Wasting less anaesthetic gas to reduce hospital emissions

By Catriona May

Hospitals are typically not environmentally-friendly places, with their reliance on single-use plastics, incineration of clinical waste and use of greenhouse-warming anaesthetic gases.

Taken as a whole, the Australian health care system accounts for around seven per of the country’s total carbon emissions. And if health services across the world were a country, they’d be the fifth largest emitter in the world.

But a University of Melbourne team of doctors and engineers have found a way to cut hospitals’ greenhouse gas footprints by preventing unused anaesthetic gas being released into the atmosphere.

Dr Forbes McGain with lab in background
Dr Forbes McGain is keen to see environmentally friendly anaesthetic gases like Xenon used more widely

These gases have between two and three orders of magnitude more global warming potential per molecule than CO2, and they are pumped out of almost every hospital ventilation system in large quantities.

We’ve found a way to recover these gases, so they can be used by patients rather than disappearing into the atmosphere, says Professor Sandra Kentish, who led the engineering side of the team along with her colleague Dr Liang Liu.

The team developed a filter that can be retrofitted onto anaesthetic machines’ exhaust systems to retain the anaesthetic gas within the patient’s breathing circuit.

Patients only use around three per cent of the anaesthetic fed to them – the remaining 97 per cent is pumped out of the hospital’s ventilation system and into the atmosphere

Because anaesthetic machines ‘breathe’ for patients during procedures, the gases the patient exhales must be released from the breathing circuit as re-circulating them would be harmful. However, unused anaesthetic is also released in this process – and not just a little bit.

Patients only use around three per cent of the anaesthetic fed to them, says Professor Kentish.

The remaining 97 per cent is pumped out of the hospital’s ventilation system and into the atmosphere.

Filter technology

The new filter allows gases with small molecules, like nitrogen, oxygen, CO₂ and water vapour, to exit the circuit and be released via the hospital ventilation system, while trapping larger molecules like anaesthetic gas.

The researchers used sevoflurane in their initial study, which is one of the most commonly used gases in general anaesthesia. They are confident it could be used with some other volatile anaesthetic gases with high global warming potential.

A ceramic tube used as part of the new filter
The new filter features ceramic tubes which are stable at high temperatures and have the option of being sterilised and re-used.

The filter physically blocks the anaesthetic gas from leaving the circuit, explains Professor Kentish. The patient can keep breathing the same anaesthetic, which means less is required overall.

Professor Kentish specialises in filters, or membranes, with experience across a wide range of applications, including CO2 capture, water treatment and dairy processing.

The new filter uses a relatively new technology, which was originally developed for separating CO₂ from other gases like hydrogen. It is ceramic, which means it is stable at high temperatures and can be sterilised and reused. It also performs well in the high humidity conditions of the anaesthetic machine breathing circuit.

We only need to modify the back end of the existing anaesthetic machine, which we think makes economic sense given the potential savings

The team is now looking for an industry partner with whom they can develop a prototype. Professor Kentish anticipates the most challenging aspect will be finding a way to modify the exhaust system without interfering with the atmospheric pressure the patient experiences.

We only need to modify the back end of the existing anaesthetic machine, which we think makes economic sense given the potential savings, she says

Money saving benefits

By dramatically reducing the volume of wasted anaesthetic gas, the filter could not only cut hospital emissions, but could also save millions of dollars in wasted anaesthetic gas in Australia alone.

These savings could be directed towards using greener, more effective anaesthetic gases, says Associate Professor Forbes McGain, an anaesthetist and intensive care physician at Western Health who had the original idea to reduce wasted anaesthetic gas.

Gases like Xenon do a better job than many of the gases we currently use, and they are more environmentally friendly, too says Associate Professor McGain, who worked with Professor Kentish and her team on the filter’s development.

The downside is that they’re very expensive.

A new environmental mission

Associate Professor McGain, who also holds an honorary position at Melbourne Medical School’s Department of Critical Care, has a proven track record in hospital sustainability. In 2009 he piloted the PVC Recovery in Hospitals Program at Western Health, which recycles used PVC medical products into useful new products. It has since been adopted by 55 healthcare settings around the world.

Reducing anaesthetic gas emissions has become his next mission.

There has been a step change in the past five years to research and develop methods to reuse or recycle anaesthetic gases, he says.

But most of these approaches involve gas capture, then reprocessing offsite. The beauty of membrane capture is that we can reduce hospital anaesthetic gas wastage, whilst not requiring gas reprocessing offsite.

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