. Hyattsville, MD: National Center for Health statistics; 1996.
3. Kahn A, et al. Prone or supine body position and sleep characteristics in infants. Pediatrics 1993;91:1112-1115.