Sleep apnea treatment

How CPAP works: follow the air from the machine to the lungs.

A CPAP machine does one simple thing. It fills your airway with air under a little pressure, so the throat cannot fall shut while you sleep. Follow that air from the filter at the back of the machine to the bottom of the windpipe, then change the pressure and watch what it does to the throat.

Seven stops

Follow the air

CPAP, which stands for continuous positive airway pressure, sends room air along one path: through a filter, a fan, a water tank, a hose and a mask, then through the nose to the back of the throat, where the pressure holds the airway open. Step through it one stop at a time.

Room air comes in

The machine pulls in ordinary room air through a filter at the back. Nothing is added to it: no oxygen and no medicine. CPAP works with plain air, pushed in under a little pressure.

The fan

A small fan, called the blower, spins fast enough to push the air out at your set pressure, here 10. Sensors inside measure the pressure and the airflow all night, which is where the hours, the mask leak and the breathing events on your morning report come from.

The water tank

The air passes over a tank of water, the humidifier, warmed by a heated plate underneath. It picks up warmth and moisture on the way, which is easier on the nose and mouth and helps prevent dryness and congestion. Use distilled water, filled to the line and no higher.

The hose

The hose carries the air to the mask. The pressure is about the same from one end to the other, so everything from the machine to the back of your throat is filled with air at your set pressure. A heated hose, if you have one, keeps the moisture from collecting as water drops on the way.

The mask and its vent

The mask seals around the nose. On it is a row of small holes, the vent, and air streams out of them all night. That is on purpose: most of each breath you breathe out leaves through the vent, carried away by that stream, so the next breath in is fresh air from the machine. Never cover or tape over the vent.

The back of the throat

From the nose, the air fills the back of the throat, where CPAP does its work. In sleep the muscles here relax, and in obstructive sleep apnea the soft palate and the tongue fall back and close the airway. The air pressure pushes them forward and holds the airway open, like a splint made of air. Turn the pressure down to watch the throat narrow.

Into the lungs

Past the voice box, the air goes down the windpipe to the lungs. You still do your own breathing; the machine does not breathe for you. Breathing out against the pressure can feel strange for the first few nights, and many machines can lower the pressure a little each time you breathe out.

Follow the air. CPAP, which stands for continuous positive airway pressure, sends room air along one path: through a filter, a fan, a water tank, a hose and a mask, then through the nose to the back of the throat, where the pressure holds the airway open. Step through it one stop at a time.

Throat: openBreathing in

At 10, the throat stays open through every breath, even with the muscles fully relaxed.

Mask

Pressure10 cm H₂O

420

The sleeper in this picture needs about 10 to keep the throat open. Your own number is set for your airway.

What the number means

CPAP pressure is measured in centimeters of water, written cm H₂O. A pressure of 10 is the push it would take to hold up a column of water 10 centimeters tall. For most people the number falls somewhere between about 5 and 15. It is set for your own airway, either from a sleep study in which the pressure is raised through the night until the breathing events stop, or by an automatic machine that adjusts the pressure on its own within a range your doctor sets.

Too little pressure does not leave the throat fully blocked or fully open. It leaves it narrow, and the snoring and shallow breaths come back before the full blockages do. That is why the pressure is set high enough for the hardest part of the night, such as sleeping on the back or REM sleep, the stage when most dreaming happens, when the throat muscles are at their most relaxed.

Why it has to be every night

CPAP does not change the throat. It holds it open for as long as the mask is on, and on a night without it the airway behaves exactly as it did before treatment. That is why we ask people to wear it whenever they sleep, naps included. The CPAP setup guide covers masks, comfort settings and the first few weeks.

To see what happens inside the airway when it does close, and how a sleep study picks it up, try the exhibit on how sleep apnea happens.

A drawing and a simple model, not a recording. The machine is drawn opened up and not to scale, and real machines differ in where the filter, the fan and the water tank sit. The throat model is one sleeper whose airway needs about 10 cm H₂O to stay open; your own pressure may be higher or lower. Nothing on this page diagnoses anything or replaces the settings your doctor prescribed.

Embed this exhibit

Free to put on a clinic, practice, teaching or patient-education site. Paste the code where you want the exhibit to appear. Keep the credit line under it, and leave the exhibit itself as it is. The frame sizes itself to the exhibit; if your site strips scripts, it stays at a fixed height and scrolls instead.

<iframe src="https://bettersleepproject.com/embed/how-cpap-works/" title="How CPAP works: follow the air" width="100%" height="2450" style="border:0;max-width:100%;display:block" loading="lazy"></iframe>
<p>Interactive exhibit: <a href="https://bettersleepproject.com/exhibits/how-cpap-works/">How CPAP works: follow the air</a>, from Better Sleep Project.</p>
<script>window.addEventListener("message",function(m){if(m.origin!=="https://bettersleepproject.com"||!m.data||m.data.source!=="bsp-embed")return;document.querySelectorAll('iframe[src^="https://bettersleepproject.com/embed/"]').forEach(function(f){if(f.contentWindow===m.source){f.style.height=m.data.height+"px";}});});</script>

Not for pages that sell a sleep product or run advertising around it. If you use it, an email to matthew@bettersleepproject.com is welcome but not required.