Obstructive Sleep Apnea, Anesthesia, and Ambulatory Surgery

There are 3 different pharynxes (Figure 1). The nasopharynx, or velopharynx, is the airspace posterior to all of the soft palate; the oropharynx is the airspace posterior to the tip of the uvula to the tip of the epiglottis; and the laryngopharynx, or hypopharynx, is the airspace posterior to the tip of the epiglottis to the vocal cords. The adult human is one of only a few animals to have a space between the uvula and the epiglottis. All other mammals, and even the newborn human, have an interlocking uvula and epiglottis. As the newborn grows, the larynx descends and the space between the uvula and epiglottis increases. As a result, only the adult human has an upper airway that is essentially a long, soft-walled tube with no bony support anteriorly or laterally. This long, soft-walled tube gives the adult human the potential to have obstructive sleep apnea (OSA). The tube remains open as a result of the action of muscles. The muscles that keep the upper airway open are as follows: • The tensor palatine retracts the soft palate away from the posterior pharyngeal wall, thereby maintaining the patency of the retropalatal nasopharynx. • The genioglossus moves the tongue anteriorly to open the retroglossal oropharynx. • The geniohyoid, sternohyoid, and thyrohyoid muscles move the epiglottis forward by tensing the hyo epiglottic ligament, thereby enlarging the retroepiglottic laryngopharynx. When we sleep, muscle tone throughout the body is lost; the deeper the sleep, either natural or pharmacologically induced, the greater the relaxation. The 2 major stages of sleep are non–rapid eye movement (NREM) and rapid eye movement (REM). Within NREM sleep are 4 substages that represent progressive slowing of brain electrical activity. Deep and restorative sleep occurs during the deeper NREM and REM sleep stages, and it is nocturnal deep and restorative sleep that allows one to function the next day without requiring diurnal sleep. During deep and restorative sleep, the pharyngeal muscles participate in the loss of muscle tone. The loss of pharyngeal muscle tone always causes some extent of pharyngeal collapse. If the loss of pharyngeal muscle tone and pharyngeal collapse is partial but sufficient to cause the inspired air to flutter around the uvula, tongue, or epiglottis, snoring and hypopnea (formally defined as a decrease in air flow more than 50% of awake value for more than 10 seconds) will result. If the loss of pharyngeal muscle tone and pharyngeal collapse leads to complete obstruction, the result will be silence and apnea. Apnea, defined as no air flow for longer than 10 seconds despite continuing ventilatory effort, and hypopnea during sleep are considered sleep-disordered breathing (Figure 2). To survive each obstructive episode, the patient must have some sort of arousal. In the vast majority of instances, the period of arousal is brief. These periods of “mini-arousal” are expressed in the brain as bursts on an electroencephalograph, as extremity movement or turning, vocalization, or some combination of the 3. ANUSHIRVAN MINOKADEH, MD