Gas and carbon – Maccgas

Restriction of use of desfurane and nitrous oxide

Use of desflurane and nitrous oxide was phased out at Macclesfield Hospital in early 2021. Desflurane vaporisers are not available and the piped nitrous oxide system has been disconnected and decommissioned. Cylinders are, for the time being, still available. The piped N₂O/O₂ system is separate and still available for maternity.

This was a group decision, not an imposition. If you disagree, please feel free to make your views known either on the WhatsApp group or in confidence to Dr Banks at davidbanks@nhs.net.

There has been much debate about the effect of anaesthetic gases and vapours on the environment. The facts are now becoming more accepted and established. Notwithstanding debate about upstream manufacturing and transport CO₂ implications of alternatives, it is clear that restricting or stopping the use of desflurane and nitrous oxide are the big “low hanging fruits”.

Sustainable healthcare requires balancing patient outcomes with economic, environmental, and social costs. Primum non nocere- “at first, do no harm”.

Desfurane is about SIXTY TIMES as bad as sevoflurane. Its radiative forcing and atmospheric persistence are worse and you need three times as much of it. Its advantages are very marginal and it is definitely worth avoiding. Use of desflurane with fresh gas flow of 1 litre per minute for an all day list causes as much greenhouse effect as driving a car 1000 Km.

Nitrous oxide is an ozone depleter as well as a greenhouse gas.

Carbon equivalent calculator:

How much is your anaesthetic gas / vapour use contributing to the greenhouse effect? How much is your volatile agent costing every minute? Enter your fresh gas flow and vaporiser setting here. If you are using desflurane you can leave the defaults for the other values, or you can modify them for other agents.

fresh gas flow:		l/min (the combined setting on all the rotameters)
setting on vaporiser:		% set on the top dial
molecular weight:		daltons (168 for DES, 184.5 for ISO, 200 for SEVO)
density of liquid:		relative (to water, which is 1.0)
cost of a bottle:	£	£137.30 for DES
volume of a bottle:		mls
global warming potential:		CO₂ multiples (2540 for DES, 510 for ISO, 130 for SEVO)

Now press

Cost is £ per minute.
In global warming terms you are emitting the equivalent of Kg of CO₂ per minute.

Doing that for an hour would cause as much greenhouse effect as driving miles in an average car.

So how do you work that out?

1. First, work out how much vapour is coming out of the common gas outlet. This will be the percentage set on the vaporiser as a fraction of the fresh gas flow. This is mls/min of vapour. At the moment you only know the cost per ml of liquid, so we need a few more steps.

2. You then have to convert this into moles per minute. Avagadro’s law states that a mole of any gas at room temperature and pressure occupies 24 litres. So divide your mls/min by 24000mls to get moles per minute.

3. The next step is to convert this to weight of liquid per minute. A mole of any substance is its molecular weight in grams, so simply multiply the previous result by the molecular weight to get grams per minute.

Multiplying this result by the relevant “global warming potential” gives the amount of CO2 that would be equivalent to. The result is grams, so you have to divide by 1000 to get Kg.

4. Almost there! We now need mls per minute. Grams per minute would be the same as mls per minute if it was water, but its not. Sevoflurane, for example is 1.5 times heavier than water, so you are using mls of liquid per minute. Then you can easily work out cost per minute.