Advice for new users wanting to "tweak" settings

[zimlich]

You can't simulate apneas. Why would you want to? You completely skew your true results.

[archangle]

Actually, a true central apnea is a non-obstructive hypoxic event, where not only is there a total cessation of breathing for at least 10 seconds, but along with that there is a drop in the oxygen saturation of at least 10 percent of the baseline saturation. In order to know if the "clear away or open airway" events that your machine records were significant, you would need to have simultaneous pulse ox data. That isn't wouldn't be that difficult to do, especially with SleepHead software and an inexpensive CMS50 D+ pulse oximeter. Frequent significant drops in your pulse ox, whatever the cause, obstructive or central, aren't good for you.

Does it have to be an SpO2 desaturation, or can it also be an EEG arousal? Obstructive apneas can be either one.

[archangle]

The problem with the assumption that CPAP pressure induces true centrals is that data-recording CPAP and APAP machines have no way to accurately determine a true central event. That's why some record it as a "open airway" event.

True central sleep apnea is a neurological event and can't really be effectively determined without being hooked up in a sleep lab.

Until someone, somewhere does a comprehensive study proving that higher CPAP pressures actually induce true central events, I remain sceptical that the relatively small number of centrals that are reported by CPAP machines are something to be overly concerned about. Now, if the machine reports a huge number of centrals, yes, that might need to be looked at.

Again, just my opinion, of course.

More importantly than all that, central apneas will still show up as apneas on any data capable CPAP I'm aware of. If increasing your pressure worsens your apnea, the machine will show it, even if it can't really tell central from obstructive.

Central or obstructive, the harm to your body is probably the same. Being able tell a central from an obstructive apnea helps you figure out what to try in order to treat your apnea.

I don't think centrals are more harmful than obstructives, just harder to treat.

One warning though, look at the airflow waveforms and see how long and how severe the apneas are. AHI counts number, not duration or severity.

[SWAMI]

What is a cm H2O, anyway? Well, it turns out that it is pretty close to a couple of pounds per square foot air pressure exerting force on your lungs, give or take. Since many machines have an adjustment resolution of one half of one cm H2O, when you tweak the settings, each increment of adjustment increases or decreases the pressure internally on your lungs by about a pound per square foot of area. That's quite a bit and that's all you really have to know. If you're curious, here's how you get to that value. Every certified SCUBA diver knows that one atmosphere of air pressure is about 14.7 (14.696 more precisely) pounds per square inch at sea level and when you descend 33 feet or about 10 meters (10.332 meters more precisely) in the water you are at two atmospheres of total pressure, one from the atmosphere and one from the water. Times a hundred gives centimeters, so 1,033.2 centimeters of water is 14.7 psi or times 144 (12 in. x 12 in.) is 2116.2 pounds per square foot. Dividing, you get 2.05 pounds per square foot per centimeter of water.

[archangle]

What is a cm H2O, anyway?

1 cmH2O is
0.014 psi.
0.74 mmHg
0.001 atmosphere

As an example of how low CPAP pressure is, imagine this.

20 cm is about 8 inches.
Get a glass of water, and fill it 8 inches deep.
Stick a straw in the glass that reaches the bottom, and gently blow air into it until water just starts to bubble out the bottom of the straw 8 inches under the water.

That's the maximum pressure a regular CPAP machine will do.

[SWAMI]

Those small numbers are deceptive because the area of the straw outlet is so miniscule compared to the area of your lungs. It's the same reason a hypodermic needle pierces your skin so easily. The reason for my post was to put the numbers into a perspective that people can relate to. Your lungs have several square feet of surface area so pressure expressed in pounds per square foot is more realistic and easier to relate to. These machines push large volumes of air that works on your entire lung area, not just a small spot. Try breathing under water with a snorkel to the surface and see how deep you can go - I've tried it, not very deep!

[Al Gore wants to SELL carbon footprint 'licenses' internationally, he must have invented CPAP 'prescriptions' after watching "True Lies"]

[Sleepster]

Every certified SCUBA diver knows that one atmosphere of air pressure is about 14.7 (14.696 more precisely) pounds per square inch at sea level and when you descend 33 feet or about 10 meters (10.332 meters more precisely) in the water you are at two atmospheres of total pressure, one from the atmosphere and one from the water.

If instead of descending 10 meters you descended only one centimeter, you'd have an increase in pressure of only one centimeter of water.

Everyone who has gone swimming has felt the increase in pressure with depth. It's easy to feel on your ears if you've gone to a depth of a meter or two, but going to a depth of only one centimeter is a relatively small increase in pressure. Way too small to feel.

It's a tiny amount of pressure. If you want to relate it to something people do with their lungs, compare it to blowing up a balloon. With your CPAP machine at it's maximum pressure of about 20 cm, it couldn't even blow up a balloon. Air at a pressure of one centimeter exerts a force on your lungs that's tiny compared to the force your lungs exert on the air when you blow up a balloon.

In the example you gave of one centimeter being roughly equivalent to two pounds per square foot, let's suppose your lungs have an effective area of one square foot, then air at a pressure of one centimeter would exert a force of two pounds. But when you blow up a balloon you exert a force that's more than 20 times larger. That's more than 40 pounds! And it usually takes way more than that to blow up a balloon, especially a balloon that's never been blown up before.

If you want to relate it to something people experience with pressure readings compare it to the reading on the gauge you use to check your vehicle's tire pressure. The smallest divisions readable on that gauge are on the order of one psi, wheras one centimeter of water is only 0.014 psi.

Or if you prefer kilopascals, the smallest divisions on the tire gauge are about 10 kPa, and one centimeter of water is only about 0.1 kPa.

It's way too small to show up on the tire gauge.

[archangle]

Those small numbers are deceptive because the area of the straw outlet is so miniscule compared to the area of your lungs.

You have no idea what you're talking about. Blowing into the straw puts the same pressure in your lungs as a CPAP machine.

[Sleepster]

This is the standard misconception about the relationship between force and pressure. Look up the hydrostatic paradox and you'll see what I mean.

The pressure in the straw is the same as the pressure in your lungs. The straw has a small cross-sectional area compared to the effective surface area of your lungs. Therefore the force on the water in the straw is small compared to the total force your lungs exert on the air. In fact the ratio of the areas equals the ratio of the forces. But imagine a small patch of the surface of your lungs the same size as the cross-sectional area of the straw. The force on this small patch of lung surface is the same as the force on the water inside the straw.

Moreover, even though the total force your lungs exert on the air is larger than the force on the water in the straw, it's still very very small compared to the force your lungs exert when you do something like blow up a balloon.

[archangle]

Moreover, even though the total force your lungs exert on the air is larger than the force on the water in the straw, it's still very very small compared to the force your lungs exert when you do something like blow up a balloon.

And both are a heck of a lot less than the force on your lung when you sneeze.