Even if you are familiar with the condition called sleep apnea, you may not intuitively understand what is happening in the body. How does sleep apnea occur? What exactly happens to the airway during the characteristic pauses in breathing? Depending on the severity, how can the same process result in snoring, hypopnea, or apnea?
In order to answer these questions, let’s explore an excerpt from UpToDate -- a trusted electronic medical reference used by health care providers and patients alike. Then, continue reading below about what this information may mean for you.
"The throat is surrounded by muscles that control the airway for speaking, swallowing, and breathing. During sleep, these muscles are less active, and this causes the throat to narrow. In most people, this narrowing does not affect breathing. In others, it can cause snoring, sometimes with reduced or completely blocked airflow. A completely blocked airway without airflow is called an obstructive apnea. Partial obstruction with diminished airflow is called a hypopnea. A person may have apnea and hypopnea during sleep.
"Insufficient breathing due to apnea or hypopnea causes oxygen levels to fall and carbon dioxide to rise. Because the airway is blocked, breathing faster or harder does not help to improve oxygen levels until the airway is reopened. Typically, this requires the person to awaken to activate the upper airway muscles. Once the airway is opened, the person then takes several deep breaths to catch up on breathing. As the person awakens, he or she may move briefly, snort or snore, and take a deep breath. Less frequently, a person may awaken completely with a sensation of gasping, smothering, or choking.
"If the person falls back to sleep quickly, he or she will not remember the event. Many people with sleep apnea are unaware of their abnormal breathing in sleep, and all patients underestimate how often their sleep is interrupted. Awakening from sleep causes sleep to be unrefreshing and causes fatigue and daytime sleepiness."
There are muscles throughout your body, even in unexpected places like your throat. There are two basic types of muscle, skeletal muscle and smooth muscle. Skeletal muscle is found in your mouth and upper throat, while smooth muscle is lower, lining your esophagus and trachea. One of the key distinctions is that skeletal muscle is under your conscious control. This means that you are able to contract it at your will, such as when you decide to swallow or cough. It may also be activated reflexively. Together these muscles help to keep your airway open and prevent its collapse.
During sleep, however, the muscles of the tongue and airway relax. This is especially true during rapid eye movement (REM) sleep when the mind is active but the body is paralyzed. A few muscles are spared from paralysis, most notably those that move the eyes and the diaphragm, which acts as a bellows to fill the lungs. Depending on your anatomy, this muscle relaxation may prove to be problematic.
If you imagine your airway as a floppy tube, the muscles keep it from collapsing when they have normal tone. However, during periods of sleep -- and if your tube is extra narrow or crowded (such as may occur in obesity) -- it may collapse more easily. Rather than a clear passage that air may move through effortlessly, the airflow becomes disrupted. When this disruption occurs, a vibration may result. This is snoring. The tube may collapse even further, either partially (resulting in a hypopnea) or completely (resulting in apnea).
When your breathing becomes compromised due to the collapse, your lungs do not get enough oxygen and you cannot blow off carbon dioxide. The solution is to open the tube. This often requires the affected person to awaken into a lighter stage of sleep, if only briefly. This arousal may go unnoticed by someone with apnea as it occurs, except for the fact that these chronic shifts into lighter sleep lead to excessive daytime sleepiness and other consequences.
The body responds in a panic to these pauses in breathing. Just as you would thrash if you were drowning underwater, your body does everything it can to reset your breathing. There is a burst of stress hormone (called cortisol) and your heart rate and blood pressure may spike as your body raises the alarm. As you awaken, the muscles in your airway regain tone and open the collapsed tube. But, as you fall back asleep, the same pressures may cause it to collapse again. Therefore, a cycle persists throughout the night as your breathing becomes compromised, your body awakens, and you fall back asleep only to disrupt your breathing again.
For those who observe someone with sleep apnea, there may be a period of silence followed by a loud snort or gasp. This is the breathing resetting itself after a period of apnea. It may occur hundreds of times per night. In a standard sleep study like a polysomnogram, the degree of breathing disruption may be quantified with a measurement called the apnea-hypopnea index (AHI).
There are some factors that contribute to sleep apnea that cannot be controlled, such as the structure of your airway anatomy. However, there are certain choices that might also lead to the collapse of the airway, such as alcohol use. In addition, sleeping on your back may contribute. If you are overweight or obese, this may also lead to problems. In addition, there are effective treatment options for sleep apnea such as continuous positive airway pressure (CPAP) that work by keeping the airway open. There are also new investigations into the use of devices such as hypoglossal nerve stimulators that activate the airway muscles during sleep.
If you are concerned about the breathing of yourself or a loved one during sleep, you should consult with your doctor about the condition and your treatment options.
Want to learn more? See UpToDate's topic, "Sleep apnea in adults," for additional in-depth medical information.
Schmidt-Nowara, Wolfgang. Bonnet, Michael et al. "Sleep apnea in adults." UpToDate. Accessed: January 2012.