• Breathing patterns
• Causes of central sleep apnea
• Heart failure and Cheyne-Stokes breathing
• References and notes

Unlike the heart, which continues beating no matter what the brain does, the breathing muscles work only when the brain gives them a "breathe now" command.

We are all familiar with several types of breathing patterns commanded by the brain:

		Normal breathing (schematic representation)
		Breathing during exercise
		Normal breathing, with sigh

In theory, many abnormal types of breathing patterns could meet the criteria for central sleep apnea. For example, simple repetitive breath-holding would meet the criteria -- if it could be done while asleep. In practice, however, the second pattern pictured below is the most commonly seen form of central sleep apnea.

		Twice holding breath, in mid-breath
		Typical pattern in central sleep apnea

The second pattern is so distinctive that is has its own name: Cheyne-Stokes breathing.

Cheyne-Stokes breathing has alternate periods of no breathing (apnea) changing smoothly into periods of hyper-breathing, which smoothly change back to no-breathing.

Cheyne-Stokes breathing is abnormal.  It may occur during sleep or wakefulness. It is generally a sign of more advanced disease when it occurs during wakefulness.

If a sleeping person has Cheyne-Stokes breathing, and if the duration of the apneas are 10 seconds or longer, and if there are more than 5 such apneas per hour of sleep, then the person has central sleep apnea.

Because the brain controls breathing patterns, brain damage can result in central sleep apnea. This is the simplest cause to understand, but it occurs far less often than the most common cause.

Heart failure is the most common cause of central sleep apnea. About 40% to 60% of persons with heart failure have central sleep apnea.

About heart failure

Heart failure is present when the heart is too weak to pump enough blood to meet the demands of the body. Heart failure is not the same as a heart attack. Heart attacks damage the heart muscle. If enough damage accumulates, heart failure occurs. As more people survive heart attacks, heart failure is becoming more and more common. High blood pressure is the other major cause of heart failure. There are dozens of less common causes.

Why does the brain command abnormal breathing when the heart is weak?

A partial answer is as simple as your bathroom shower. (The complete answer is still debated.) But, first, some background on the control of breathing.

The brain continuously monitors the body's status and continuously decides the proper rate and depth of breathing to command. In particular, the brain continuously monitors how much carbon dioxide is contained in the bloodstream.

Breathing is normally controlled by a simple cycle of events:

1. When the level of carbon dioxide gets too high, the brain sends a "breathe now" command to the breathing muscles.
2. The act of breathing lowers the level of carbon dioxide in the blood.
3. There is no stimulus to breathe until the level of carbon dioxide rises again to the "too high" level.
4. Since the body continuously produces carbon dioxide, the "too high" level is reached again in a few seconds. The cycle starts again at step 1).

Heart failure disrupts this cycle, and Cheyne-Stokes breathing is the result. Heart failure is disruptive because the blood circulates more slowly through the body in heart failure. As a result, the carbon dioxide levels seen by the brain are not current information -- they represent carbon dioxide information from several seconds earlier.

The effect of having old information about carbon dioxide is the same as trying to adjust the water temperature in a shower. We all know that in some showers, it can take quite awhile before we can feel the effect of turning the "hot" knob up. This slow circulation of water in the plumbing of the house can lead to large swings in the temperature of the shower water.

An impatient person trying to adjust one of these showers will turn the "hot" knob, wait a second, feel no change, turn it up more, feel no change, turn it up again, and finally be scalded when a torrent of hot water finally comes out of the nozzle. The person will then turn the hot knob way down, feel no change, turn it down again, and finally be rewarded with a blast of ice water.

As the person gets familiar with how quickly the water temperature responds to changes in the "hot" knob, the swings in water temperature decrease.

The situation in a patient with heart failure is similar. The brain is acting on old information, and large swings in breathing depth and rate occur. The difference is that the brain does not learn about the delay in carbon dioxide levels, and that the swings in breathing continue.

The best treatment for Cheyne-Stokes respiration is to treat the heart failure. The circulation of blood becomes shorter. It is always easier to adjust the temperature of a shower where the water temperature changes the instant the "hot" knob is turned.