Inspired CO2and O2in sleeping infants rebreathing from bedding: relevance for sudden infant death syndrome

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Abstract
Some infants sleep facedown for long periods with no ill effects, whereas others become hypoxemic. Rebreathing of expired air has been determined by CO2 measurement; however, O2 levels under such conditions have not been determined. To evaluate this and other factors influencing inspired gas concentrations, we studied 21 healthy infants during natural sleep while facedown on soft bedding. We 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 were1) a variable seal between bedding and infants' faces and2) 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 while 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 CO2and O2 during unsteady conditions.