Interaction of cross-sectional area, driving pressure, and airflow of passive velopharynx

Previous studies have shown that, when the pharyngeal muscles are relaxed, the velopharynx is a highly compliant segment of the pharynx. Thus, under these circumstances, cross-sectional area of the velopharynx (A(VP)), driving pressure across the velopharynx (Delta P), and inspiratory airflow ((V) over dot I) will be mutually interdependent variables. The purpose of the present investigation was to describe the interrelation among these three variables during inspiration. We studied 15 sleeping patients with obstructive sleep apnea/hypopnea when the pharyngeal muscles were rendered hypotonic by applying continuous positive airway pressure to the nasal airway. A(VP), determined by endoscopic imaging, was significantly greater at onset of (V) over dot I limitation than at minimum oropharyngeal pressure (P < 0.01). Snoring was never observed during (V) over dot I limitation. In a subgroup of six patients, values for Delta P, (V) over dot I, and A(VP) were obtained at 0.1-s intervals at various levels of mask pressure. For these six patients, the mathematical expression (V) over dot I = 0.657(A(VP)/A(max)).Delta P-0.332, where A(max) is maximal A(VP), described the relationship among the three variables (R-2 = 0.962) for flow-limited and non-flow-limited inspirations. The impedance of the passive velopharynx, defined as Delta P-0.33/(V) over dot, was inversely related to A(VP) and increased dramatically when A(VP) was <0.3 cm(2). In summary, we observed a progressive decrease in A(VP) during flow-limited inspiration in patients with obstructive sleep apnea. This constriction of the velopharynx contributes to an increase in velopharyngeal impedance that, in turn, counterbalances the increase in Delta P during flow limitation.