RE: FGF rates for closed GA circuit

James Colver Cert. Ed, RVN — Sun, 10 Mar 2019 19:20:54 GMT

It's not something I have ever attempted or felt the need to attempt in 20 years! But Susanna Taylor posted the following in response to the isoflo shortage and has the info you are looking for..

Given the alert about upcoming shortage in the UK of Isoflurane, I thought I would jump the gun and post some info about Low Flow anaesthesia.

Details about the supply issue can be found here:

https://www.gov.uk/government/news/veterinary-medicine-supply-problem-isoflurane

Here we go...

How will Low flow anaesthesia help?

Isoflurane usage is mostly reliant on how much gas flows through the vaporiser. The higher the flow the more Iso is picked up. 2% of 1L/min is 20mLs, 2% of 4L/min is 80mL etc. The lower the flow of gas, the longer the vaporiser contents (and therefore your precious Iso bottles) will will last!
Also, apart from minimising disruption in times of shortage, Low flow anaesthesia has other benefits: cost reduction of oxygen and inhalation agent, reduced waste anaesthetic gas, reduced risk of (theatre) environmental pollution, reduced atmospheric pollution (Greenhouse gases etc), and better ability to retain warmth in the breathing system for your patient.

How much oxygen do our patients actually need?

Metabolic oxygen usage is about 8-15ml/min (so for a 30kg lab, roughly 300mL a minute) - not much really
But we do need it in enough VOLUME to reach the lungs - i.e. Tidal volume (10-15mL/kg) - so a bit more
Our flowmetre is calibrated as L/min so we use minute volume (TV x RR) to know how much volume is to be set.
we breathe 21% room air normally, and under anaesthesia we need at least 30% - nowhere near 100%

Why do we use such high flow rates and 100% oxygen in veterinary anaesthesia then?

Flow rates are dependant on the breathing system. Remember 'circuit factors'? these ensure the breathing system works efficiently at removing the CO2 in time before the patients next breath - its not really much to do with giving enough oxygen for the patient to use (as we are actually giving loads), its mostly just about 'blowing away' the CO2 from the system in the respiratory pause.
We use 100% oxygen because its easy - most of us have machines that only have oxygen flowmetres, so we can only give 100%. Human machines nearly always have medical air connections, so the %O2 can be adjusted as needed. Similar if N2O is used. Actually, using 100% oxygen on longer cases may even be detrimental.

What's the aim of Low Flow anaesthesia?

To JUST give enough gas volume to meet oxygen demand and remove CO2, and not waste any other oxygen.

How do I use my circle system to do Low Flow anaesthesia?

Circles are designed to reuse expired oxygen, and inhalation agent and they do this by removing the CO2 within the system. When we use high flow rates (2L and above) we are driving the expired gas out the APL valve and off to scavenging before it gets chance to be recycled. We do NEED to do this initially, so that we fill up the system with enough inhalation agent, and also remove all the patients Nitrogen (from breathing room air, about 70%). We can also use these higher flow rates to rapidly change the concentration of inhalation agent, such as at recovery.
But once the patient and system are settled, we can start to recycle the oxygen and inhalation agent:
To use Low Flow with a circle, you just need to lower your oxygen, (after the first 10mins,) to about 1-1.5L. Then keep an eye on the rebreathing bag; if it goes a bit flat, close the valve a bit until it stays round (but of course not taut.).
Thats it!

What if I dont have a circle?

A Lack is better than a Bain (as I will discuss for mini lack vs T piece), or even a trusty Magill is better.

How about smaller patients?

Commonly we dont use circle systems in less then 10kg animals as we worry they can't move the valves well enough BUT there are circle systems available on the market for this size range, or you can adjust your current system with smooth bore, narrower tubes. The key is to watch a capnograph and look for rebreathing (FICO2 >1); if there isn't, the patient is probably fine.
The next best low flow system for <10kg is the MINI LACK. As this system has the reservoir bag on the inspiratory limb, there is less risk of rebreathing, and so the circuit factor is only x1 (i.e. Minute volume, - nothing extra is needed). If used carefully, with a capnograph, circuit factor can actually be reduced to x0.8 as reduced pressures in the system allow dead space gas to return to the inspiratory limb. This is a HUGE reduction compared to standard T Piece flow rates

What if I only have a T-piece?

T-pieces require a higher flow rate, circuit factors of 2.5-3. because the bag is on the expiratory limb. SO firstly, start calculating flowrate based on your actual patients weight and RR. It might be less then you are using. If you have a capnograph you can get even better...

How can a capnograph help me?

AS i said, our flow rates are really about removing CO2. Capnographs tell us exactly how much CO2 a patient is breathing IN and out. As long as they are breathing IN 0, our flow rate is ok...OR too HIGH! We can slowly turn our flow rate down and monitor FiCO2 - if it stay as 0 its , and we can turn down a bit more. When they start to breathe in CO2 we know we've gone too far, and we need to turn up a little again. But at this point we can pat ourselves on the back and know we are using the lowest possible safe flow rate

Is there anything else I should be wary of?

Yes; some vaporisers are not very well calibrated when under 1L of gas flows through them, so you might get less or more Iso then you want. Its likely not to be a dramatic change, and so careful monitoring of depth is all thats needed, BUT you may prefer to play it safe and decide 1L is as low as you go. IF you are lucky and have a posh monitor that tells you inspired Iso levels, you don't need to worry, just keep an eye on this.

Where does the Humphrey ADE fit in?

The ADE works commonly as a Mini Lack, or a circle, so all the above applies. The fancy APL valve makes it even more suitable for low flows, so feel happy if you have one of these.

Anything else?

Embracing MAC reducing anaesthetic techniques (CRIs, PIVA, Local anaesthesia, etc) will also reduce the patients requirement for iso and will also play a part in lengthening reserves, But thats a whole other CPD subject!

What next?

Use this as an opportunity to embrace low flow and reassess your current practices. Even if you aren't short of Iso yourself, others might be, and anyway its better for our patients, our pockets, and the earth
Invest in some new systems - Mini lacks and narrow bore tubes are not that expensive, Or convince your boss to invest in a new circle and a capnograph - used properly they save money in the long run.
Please ask any questions - this a nice diversion from the work Im actually meant to be doing! . and of course constructive criticism is always welcome too.

RE: FGF rates for closed GA circuit

Nicola Smith — Sun, 10 Mar 2019 17:53:44 GMT

I've not come across any practices using circle fully closed, generally used with valve slightly open. I think to run fully closed you need to calculate animals individual metabolic oxygen consumption then would need very close monitoring.

RE: FGF rates for closed GA circuit

Jessica Atkins — Sun, 10 Mar 2019 10:05:27 GMT

Hiya - I'm seeking info for any closed circuit used with a fully closed APL valve. I assume that the circle is the most commonly used as I don't know any practices that even own a to-and-fro though that's the only other one I can find in the textbooks.

RE: FGF rates for closed GA circuit

James Colver Cert. Ed, RVN — Sat, 09 Mar 2019 20:39:10 GMT

Hi, can you be more specific regarding which circuit you are referring to and whether you intend on using it fully closed?