copper kettle equation

Here is an excerpt from Chapter 7 Military Anesthesia Machines (the old combat anesthesia machine was a copper kettle vaporizer).

"agent output is determined by the same principles that govern the copper kettle. Each bubble of oxygen is saturated with the vapor of the liquid anesthetic. The large surface area of the oxygen bubble allows ample time for the liquid anesthetic agent to evaporate inside the bubble and to fully saturate the oxygen flowing throught the vaporizer. Eventually, the oxygen entering the liquid anesthetic plus the anesthetic vapor exit the top of the vaporizer and are diluted by the metabolic oxygen and nitrous oxide in the common gas outlet. At least 50 ml of anesthetic liquid must be in the vaporizer for the anesthesia provider to be certain the the desired vapor concentration is delivered.
Many formulas can be used to calculate the final output of anesthetic agent concentration in the inspiratory limb of the breathing circuit. Approximate calculations of output are accurate enough for clinical application, of which the following is one method:
DERIVATION:

VF[ml/min] x [Pa/[Pb-Pa]] x 100

% = -------------------------------

TF [L/min] x 1000

where % represents the percentage concentration of anesthetic in the inspiratory limb of the breathing circuit, VF represents the flow of oxygen through the vaporizer (in ml/min), TF represents the total gas flow (in L/min), Pa represents the vapor pressure of volatile agent (in mmHG), and Pb represents barometric pressure (in mmHg).

To simplify the calculation of vaporizer output concentration, a Verni-trol (manufactured by Ohmeda) anesthetic flow calculator ("whiz wheel") is included by the manufacturer (for the Field Anesthesia Machine 885A)."

A picture of the "whiz wheel" is here: www.wramc.amedd.army.mil/fieldmed/885atutor/Theory.htm

If I can help more, let me know
Mike

Good grief .......this post just singlehandedly ruled out anesthesia of any sort as a specialty for me and guaranteed I will avoid all physics classes like the plague.........LOL......Kudos to anyone who understands this stuff.....

Don't let this get you down, break it down one sentence at a time and just practice the problems. Thats why are school is as long as it is, you will get taught what you need

Mike

If I remember correctly, there is also a temperature component of the copper kettle type vaporizer. The wheel you refer to has temperature, total flow components to calculate the gas flow through the vaporizer to determine concentration. Check out Boyle's Law. Modern vaporizers are temperature compensated.

This is from an old CRNA who hasn't thought much about the gas laws for years, but who still understands the concepts.

Yoga

Wasn't there an old rough estimate technique using vernitrols? Something along the lines of using a 5 liter fresh gas flow with ethrane, and 3 liter flow with halothane (or was it the other way around), and reading "directly" from the sidearm using 100cc/min as 1%? Does anyone else remember something along this line? I remember it being easy back then when we did it all the time, but OMG, it's been a long time since copper kettles and vernitrols. We never even thought about end-tidal agent concentrations since we had no gas monitors of any kind to measure them. Sure am happy we have all these modern conveniences now.

I still have an anesthesia machine with the side-arm vernitrol and slide rule calculator. It is in storage and I will probably trash it, when I figure out how to get rid of it. You had to know how to use the machines, but the concept was simple. You could put any agent in the vaporizer and just calucate the concentration. I reviewed one lawsuit where the anesthetist did a wrong calculation and delivered a lethal dose of the agent.

Modern anesthesia has many more gadgets, but it is a lot easier.

By the way, a sound understanding of the basic physics involved is essential to knowing what you are doing when you use an anesthesia machine. And, just as important--how to troubleshoot.

Yoga

Ditto for me.......I feel utterly stupid now.

Yeah, me too!
After my BSN I'm going to CRNA school (in approx. 1-2yrs). I hope all of you are still around then!!! I may need some serious help!!