• The human airway
• The airway during sleep
• The role of weight
• Other factors

The Human Airway

Obstructive sleep apnea (OSA) occurs when air is physically blocked from entering the lungs intermittently during sleep. Why does this happen?

The answer is not completely known. For air to get to the lungs, it must first go through the set of structures known as the "upper airway," which we call "airway" for short.

The diagram below shows the airway of a human lying on his or her back. The airway includes the nose, the mouth, and the parts of the mouth behind the tongue that cannot be seen without special instruments.

The airway is similar to a tube. Like a straw used to drink a milkshake, some parts of this tube have a tendency to collapse shut. The red "X" in the diagram below shows a common site of collapse -- behind the tongue. When collapse occurs here, air cannot get from the nose or mouth into the lungs.

To prevent collapse, there are muscles in the airway with the job of keeping the airway open. These muscles are 100% on-duty during wakefulness, but when sleep comes, they relax.

The Airway During Sleep

Muscles throughout the body relax during sleep. In normal people, the airway stays open even when the airway muscles relax. In people with obstructive sleep apnea, however, the airway collapses, or is close to collapsing, when airway muscles relax.

Here's a diagram of what happens in obstructive sleep apnea:

	Sleep
	Airway muscles relax
	Airway collapses
	Reduced airflow
	Distress
	Brain awakens
	Airway muscles energized
	Airway opens
	Normal airflow
	Distress ends
	Brain goes back to sleep

As you can see, the process of airway opening and collapse is dynamic -- it is a process that repeats multiple times.

The distress caused by reduced airflow is similar to being choked. This distress awakens the brain, which turns on the airway muscles, which permits normal airflow to occur and ends the distress. It's usual at this point for the brain to fall right back asleep. It falls back asleep so fast that no memory of the event is formed. Most persons with sleep apnea are not aware of their problems breathing during sleep.

If you have heard of unconscious people "swallowing their tongue" and choking to death, the situation is exactly the same as in obstructive sleep apnea, except that the brain does not awaken and the airway muscles are not roused to open the airway.

The Role of Weight

Putting on weight increases the tendency for the airway to collapse. The reasons are incompletely known.

Fat can accumulate in the tissues surrounding the airway, just as fat accumulates in other parts of the body. This accumulation of fat can change the shape of the airway, rendering it more likely to collapse.

Thus, it is not surprising that some studes have found that persons with a large neck are at higher risk for sleep apnea.

Other Factors

In addition to weight, several other factors can influence the presence and severity of obstructive sleep apnea:

• The shape of the jaw is important. When the jaw is small, the tongue sits further back in the mouth. This increases the tendency for the tongue to pinch off airflow during sleep.

• When lying on your back, gravity pulls the tongue toward the back of the mouth. This, too, increases the tendency for the tongue to pinch off airflow during sleep. Some people have obstructive sleep apnea only when they are sleeping on their back, or have more severe sleep apnea on their back.

• Alcohol and some medications relax airway muscles. Drinking alcohol can worsen or tip someone into obstructive sleep apnea.

• In children especially, large tonsils or adenoids can partially block the airway.

• Smoking irritates the tissues of the upper airway, and can cause them to swell. Swollen tissues increase the chances of physically narrowing the airway.

• All sleep is not the same. There are different types of sleep, known as sleep stages. Dreaming sleep, also known as REM sleep, is one such stage. During dreaming sleep, body muscles are normally more relaxed than during other stages of sleep. Some people will have obstructive sleep apnea only during dreaming sleep.

All of these factors can occur in combination. For example, a person may have sleep apnea only during dreaming sleep when sleeping on their back, after drinking alcohol.

Thus, many factors promote collapse of the upper airway. No acute means of reducing the tendency to collapse is known.

Next:   Details about central sleep apnea.