My wife asked me exactly what does cm H2o mean? Can we compare it to Pounds per Square Inch (PSI) ?

I can read the words, but do not work with any other systems using it, so it is some nebulious term tossed out by an engineer with a propensity for accuracy.
It is a "relative term" to me, meaning...20 cm H2o is more than 10 cm H2o.

BUT is the scale linear or logarithmic?

Does 20 cm H2o roughly equal 20 PSI?

For those whose minds are clear and functioning within warranty specs, does it really matter for most of us?

Should it be construed as a simple scale for our use, not of any real importance to us?

Sometimes I like to "just know" what something means. Is there a simple explanation? BTW, one word answers don't get credit on today's exam

What the "xxCMH2O" referes to is the number of centimeters in height that the pressure of the air wil push a column of water to. In your case, it means that you are using pressure that will push a colum of water to 20 centimeters. It is NOT equivalent to PSI (MUCH less). It is more analogous to barometric presssure which is measured in inches of mercury; 30.00 in. on the barometer means that the pressure of the air will raise a column of mercury to 30.00 inches.

Our beloved "xxCMH2O" is simply the metric we use to set and monitor the pressures that we live with.

Hope this helps!

Chuck

Just to add to what goofy said, it is a linear scale of measurement. 10 cm is twice as much volume of air as 5 cm. If you've ever seen a manometer, it works like that.

dllfo, I found several explanations in these topics helpful:

Feb 28 2005 subject: Science behind CPAP Pressure
rwguinn wrote:
I have posted what the pressures mean elsewhere.
for example, 25cm H2O is about 10 inches of water. That is a low pressure. To demonstrate:
The center of my lungs is about 10 inches below my chin. If I stand in a swimming pool, with the water up to my chin, the water exerts a pressure exactly equal to what a person on maximum CPAP therapy of 25cm H20 would feel. The difference is that it helps me exhale, and I have to inhale against it.
CPAP helps me inhale, and I have to exhale against the pressure.
Neither one helps me breathe. It is static pressure.

In that same thread, -SWS wrote:
The static pressure causes a slight inflation throughout the respiratory tract. That slight inflation increases the diameter or clearance at the point of obstruction and througout the rest of the respiratory tract. That slight inflation even reaches into the alveoli.

To my knowledge there are no studies showing the long-term effects of CPAP static air pressure on the respiratory tract. However, I also suspect the typical healthy human body is resilient enough to cope with 20 cm pressure with no significant side effects. The respiratory tract is, after all, designed for inflation and deflation. However, there are chronic pulmonary conditions in which even 20 cm of inflationary pressure or less is clearly undesireable. Read the patent descriptions of some of the more advanced machines such as the ResMed Autoset CS 2 and you can see scientists and engineers addressing the issue of inadvertent overinflation.

Thanks Roger for your excellent post. We are very lucky to have your expertise here!

Aug 08 2005 subject: Who thinks what? And what the experts say? Does the pressure

May 02 2005 subject: Formula for calculating volume of air for given pressure

Deleted

Thanks for those links RG-I'm printing them out for this evenings reading. I've been wanting to compare cpap with the ventilators used in ICU.

A great link to convert all maner of units!!!

http://www.davidson.com.au/tools/convert/default.asp

It's another thing that isn't Rocket Science, the pressure we use to keep our airways open is very low compared to the pressure we use to air the tires on our car.

So we have different scales to do different jobs, it's easier to control if the scale fits the job. We use three main units of measure for pressure.

One is Pounds Per Square Inch, the most used measure of pressure.

Another is Inches of Mercury Pressure, 1 cm of Hg, is the pressure required to raise a column of mercury 1 cm. Mercury is a heavy element, so the pressure it measures is greater than lifting water.

Another is Inches of Water Pressure, 1 cm of H20, is the amount of pressure required to raise a column of water 1 cm in height.

1 C.M. of H20 = 0.036127 P.S.I. 1 C.M. of H20 = 0.073556 Inches of Hg

Jim

Just to put it in writing.
From HG45's web site: 20 CMH2O = .28 PSI

So, your 20 cm of H2O is roughly equal to 0.3 psi or 1/100 of the pressure in your car tires. The calibration can be checked by inserting a plastic tube in the humidifier tank and restricting the air flow out of the tank until the column of water in the tube stops rising. That is the limit of the pumps ability to lift the water. Measure from top of water in tank to top of water in tube and the difference is the pressure produced.

TerryB

Here is a link to an article that explains how pressure measuring devices work, such as the one in question. A manometer can use mercury, but for low pressures like the ones we use, a water column, using the same principles is used.

http://en.wikipedia.org/wiki/Manometer#Hydrostatic

dllfo wrote:My wife asked me exactly what does cm H2o mean? Can we compare it to Pounds per Square Inch (PSI) ?

I can read the words, but do not work with any other systems using it, so it is some nebulious term tossed out by an engineer with a propensity for accuracy.
It is a "relative term" to me, meaning...20 cm H2o is more than 10 cm H2o.

BUT is the scale linear or logarithmic?

Does 20 cm H2o roughly equal 20 PSI?

For those whose minds are clear and functioning within warranty specs, does it really matter for most of us?

Should it be construed as a simple scale for our use, not of any real importance to us?

Sometimes I like to "just know" what something means. Is there a simple explanation? BTW, one word answers don't get credit on today's exam

The short answers to your questions are as follows:

A cm of h2o means the amount of pressure needed to raise any column of h2o a height specified in centimeters.

The unit of cm of h2o can be converted to units of mm of Mercury, bars, pascals, atmospheres, as well as pounds per square inch.

The measurement scale is linear and absolute.

1 cm h2o = 0.014 lbs per square inch.

For whatever reason, the CPAP industry has chosen this unit of measure … perhaps it has to do with what units are used for other medical devices?

It is a very simple scale for us to use with our treatment.

Yes, it is a very simple explanation … do I pass the test?

For a more detailed definition of pressure in the context used for CPAP, continue below:

Pressure, in particular hydrostatic pressure (also referred to as head pressure), is measured as the force per unit area needed to raise a column of fluid to a specified height referenced from sea level … it is based on Pascal’s Law (Blaise Pascal, 1600’s) which states that:

Pressure = gravity * density of column of fluid * height of the fluid column

So basically, the pressure of your CPAP machine exerts a force distributed equally and perpendicularly outward against the entire surface area of your respiratory system that is equivalent to raising a column of water to the setting of your CPAP machine in centimeters.

That is all for today’s lesson on hydrostatic pressure ... tomorrow's lesson ... how to build a CPAP system with advice from your friendly cpaptalk members.

- roberto

That is all for today’s lesson on hydrostatic pressure ... tomorrow's lesson ... how to build a CPAP system with advice from your friendly cpaptalk members.

Correct me if you think I'm wrong, but you really could create your own basis cpap if you had time on your hands and the inclination. For a motor you'd use a vacuum cleaner motor. You'd needd any type of reastat to control the speed of the motor. A home made water column to measure and adjust the flow and some duct tape to attach to your interface hose. You'd need a few other assorted hoses and clamps, but you really could do it. You may not want to actually use what you built but it would be a fun project for the Mc Gyver types.

Sleepless in St. Louis wrote:

That is all for today’s lesson on hydrostatic pressure ... tomorrow's lesson ... how to build a CPAP system with advice from your friendly cpaptalk members.

Correct me if you think I'm wrong, but you really could create your own basis cpap if you had time on your hands and the inclination. For a motor you'd use a vacuum cleaner motor. You'd needd any type of reastat to control the speed of the motor. A home made water column to measure and adjust the flow and some duct tape to attach to your interface hose. You'd need a few other assorted hoses and clamps, but you really could do it. You may not want to actually use what you built but it would be a fun project for the Mc Gyver types.

I think I would stick to something smaller and quiter like an air pump for a fish tank ... a vaccum cleaner motor might cure you of the CPAP treatment for good by exploding your lungs.

- roberto

sinkem wrote:Just to put it in writing.
From HG45's web site: 20 CMH2O = .28 PSI

From http://www.1728.com/convpres.htm?b0=1++
20 cmh20 = 0.72255 psi FALSE!!!

EDIT for False info 0.28902 cmis correct as it's in cm, my number is for inches that is a 2.5 difference converting inches to cm.

You don't suppose they were using distilled Water. Jim