The relationship between bedding and face-down death in infancy: Mathematical analysis of a respiratory simulation system using an infant mannequin to assess gas diffusibility in bedding

Rebreathing is a model for the relationship between a prone sleeping position and sudden infant death syndrome. This study used a mechanical simulation model to establish the relationship between types of bedding and rebreathing potential for an infant placed prone (face down) at different postnatal ages. The infant mannequin was connected to a respirator set to deliver physiologically appropriate combinations of tidal volume (V_T) and respiratory rates (RR) across a range of postnatal ages (0-18 months). Before measurements were made, CO₂ flow was regulated to 5 ± 0.1% of end-tidal PCO₂ (EtCO₂). After the model was placed in a prone position, any increase in the fractional concentration of inspired CO₂ (FiCO₂) was measured. FiCO₂ increased immediately and rapidly, and reached a maximum value within a few minutes. The maximum FiCO₂ ranged from under 2% to over 10%, depending on the bedding. FiCO₂ was also affected by V_T and RR. This model is not applicable to actual infants because of the large tissue stores of CO₂ in infants; however, it is useful for evaluation of gas diffusibility of bedding and will simplify the investigation of sleeping environments when a baby is found dead with its face covered by soft bedding. In general, the higher the FiCO₂, the greater the rebreathing potential. Theoretically, considering the paucity of body stores of O₂, changes in FiO₂ would be affected not by changes in FiCO₂, but by CO₂ production and gas movement around the infant's face. The rapid decrease of FiO₂ is approximated at the inverse of the FiCO₂ timecourse, suggesting the significance of not only CO₂ accumulation but also O₂ deprivation in the potential space around the baby's face.