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Chapter: Medical Physiology: Regulation of Respiration

Sleep Apnea

The term apnea means absence of spontaneous breathing.

Sleep Apnea

The term apnea means absence of spontaneous breathing. Occasional apneas occur during normal sleep, but in persons with sleep apnea, the frequency and duration are greatly increased, with episodes of apnea lasting for 10 seconds or longer and occurring 300 to 500 times each night. Sleep apneas can be caused by obstruction of the upper airways, especially the pharynx, or by impaired central nervous system respiratory drive.

Obstructive Sleep Apnea Is Caused by Blockage of the Upper Airway. The muscles of the pharynx normally keep thispassage open to allow air to flow into the lungs during inspiration. During sleep, these muscles usually relax, but the airway passage remains open enough to permit adequate airflow. Some individuals have an especially narrow passage, and relaxation of these muscles during sleep causes the pharynx to completely close so that air cannot flow into the lungs.

In persons with sleep apnea, loud snoring and laboredbreathing occur soon after falling asleep. The snoringproceeds, often becoming louder, and is then inter-rupted by a long silent period during which no breath-ing (apnea) occurs. These periods of apnea result in significant decreases in PO2 and increases in PCO2, which greatly stimulate respiration. This, in turn, causes sudden attempts to breathe, which result in loud snorts and gasps followed by snoring and repeated episodes of apnea. The periods of apnea and labored breathing are repeated several hundred times during the night, result-ing in fragmented, restless sleep. Therefore, patients with sleep apnea usually have excessive daytime drowsi-ness as well as other disorders, including increased sym-pathetic activity, high heart rates, pulmonary and systemic hypertension, and a greatly elevated risk for cardiovascular disease.

Obstructive sleep apnea most commonly occurs in older, obese persons in whom there is increased fat dep-osition in the soft tissues of the pharynx or compression of the pharynx due to excessive fat masses in the neck. In a few individuals, sleep apnea may be associated with nasal obstruction, a very large tongue, enlarged tonsils, or certain shapes of the palate that greatly increase resistance to the flow of air to the lungs during inspira-tion. The most common treatments of obstructive sleep apnea include (1) surgery to remove excess fat tissue at the back of the throat (a procedure called uvu-lopalatopharyngoplasty), to remove enlarged tonsils oradenoids, or to create an opening in the trachea (tra-cheostomy) to bypass the obstructed airway during sleep, and (2) nasal ventilation with continuous positiveairway pressure (CPAP).

“Central” Sleep Apnea Occurs When the Neural Drive to Respiratory Muscles Is Transiently Abolished. In a few personswith sleep apnea, the central nervous system drive to the ventilatory muscles transiently ceases. Disorders that can cause cessation of the ventilatory drive during sleep include damage to the central respiratory centersor abnormalities of the respiratory neuromuscular appa-ratus. Patients affected by central sleep apnea may havedecreased ventilation when they are awake, although they are fully capable of normal voluntary breathing. During sleep, their breathing disorders usually worsen, resulting in more frequent episodes of apnea that decrease PO2 and increase PCO2 until a critical level is reached that eventually stimulates respiration. These transient instabilities of respiration cause restless sleep and clinical features similar to those observed in obstructive sleep apnea.

In most patients, the cause of central sleep apnea is unknown, although instability of the respiratory drive can result from strokes or other disorders that make the respiratory centers of the brain less responsive to the stimulatory effects of carbon dioxide and hydrogen ions. Patients with this disease are extremely sensitive to even small doses of sedatives or narcotics, which further reduce the responsiveness of the respiratory centers to the stimulatory effects of carbon dioxide. Medications that stimulate the respiratory centers can sometimes be helpful, but ventilation with CPAP at night is usually necessary.

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