Sleep apnea is a disorder characterized by a reduction or cessation (pause of breathing, airflow) during sleep. It is common among adults but rare among children. There are two types of sleep apnea, the more common obstructive sleep apnea and the less common central sleep apnea, both of which will be described later in this article. Although a diagnosis of sleep apnea often will be suspected on the basis of a person's history, there are several tests that can be used to confirm the diagnosis. The treatment of sleep apnea may be either surgical or nonsurgical.
An apnea is a period of time during which breathing stops or is markedly reduced. In simplified terms, an apnea occurs when a person stops breathing for 10 seconds or more. So, if normal breath airflow is 70% to 100%, an apnea is if you stop breathing completely, or take less than 25% of a normal breath (for a period that lasts 10 seconds or more). This definition includes complete stoppage of airflow. (Other definitions of apnea that may be used include at least a 4% drop in the saturation of oxygen in the blood, a direct result of the reduction in the transfer of oxygen into the blood when breathing stops.)
Apneas usually occur during sleep. When an apnea occurs, sleep is disrupted. Sometimes this means the person wakes up completely, but sometimes this can mean the person comes out of a deep level of sleep and into a more shallow level of sleep. Apneas are usually measured during sleep (preferably in all stages of sleep) over a two-hour period. An estimate of the severity of apnea is calculated by dividing the number of apneas by the number of hours of sleep, giving an apnea index (AI). The greater the AI, the more severe the apnea.
A hypopnea is a decrease in breathing that is not as severe as an apnea. So, if normal breath airflow is 100% to 70%, a hypopnea is 69% to 26% of a normal breath. Like apneas, hypopneas are associated with a 4% or greater drop in the saturation of oxygen in the blood and usually occur during sleep. Also like apneas, hypopneas usually disrupt the level of sleep. A hypopnea index (HI) can be calculated by dividing the number of hypopneas by the number of hours of sleep.
The apnea-hypopnea index (AHI) is an index of severity that combines apneas and hypopneas. Combining them both gives an overall severity of sleep apnea including sleep disruptions and desaturations (a low level of oxygen in the blood). The apnea-hypopnea index, like the apnea index and hypopnea index, is calculated by dividing the number of apneas and hypopneas by the number of hours of sleep. Another index that is used to measure sleep apnea is the respiratory disturbance index (RDI). The respiratory disturbance index is similar to the apnea-hypopnea index, however, it also includes respiratory events that do not technically meet the definitions of apneas or hypopneas, but do disrupt sleep.
Sleep apnea is formally defined as an apnea-hypopnea index of at least 15 episodes/hour in a patient without medical problems that may be related to the sleep apnea. (That is the equivalent of one episode every 4 minutes.) In a patient with high blood pressure, stroke, daytime sleepiness, ischemic heart disease (low flow of blood to the heart), insomnia, or mood disorders—all of which can be caused or worsened by sleep apnea--sleep apnea is defined as an apnea-hypopnea index of at least 5 episodes/hour. This definition is stricter because the patient may be already experiencing the negative medical effects of sleep apnea, and it may be important to begin treatment at a lower apnea-hypopnea index.
What are the types of sleep apnea?
There are three types of sleep apnea, central sleep apnea (CSA), obstructive sleep apnea (OSA), and mixed sleep apnea (both central sleep apnea and obstructive sleep apnea).
During sleep, the brain instructs the muscles of breathing to take a breath. Central sleep apnea (CSA) occurs when the brain does not send the signal to the muscles to take a breath, and there is no muscular effort to take a breath. Obstructive sleep apnea (OSA) occurs when the brain sends the signal to the muscles and the muscles make an effort to take a breath, but they are unsuccessful because the airway becomes obstructed and prevents the flow of air. The third type of sleep apnea, mixed sleep apnea, occurs when there is both central sleep apnea and obstructive sleep apnea.
What is central sleep apnea and what causes it?
Central sleep apnea (CSA) occurs when the brain does not send the signal to breathe to the muscles of breathing. This usually occurs in infants or in adults with heart disease, cerebrovascular disease, or congenital diseases but also can be caused by some medications and high altitudes.
Central sleep apnea may occur in premature infants (born before 37 weeks of gestation) or in full term infants. It is defined as apneas lasting more than 20 seconds, usually with a change in the heart rate, a reduction in blood oxygen, or hypotonia (general relaxation of the body’s muscles). These children often will require an apnea monitor that sounds an alarm when apneas occur. Central sleep apnea is not the same thing as sudden infant death syndrome (SIDS).
Under normal circumstances, the brain monitors several things to determine how often to breathe. If it senses a lack of oxygen or an excess of carbon dioxide in the blood, it will speed up breathing. The increase in breathing increases the oxygen and decreases the carbon dioxide in blood. Some people with heart disease have an increase in carbon dioxide in their blood at all times. When there is a chronic (long term) increase in blood carbon dioxide, the brain starts to ignore the oxygen level and monitors the blood carbon dioxide level to determine when to take the next breath. The control of breathing also becomes slower to respond to changes in carbon dioxide levels. Thus, if a person takes more or deeper breaths and “blows off” carbon dioxide, the drive to breathe decreases, and the rate of breathing decreases. After a while, the carbon dioxide builds back up in the blood, and the rate of breathing increases. The brain, slow to adjust, continues to signal for more rapid breathing until the carbon dioxide level drops too low. Breathing then slows down or stops until the carbon dioxide level rises again. This pattern of abnormal breathing is called Cheyne-Stokes breathing (after the men who described it). It is characterized by repetitive cycles of fast breathing followed by slow breathing and apnea. This breathing pattern happens when the person is awake or asleep, but becomes more of a problem when asleep. Some patients with heart failure have central sleep apnea associated with a Cheyne-Stokes pattern of breathing.
Central sleep apnea usually occurs in adults with other medical problems. In infants, it usually occurs with prematurity or other congenital disorders. In both patient groups it is usually suspected by the primary physician. Central sleep apnea can be diagnosed with a sleep study or overnight monitoring while the patient is in the hospital. In infants, central sleep apnea is treated with an apnea alarm. This alarm monitors the infant’s breathing with sensors and sounds a loud noise when the infant experiences an apnea. The alarm usually wakes the infant and the parents. Most infants usually “out-grow” the central apnea episodes, so the alarm monitoring is stopped after the episodes resolve. In infants with other congenital problems, apnea monitoring may be needed for a longer period. In adults with central sleep apnea, the apneas are treated by treating the underlying heart disease, medication interaction, high altitude, or other primary problem
What is obstructive sleep apnea and what causes it?
An apnea event has four components.
1. First, the airway collapses.
2. Second, an effort is made to take a breath, but is unsuccessful.
3. Third, the oxygen level in the blood drops.
4. Finally, when the amount of oxygen reaching the brain decreases, the brain signals the body to wake up and take a breath. (This is what the bed partner hears as a silence followed by a gasp for air.)
First, it is necessary to describe a "normal breath." A normal breath of air passes through the nasal passages, behind the palate, uvula, and tongue base, through the throat muscles, and between the vocal cords into the lungs.
Airflow can become diminished if a person has a deviated septum (the middle wall of the nose). A septum can be deviated to one or both sides narrowing the air passages. There are filters in the nose called turbinates that can obstruct airflow when they become swollen.
If the palate and uvula (the thing that hangs in the back of your throat) are long or floppy, they can fall backwards and close the area for breathing. The back of the tongue can also fall backwards and obstruct breathing especially when laying flat on your back. Finally the side walls of the throat can fall together to narrow or close the airway.
To break it down even further:
- The muscles of breathing work to expand the chest and lower the diaphragm to draw in a breath of air.
- The negative pressure literally sucks air into the lungs.
- The nasal passages, palate, tongue, and pharyngeal tissues can all contribute to narrowing of the airway.
- If during an attempt to breathe, the airway collapses; the tissues of the airway are sucked together by the negative pressure.
- The harder the chest tries to pull air in, the greater the negative pressure, and the more the tissues of the airway are sealed together.
- Finally, when the oxygen in the blood stream decreases, the person wakes up or the level of sleep becomes more shallow.
People with obstructive sleep apnea have an airway that is more narrow than normal, usually at the base of the tongue and palate. When lying flat, the palate is above the air passage. When the pharyngeal muscles relax, the palate can fall backwards. This can obstruct the airway.
The genioglossus muscle is located where the base of the tongue attaches to the jawbone in front. Most people have enough space behind the tongue to take a breath without needing to pull the tongue forward. However, when obstructive sleep apnea patients are awake, this muscle needs to be active to pull the base of the tongue forward to open the airway. During sleep most muscles, including the genioglossus, relax. During one particular stage of sleep, the stage of rapid eye movement (REM), the muscles completely relax. Relaxation of the genioglossus muscle during sleep allows the base of the tongue to fall backwards, and the airway closes.
Patients with obstructive sleep apnea often don’t report waking up during the night with each episode of apnea. Frequently, during the apnea, the brain only awakens from a deep sleep (stages 3, 4, or REM) to a shallow level of sleep. The genioglossus muscle then contracts and pulls the tongue forward so that a breath can be taken. The patient may remain asleep, but the deep sleep that is important to be fully rested the following day is disrupted.
What are obstructive sleep apnea symptoms?
Obstructive sleep apnea has many well-studied consequences. First, as you would expect, it disrupts sleep. Patients with disrupted sleep cannot concentrate, think, or remember as well during the day. This has been shown to cause more accidents in the work place and while driving. Thus, people with obstructive sleep apnea have a three-fold greater risk of a car accident than the general population (there aren’t many diseases that can kill the patient AND the people in the car next to him or her!).
High blood pressure
Sleep apnea causes high blood pressure and heart problems. Stopping breathing frequently during the night (every 1-4 minutes) can cause increased stress on the heart. As the oxygen saturation in the blood decreases and the apnea continues, the sympathetic system (“Fight or Flight” response) is activated. This sends nerve signals and adrenaline signals to the blood vessels to constrict and to the heart to work harder. When the vessels constrict, more blood is sent to the brain and muscles. However, this increases the blood pressure, which requires the heart to work harder to pump blood through the smaller vessels. That, combined with the signal for the heart to work harder and the lower available oxygen in the blood, causes increased stress on the heart throughout the night. During sleep is the time when the heart usually has less work to do and can “rest.”
Among patients with obstructive sleep apnea that do not have high blood pressure, 45% will develop high blood pressure within 4 years. If you look at patients who have hard-to-control blood pressure, that is, taking more than one medicine for control, 80% have obstructive sleep apnea. When the obstructive sleep apnea is treated, the high blood pressure comes down. For more, please read the High Blood Pressure article.
The risk for congestive heart failure increases by 2.3 times and the risk of stroke by 1.5 times with obstructive sleep apnea. For more, please read the Congestive Heart Failure article.
Obstructive sleep apnea can complicate atrial fibrillation treatment. Atrial fibrillation is a condition in which the upper part of the heart (atrium) is beating out of coordination with the lower part (ventricle). The treatment is to cardiovert the heart, which resets the atrium and allows it to synchronize with the ventricle. After cardioversion, 50% of patients have a recurrence of atrial fibrillation, but patients with obstructive sleep apnea have an 80% recurrence. Finally, obstructive sleep apnea can increase the risk of sudden death. For more, please read the Atrial Fibrillation article.
Obstructive sleep apnea has many consequences, some of which can kill patient and their loved ones. Again, it is estimated that only 10% of people with obstructive sleep apnea are being treated. Just treating the obstructive sleep apnea would improve many other areas of their lives.