Hypoxemia, rather than hypercarbia, may be the more important factor when death occurs in infants sleeping with their faces covered by soft porous bedding
Bethesda, MD – The incidence of sudden infant death syndrome (SIDS) has decreased markedly over the past decade since recommendations were made in several countries to place infants in a nonprone position when sleeping. However, SIDS remains the leading cause of infant death beyond the neonatal period in the United States, and 20 percent of US infants continue to sleep prone. Despite the established increased risk of SIDS with prone sleep, the cause of death is still under debate. One proposed reason for infant mortality is that when an infant sleeps facedown, “rebreathing” of expired air caught in the soft porous bedding occurs.
Background
The rebreathing theory has been criticized because, although increased CO2 is biologically significant, this condition is unlikely to cause nonspecific reversible depression of neuronal excitability or rapid death. Another condition, hypoxemia (subnormal oxygenation of arterial blood) has been noted in animal models of rebreathing; however, no direct measurements of environmental oxygen (O2) in animal or human models have been made. O2 content of inspired air during rebreathing has been assumed to reciprocate CO2 levels, such that inspired O2 = room air O2 minus end-inspiration CO2. The effects of factors such as the respiratory exchange ratio on CO2 and O2 are unknown.
The observations found in previous studies suggest that complex interactions of several factors influencing gas exchange between infant and environment may influence the degree of hypercarbia and hypoxia that develop in inspired air. To further explore the consequences of infants sleeping facedown on soft bedding, a team of researchers evaluated four aspects of gas exchange: (1) infants' gas exchange with the external environment through air-channel formation in bedding, (2) infants' gas exchange with the bedding as affected by gas gradients, (3) infants' ventilatory responses to hypercapnia and hypoxia during rebreathing and gas exchanging efficiency of different respiratory patterns, and (4) direct measurement of CO2 and O2 in the rebreathing environment during periods of rapid and slow change in the gas concentration of inspired air.
The authors of the study, “Inspired CO2 and O2 in Sleeping Infants Rebreathing From Bedding: Relevance for Sudden Infant Death Syndrome,” are Aloka L. Patel, Kathy Harris, and Bradley T. Thach, all from the Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri. The study is published in the December 2001 edition of the Journal of Applied Physiology.
Methodology
Twenty-one healthy infants younger than six months old from the St. Louis community were enrolled between September and November 1999. The subjects were 5-24 weeks old. There were 8 males and 13 females. Nineteen of the 21 were born at term. At home, 4 (19 percent) slept prone or face down, 4 (19 percent) slept supine (face up) or on the side, and 13 (62 percent) slept supine. Seventeen infants (81 percent) received routine periods of prone positioning while awake and supervised.
Infants were studied during natural sleep during a morning or afternoon nap after a feeding. They were placed in a plastic crib. Bedding used in this study consisted of a corrugated foam pad covered by a polyester-filled comforter folded double (thickness of doubled comforter = 3 cm). A shallow depression (12.5 × 12.5 cm at surface, 4.5 cm deep) was cut into the foam mattress directly beneath the infant's head.
The research team measured gas exchange with the environment and bedding, ventilatory response to rebreathing, and concentrations of inspired CO2 and O2. Two important factors influencing inspired gas concentrations were (1) a variable seal between bedding and infants' faces and (2) gas gradients in the bedding beneath the infants, with O2-poor and CO2-rich air nearest to the face, fresher air distal to the face, and larger tidal volumes being associated with fresher inspired air.
During the study, it became evident that gas concentrations in bedding during rebreathing are influenced by a number of factors. To estimate the rate of flow out of the bedding, a mixture of 5 percent CO2, 13 percent O2, and balance N2 was introduced into the bedding until it was saturated. A water-filled container with a shape and mass approximating that of an infant's head was placed over the sampling catheter. Half-lives for decrease in CO2 and increase in O2 were measured during continuous sampling.
Next, the rate of CO2 flow into the bedding was estimated by determining the maximum rates of rise of CO2 when infants first assumed the facedown position. Half-life was calculated for this rate of rise. Knowing the rates of flow into and out of the bedding allowed the researchers to mathematically determine the differences in gas concentration in the bedding for continuous vs. intermittent sampling.
Results
Preliminary data revealed that there are often air channels around the infant's face, while sleeping facedown, which allow gas exchange with the environment and that they are hidden, since direct inspection does not suggest their presence. In addition, it was noted that slight movements of the infant's head could increase or decrease flow through these channels. This principle was also demonstrated by the significant rise in CO2 after the placement of cloths near infants' faces, thus increasing the bedding's effect on rebreathing.
When the infant has the ability to exchange air via air channels, the degree of rebreathing may be limited. However, this also highlights the subtleties of infant and bedding positioning, which are not immediately visible, these subtleties may explain why some infants can sleep facedown, whereas others who do so may experience dangerous asphyxia.
Conclusions Two important factors influencing inspired gas concentrations were (1) a variable seal between bedding and infants' faces and (2) gas gradients in the bedding beneath the infants, with O2-poor and CO2-rich air nearest to the face, fresher air distal to the face, and larger tidal volumes being associated with fresher inspired air. Minute ventilation increased significantly during rebreathing because of an increase in tidal volume, not frequency. The measured drop in inspired O2 was significantly greater than the accompanying rise in inspired CO2. This appears to be due to effects of the respiratory exchange ratio and differential tissue solubilities of CO2 and O2 during unsteady conditions.
These findings further advance the concept that hypoxemia, rather than hypercarbia, may be the more important factor when death occurs in infants sleeping with their faces covered by soft porous bedding.
Source
December 2001 edition of the Journal of Applied Physiology.
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.
Journal
Journal of Applied Physiology