|M.Sc Student||Itzhak Nadav|
|Subject||Aerodynamic and Geometric Factors Affecting Rebreathing in|
|Department||Department of Mechanical Engineering||Supervisor||Professor David Greenblatt|
|Full Thesis text|
The rebreathing of expired air, with high carbon dioxide and low oxygen concentrations, has long been implicated in unexplained Sudden Infant Death Syndrome (SIDS) when infants are placed to sleep in a prone (facedown) position. Systematic in-vitro experiments were performed to determine the physical mechanism and to elucidate parameters associated with rebreathing in sleeping infants. The effect of the aerodynamic parameters: Reynolds number, Strouhal number, and Froude number as well as different anatomical (e.g. tidal volume, nostril diameter), physiological (e.g. breathing frequency) and environmental (e.g. temperature, distance from the surface, nasal orientation, bedding texture) factors were considered. A nasal module was designed to serve as a simplified geometric representation of an infant's nostrils and placed above a bedding surface. Quantitative and flow visualization experiments were performed to measure rebreathing using water as the working medium, under conditions of dynamic similarity.
The main conclusions were as follows: In the prone position, the interaction of the exhaled air with the bedding surface results in forming vortices in the vicinity of the nares. Increases in Strouhal number (faster, shallower breathing), always produced higher rebreathed percentages, because rolled-up vortices in the vicinity of the nostrils had less time to move away by self-induction. Positively and negatively buoyant flows resulted in significant rebreathing. In the latter case, consistent with a warm environment or a high percentage of rebreathed CO2, denser gas pooled in the vicinity of the nostrils. Reynolds numbers below 200 also dramatically increased rebreathing because the expired gas pooled much closer to the nostrils. The surface texture (bedding) also had a profound effect on rebreathing. In the case of a simulated woolen surface (polyester mat), the mat fibers prevent the rollup of large vortices, producing smaller seemingly random vortices that are not transported away from the nares. Rebreathing percentages consistently increased with increases in infant nostril diameter. This finding strongly suggests that infants with larger nostril diameters, perhaps associated with certain racial and ethnic groups, are at a much greater risk of rebreathing. Furthermore, the results offer plausible explanations of why a high-temperature environment and low birthweight are SIDS risk factors.
This research has important ramifications for the study of SIDS because the dependence of rebreathing on aerodynamic parameters, based on physiological, anatomical and environmental factors has been quantified for the first time. Our results provide clear quantitative evidence of the factors that place prone-sleeping infants at risk of rebreathing and illustrate multiple pathways to rebreathing. Moreover, the results suggest the existence of a new risk factor, namely nostril diameter, for rebreathing.