Ventilator concepts of volume and pressure

[Nurse2Bforever]

I'm having a hard time understanding the difference between volume and pressure on a ventilator. I know that the "amount" of air that the machine gives to the patient is in mL's, which would mean that it is the volume. Then what/where is the pressure on the vent? I don't understand!

Also, I know that minute ventilation is tidal volume times respiratory rate. Is there any significance to this calculation? My med surge book just has the formula, but it does not indicate when this calculation would be Used.

[EricJRN, MSN, RN]

Basically, when you talk about ventilator pressures, you're referring to the pressure generated in the patient's airways.

If I have nice, elastic, healthy lungs, you can give me a pretty large tidal volume of air without generating excessive pressures. However, if I have poor lung compliance from chronic or severe disease, relatively little air may cause high pressures. (You might think of it as similar to the relationship between stroke volume, blood vessel tone and BP.)

Ventilators can either work in pressure-control or volume-control modes. In volume control, you're delivering a set volume but the pressure will vary depending on several factors (lung compliance and agitation, for example). You should still see a screen of measured values where you can find out things like the peak inspiratory pressure (PIP) generated with each breath.

Likewise, in pressure control you're setting a pressure (usually PIP), so the tidal volume will vary and you can access it by clicking over to the appropriate screen. Alarms may sound in pressure control if an appropriate tidal volume isn't being delivered with the current set pressures. That alarm may say "minute volume low" or "tidal volume low" or something similar.

That's a very, very basic description. There are many individual vent modes, with some even combining the advantages of pressure ventilation and the advantages of volume ventilation. The specific place to find all of your values is going to depend on the specific mode and the model of the vent.

Are you going to spend time in an ICU where you might be able to get hands-on time with vents? This may make more sense once you have a bedside nurse, instructor or RT to show you the values on an actual ventilator.

[kgh31386, BSN, MSN, RN]

Don't forget about CPAP and PEEP. They involve pressure as well. PEEP is a little puff of air at the end of your breath and helps to keep a higher volume of gas in the lungs at the end of expiration. That helps to prevent the pulmonary shunting and improves your gas exchange. CPAP does the same thing except it's continuous. They prevent your aveoli from collapsing and improve gas exchange.

[EricJRN, MSN, RN]

Yes, that's a great point, kgh. Even with volume control ventilation, you still set a positive end expiratory pressure. (We use primarily SIMV-pressure control/pressure support and high frequency ventilation in my setting, so I knew I'd forget something. Good call!)