ABSTRACT
The clinical defect in patients with obstructive sleep apnea (OSA) was primarily described as an inspiratory event four decades ago by Gastaut (1) as both airway closure and increased effort became evident during this phase of breathing. Other earlier studies noted increased total pulmonary resistance in the breaths preceding an apneic event and also reported that airway disturbances occur during both the expiratory and the inspiratory phases of ventilation (2,3). It is known that there must be a balanceof opposing forces that either support pharyngeal patency or encourage the pharynx tocollapse (4). The genioglossus and other major pharyngeal dilator muscles are phasically activated muscles that show a burst of inspiratoryactivity followed by a reduction in tone duringexhalation. During early exhalation, the elevated lung recoil pressure maintains pharyngeal patency but at end-exhalation when positive pressure is at a minimum,the pharynx is most susceptible to collapse (5). Other elegant imaging studies have also revealed that there is a crucial timing for positive airway pressure (PAP) therapy in patients with OSA and occurs most criticallyat the end of exhalation (6). The above studies make it easy to recognize that there are important factors occurring throughout all phases of respiration.
