I want to understand CPAP pressure in terms I can relate to. What I really want to know is how much pressure is my machine delivering, as compared to not using any CPAP at all. Before you jump in with a trigger reply, read the rest of this post.
I started my research by finding out what normal atmospheric pressure is on the planet Earth. Atmospheric pressure varies a bit due to weather and altitude, but I found out that the average atmospheric pressure at sea level is generally somewhere around 100kPa (a meteorologist would call this about 30 inches of mercury.) What's a kPa? --It's a kilopascal, or 1000Pa. So what's a Pa? It's a "Pascal Unit" which is a scientifically standardized measure of pressure.
I don't really need to know what a Pascal Unit is, or how it's measured, all I want to know is what's the pressure level of the air in my back yard. And the answer is--around 100kPa (which by the way is around 14-1/2 PSI). Good enough. But our CPAP machines deliver IPAP pressures in the neighborhood of 5 to 20 centimeters of water (cmH2O). So how many centimeters of water equals 100kPa? --Easy enough to Google that.--
Here's where it gets weird. 100kPa is around 1000cmH2O!! So if my CPAP is delivering 15cmH2O, then that's like breathing whisper-thin air in a low Earth orbit, which means I would suffocate and die within minutes. (15cmH2O is about 5% of the air pressure on the top of Mt. Everest.) So my original question remains, what are CPAP manufacturers talking about when they talk about centimeters of pressure?
Then I thought, maybe they are talking about centimeters of pressure "on top of" or "in addition to" ambient atmospheric pressure. But that doesn't make sense either because, let's say, my IPAP pressure is set to 15cm. 15cm is equal to about 1-1/2kPa. One and one-half kPa is equal to (approximately) the difference in normal atmospheric pressure between getting into an elevator on the 10th floor and taking it down to the lobby. Which is to say, negligible. So that can't be it.
Which brings me back to my original question. What I really want to know is, how many cmH2O do I set my CPAP machine to so that it delivers NO pressure, or basically SEA LEVEL pressure? In other words, how many cmH2O is the equivalent of 100kPa?