Phasic respiratory pharyngeal mechanics by magnetic resonance imaging in lean and obese Zucker rats

Rationale: Although obstructive sleep apnea is strongly associated with obesity, we have little understanding of how obesity may alter the mechanical properties of the pharynx and the role of obesity in the pathogenesis of sleep apnea. Objectives: The overall objective of this study was to determine the effect of obesity on pharyngeal airway size and pharyngeal wall tissue strain in lean and obese Zucker rats. Methods: Respiratory-gated magnetic resonance imaging with non-invasive tissue tagging was performed in anesthetized, spontaneously breathing lean (n = 9) and obese (n = 9) Zucker rats. Images acquired during expiration and inspiration of the rostral, mid-, and caudal pharynx were analyzed for airway size and pharyngeal wall tissue strain, using planimetry, optical flow, and finite element analyses. Differences in cross-sectional airway area, lateral and anteroposterior airway diameters, and tissue strain (stretch, compression, and direction of stretch) in the lateral and ventral pharyngeal walls were compared by analysis of variance (significance at p < 0.05). Measurements and Main Results: Compared with their lean littermates, obese rats had the following significant findings: reduced pharyngeal airway cross-sectional area during inspiration and expiration, smaller increases in airway area during inspiration, and decreased lateral airway dilation during inspiration. Tissue strain in the pharyngeal walls showed no significant differences between obese and lean rats. Conclusions: These findings suggest that obesity results in a mechanical abnormality that decreases pharyngeal airway size and prevents a normal airway response to a given change in pharyngeal wall tissue strain.