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Sleep apnea or sleep apnoea is a sleep disorder characterized by pauses in breathing during sleep. Each episode, called an apnea lasts long enough that one or more breaths are missed, and such episodes occur repeatedly throughout sleep.[1] The standard definition of any apneic event includes a minimum 10-second interval between breaths, with either a neurological arousal (a 3-second or greater shift in EEG frequency, measured at C3, C4, O1, or O2) or a blood oxygen desaturation of 3-4% or greater, or both arousal and desaturationSleep apnea is diagnosed with an overnight sleep test called apolysomnogram, or a “sleep study”.

Clinically significant levels of sleep apnea are defined as five or more episodes per hour of any type of apnea from the polysomnogram. There are three distinct forms of sleep apnea: central, obstructive, and complex (i.e., a combination of central and obstructive) constituting 0.4%, 84% and 15% of cases respectively.[2] Breathing is interrupted by the lack of respiratory effort in central sleep apnea; in obstructive sleep apnea, breathing is interrupted by a physical block to airflow despite respiratory effort. In complex (or “mixed”) sleep apnea, there is a transition from central to obstructive features during the events themselves Regardless of type, the individual with sleep apnea is rarely aware of having difficulty breathing, even upon awakening Sleep apnea is recognized as a problem by others witnessing the individual during episodes or is suspected because of its effects on the body (sequelae). Symptoms may be present for years (or even decades) without identification, during which time the sufferer may become conditioned to the daytime sleepiness and fatigue associated with significant levels of sleep disturbance.

In any person, hypoxia and hypercapnia have certain common effects on the body. The heart rate will increase, unless there are such severe co-existing problems with the heart muscle itself or the autonomic nervous system that makes this compensatory increase impossible. The more translucent areas of the body will show a bluish or dusky cast from cyanosis, which is the change in hue that occurs owing to lack of oxygen in the blood (“turning blue”). Overdoses of drugs that are respiratory depressants (such as heroin, and other opiates) kill by damping the activity of the brain’s respiratory control centers. In central sleep apnea, the effects of sleep alone can remove the brain’s mandate for the body to breathe. Even in severe cases of central sleep apnea, the effects almost always result in pauses that make breathing irregular, rather than cause the total cessation of breathing.[Normal Respiratory Drive: After exhalation, the blood level of oxygen decreases and that of carbon dioxide increases. Exchange of gases with a lungful of fresh air is necessary to replenish oxygen and rid the bloodstream of built-up carbon dioxide. Oxygen and carbon dioxide receptors in the blood stream (called chemoreceptors) send nerve impulses to the brain, which then signals reflex opening of the larynx (so that the opening between the vocal cords enlarges) and movements of the rib cage muscles and diaphragm. These muscles expand the thorax (chest cavity) so that a partial vacuum is made within the lungs and air rushes in to fill it.

Physiologic effects of central apnea: During central apneas, the central respiratory drive is absent, and the brain does not respond to changing blood levels of the respiratory gases. No breath is taken despite the normal signals to inhale. The immediate effects of central sleep apnea on the body depend on how long the failure to breathe endures. At worst, central sleep apnea may cause sudden death. Short of death, drops in blood oxygen may trigger seizures, even in the absence of epilepsy. In people with epilepsy, the hypoxia caused by apnea may trigger seizures that had previously been well controlled by medications[. In other words, a seizure disorder may become unstable in the presence of sleep apnea. In adults with coronary artery disease, a severe drop in blood oxygen level can cause angina, arrhythmias, or heart attacks (myocardial infarction). Longstanding recurrent episodes of apnea, over months and years, may cause an increase in carbon dioxide levels that can change the pH of the blood enough to cause a metabolic acidosis.