Help with Gas Machine

Help Needed:

Just learning the ropes of the Anesthesia Machine and I was wondering what is it that drives the machine. I know that you can control your flow volumes of air, N2O, and O2 as well as vaporizers. However, is there ever a time when you would be venting a pt with no gas flows and if so are they just getting atmospheric air (i.e 21% O2)?

You can turn your gas flow off and just run in ventilator mode. They it is very similar to the vent in that you control your TV Fio2, etc.

We use the Datex-Ohmeda at my hospital sites ... as far as I know (I've been visualizing the gas machine in my head for your question), there can be no ventilation without any gas flow. In order to fill the bellows, you need some sort of gas flow. However, if you hooked up an ambu-bag to the external flowmeter, you could ventilate the patient that way. Someone with more background in Dorsch & Dorsch please correct me if I'm wrong!

The O2 flush valve will fill the bellows at 35-75L/min (50psi), but then again, it will only fill it once without additional gas on.

Speaking of the flush valve ... how many of you actually use it during the case (say when you need to disconnect for repositioning and need a quick fill of the bellows with the reconnect)? I've been told 2 different things - if you push the valve on exhalation and watch your PIP, then there's no risk for barotrauma. On the other hand, I've been told it causes barotrauma no matter what. Anyone else been told different?

Kat,

There is a really cool website that simulates the machine:

http://vam.anest.ufl.edu/logincheck.html

If you PM me, I'll give you my login if you don't want to register.

Dave

Very breifly and in a nutshell, the answer to your question is...it depends....upon the type of anesthesia machine you are using. The textbook answer is that most machines have a minimum O2 flow of 200ml/min (even with the flow meters all the way off). This flow starts as soon as you turn the machine on. This is a safety mechanism so that you cannot deliver a hypoxic mixture at very low flows. However, some of the older narcomed machines will allow you to "turn off" all flow through the circuit. I would assume that you are delivering a general anesthetic, so if you have no flow through the circuit then you have no flow through the vaporizer and therefore have no anesthesia (unless you are doing TIVA and just using the ventilator on the machine). All the new machines have this safety mechanism, so the answer to your question is that there is always some flow through the circuit.

The other part of yor question is what "drives" the machine. Newer machines use medical air to drive the ventilators. Older machines used O2 to drive the ventilator. The drive gas is different from the gas delivered to the patient.

Hope that helps.

Thanks for the explanation skytrane ... and I used that site at the beginning of the program Dave - it's great! Good luck with classes :)