CPAP Pressure vs. Volume

[PaulaO2]

Anyone know what the inhale pressure is of the average ventilator?

[greatunclebill]

Anyone know what the inhale pressure is of the average ventilator?

i don't know. here's one that i found.

Allied AutoVent 4000 Transport Ventilator
Part Number: L760
Model: AutoVent 4000
Manufacturer: Allied Healthcare
Application: Transport Ventilator
Dimensions: 9.6 W X 6.5 H X 2.9 D Inch
Inspiratory Time: 1 to 2 Seconds
Power Source: Pneumatic Power
Pressure: 20 to 80 cm H2O
Type: Automatic
Volume: 200 to 600 ml
Weight: 3.7 Lbs

The AutoVent 4000 series transport ventilator with optional CPAP, by Allied Healthcare Products, is a pnueumatically powered, durable and light ventilator that offers the capabilities of ventilation and CPAP in one system.

Features of the Allied AutoVent 4000 Transport Ventilator:
- Pneumatic power and hard plastic case provide a sturdy, virtually unbreakable ventilator
- Independent settings for the critical ventillation parameters of breaths per minute (BPM), Inspiratory Time, Tidal Volume, and Airway Pressure Relief
- AutoVent comes with the LSP Patient Valve, with its bright green visual breath indicator, built-in pressure limit alarm and Intermittent Mandatory Ventilation providing gas flow for spontaneous breathing patients

For More Information on the Allied AutoVent 4000 Transport

[PaulaO2]

Yeah, I figured it's pressure would be higher. 20 to 80 is a huge range. I can see why folks need to be 'weaned' from one!

[Sleepster]

What is the difference between CPAP pressure and volume of air flow?

Pressure is how much force is exerted on a unit square. And of course, when it comes to CPAP machines, pressure is what's specified.

Volume flow rate (measured, for example, in liters per minute) is the volume of fluid, in this case air, that's delivered in one unit of time. It's also called flux. And if you wanted to calculate it for a CPAP hose you'd multiply the cross-sectional area of the hose by the speed of the air.

Volume flow rate is different than pressure. You could have air at a pressure of 10 cm, and it could be sitting still, not moving. Or, it could be moving slowly, or quickly. All at the same pressure of 10 cm.

In the case of a CPAP machine, the wider the hose, the greater the volume flow rate. If you bend the hose and wind it around in a bunch of turns, you might reduce the volume flow rate, but you'd not change the pressure.

The thing that impressed me the first time I saw my CPAP machine in operation was that it's able to maintain the same pressure despite a very high volume flow rate. If you think about it, this is what a CPAP machine must be designed to do. When you inhale, the volume flow rate goes up, when exhale, it reverses and becomes negative (this is where the "positive" term comes from in the phrase "positive airway pressure"). In between the inhale and the exhale, the volume flow rate is zero. Regardless of whether the flow rate is huge when you inhale quickly, small when you inhale slowly, zero between your inhale and exhale, negative when you exhale, the pressure must remain constant. It's an amazing engineering feat!

Try this experiment. Turn on your CPAP and block the hose with your thumb. Look at the pressure. Take your thumb off and the flow rate increases, but the pressure remains the same.

[Sleepster]

Actually, pressure and volume are inversely proportional. As I recall from college years ago, and from SCUBA certifications at a later age, it is based on Boyles Law.

The mathematical equation for Boyle's law is:

pV = k

Boyle's Law applies only for a fixed amount of a gas. With a CPAP machine we repeatedly inhale and exhale, so the amount of gas (air) changes drastically. In other words, Boyle's Law doesn't apply for a person connected to a CPAP machine.

Sorry, that's as far as I go with the math. I love physics, but I hate the math...

Of the two, I prefer physics. Math is no good unless you use it for something!

[Bazinga]

I used to be an ICU nurse working with vents. Settings had a few components:

1. Type of ventilation: (e.g. AC - assist control; IMV - intermittent mechanical ventilation, CMV - complete mechanical ventilation, etc, indicating how much "initiative" the vent takes for the patient)
2. Oxygen setting: FiO2 (fraction of inspired oxygen) set to achieve desired SPO2 ("saturation of peripheral oxygen") blood oxygen levels
3. Tidal volume (TV) - normal volume of air displaced between normal inspiration and expiration; typically around 500mL (give or take a couple hundred); i.e. the amount of "air" the vent pushes into the lungs with each breath
4. PEEP - (positive end expiratory pressure) to keep the alveoli open/uncollapsed between breaths, typically set around 5 cmH2O
5. Rate: Breaths per minute (e.g. 8-14 or so, depending on patient's acid/base status as measured by ABGs - arterial blood gas sampling)

"Pressure" is not something the operator sets (with the exception of PEEP, and also with "pressure support" ventilation which decreases the work of the patient breathing for himself; different from true mechanical ventilation).

With mechanical ventilation the ventilator uses whatever pressure is necessary to achieve the above settings/parameters. A bit of a different approach due to different desired outcomes (breathing for the person or assisting with inflating the lungs (ventilation) vs. splinting the airway open while he person breathes on his own (CPAP).

It's been about 7 years since I worked in that environment, so anyone else out there with more current info, please feel free to correct!!

[Bazinga]

Jumpstart, I think the idea Zimlich had about not being able to inflate a balloon (or a dirigible, or the lungs) is valid. The missing variable, I believe, in the Boyle's Law conundrum, is that with CPAP, there is an ever-flowing air exhaust port on the mask interface, so the increase in the lung pressure is nil, or volume is dependent only on the person's inspiratory effort (not the CPAP), whereas in mechanical ventilation, there is an endotracheal intubation tube that seals off the trachea by way of an inflatable balloon. That is how the ventilator is able to "blow the lungs up" like a balloon.

Great discussion everyone!

[greatunclebill]

Jumpstart, I think the idea Zimlich had about not being able to inflate a balloon (or a dirigible, or the lungs) is valid. The missing variable, I believe, in the Boyle's Law conundrum, is that with CPAP, there is an ever-flowing air exhaust port on the mask interface, so the increase in the lung pressure is nil, or volume is dependent only on the person's inspiratory effort (not the CPAP), whereas in mechanical ventilation, there is an endotracheal intubation tube that seals off the trachea by way of an inflatable balloon. That is how the ventilator is able to "blow the lungs up" like a balloon.

Great discussion everyone!

i put the balloon on the cpap air port so there was no exhalation involved. go to my post about the balloon to see the picture of the balloon on the turned on machine set at 20.

[Bazinga]

I saw it GreatUncleBill!!! Very cool...
Question: Did you have a complete seal, or just rest it on top of the tube?

[greatunclebill]

I saw it GreatUncleBill!!! Very cool...
Question: Did you have a complete seal, or just rest it on top of the tube?

resting on the top would do no good. go look again at the picture in the post to see how the balloon stretched over the opening. it's a real good picture. i didn't hold it on., there was not enough pressure to blow it off or inflate it. it basically just filled the balloon to original shape and didn't expand it.