Anesthesia

VIN News Service photo

Applying lower flow rates and selecting more efficient equipment are ways that veterinarians can cut their use of inhalants for general anesthesia. Inhaled anesthetic agents are considered potent greenhouse gases because they stay in the atmosphere longer than carbon dioxide and trap more heat.

Veterinarians increasingly are being asked to limit their use of inhaled anesthetics amid concerns the gases are making an outsized contribution to global warming.

The calls for change, such as by using lower flow rates, are coming from individual veterinarians, at least one professional association and owners of animal hospitals, including Mars Inc., the world's largest.

Inhaled anesthetic agents (IAAs) — including nitrous oxide, desflurane, isoflurane and sevoflurane — are disproportionately potent greenhouse gases. Scientists agree their presence in the atmosphere is fractionally small compared with the carbon dioxide emitted by things like coal-fired power stations, motor vehicles and aircraft. But due to their chemical structures, IAAs have hundreds to thousands of times more global warming potential per molecule than carbon dioxide.

IAAs' contribution to climate change often has been treated less seriously due to their medical importance. In the past five to 10 years, though, greater attention gradually has been paid to managing their use for environmental reasons, initially in the much larger human-medicine sector and more recently in the veterinary realm.

"We tend to view health care as some sort of bubble — that everything we do is beneficial and the environmental impact doesn't count," said Dr. Matt McMillan, a veterinary anesthesia specialist based at an independent practice in Marlow, England. "However, everything we do, all the amazing things we do, have a knock-on cost on the environment, and the further we go into this climate crisis, the more we have to think about these effects."

McMillan authored a study published last year that assessed how much environmental damage practices could prevent — and how much money they might save — by auditing and adjusting their inhaled anesthesia techniques. The research involved retrospectively reviewing the records of 100 consecutive anesthesia procedures from a typical week at the Queen Mother Hospital for Small Animals at the Royal Veterinary College in London.

McMillan found that applying a hypothetical intervention, which included using already available equipment and lowering flow rates, could reduce a practice's carbon footprint from IAAs by a whopping 63% — without compromising patient welfare.

The results, published in the Journal of Small Animal Practice, were telegraphed in a press release by the British Small Animal Veterinary Association, a large professional membership group in the United Kingdom. The journal's editor, Dr. Nicola Di Girolamo, described the study as "a good first step" toward understanding how veterinary practices could reduce IAA consumption.

Research in the animal-health realm also has been carried out by Dr. Ellie West, a clinical veterinary anesthetist and the environmental sustainability lead at Mars U.K. subsidiary Linnaeus. West last year started steering a campaign at Linnaeus that encourages its 180-some practices to lower their use of IAAs.

"While volatile anesthetic agents have a low atmospheric persistence and constitute a relatively low proportion of global carbon emissions, they are potent greenhouse gases," West said in an emailed statement to the VIN News Service. "We believe veterinary professionals have a responsibility to reduce their environmental impact."

The British campaign, which offers guidance, education and reduction targets, has been at the forefront of Mars' efforts to reduce its IAA use globally. "Looking ahead, we are exploring various avenues to continue reducing the climate impact of anesthesia around the world," Mars Veterinary Health global sustainability director Margo Mosher said in a separate email.

Mars, a conglomerate known for making candy, has a goal of reducing its company-wide greenhouse gas emissions by 27% by 2025 and achieving net-zero by 2050. "Outside of our overarching emissions reduction goal, we haven't yet set Mars Veterinary Health-wide anesthetic gas reduction targets, but we are considering it as we understand more about how anesthesia impacts and contributes to our overall carbon footprint," Mosher said.

Attempting to grasp the magnitude of the problem

Views differ on the extent to which anesthetic gases contribute to climate change.

West points to a study published in 2012 that estimated that inhaled anesthetics had a concentration in the atmosphere about 1/100,000th that of carbon dioxide. But because the agents have more global warming potential — a calculation of how long a gas remains in the atmosphere and how strongly it absorbs energy — the paper estimated they were responsible for 10% to 15% of the total human-made "radiative forcing" of climate in the industrial era. (Radiative forcing refers to the imbalance of energy entering and exiting the atmosphere. When more solar radiation enters and less infrared radiation exits, as scientists say is occurring now, the atmosphere warms.)

That 2012 paper also was cited in a review of the literature published last year and conducted by scientists at the pharmaceutical company AbbVie. They concluded that while IAAs contribute to greenhouse gas emissions, the magnitude of the contribution is "lower than those of other human-produced substances." Some other scientists question the radiative-forcing power of IAAs, arguing it is "vanishingly small compared with CO2."

Regardless, many professional bodies encourage reducing the use of IAAs in human medicine. In September, the World Federation of Societies of Anaesthesiologists released a global consensus statement calling for anesthesia providers to limit use of inhaled gases. Educational institutions are getting in on the act: The University of British Columbia in Canada, for instance, has purchased modern, low-flow anesthesia machines for the express purpose of slashing its greenhouse gas emissions.

The veterinary sector accounts for a smaller proportion of IAA use compared with human medicine; exactly how much is hard to determine. As a cursory example, the Animal Health Institute trade group estimates the U.S. animal pharmaceutical market comprises roughly 2% of the country's total pharmaceutical market, measured by dollar value. Still, veterinary medicine is a multibillion-dollar industry responsible for executing many thousands of surgical procedures each year.

"A lot of compromise by nations, big and small business and individuals is required for us to meet climate change targets," McMillan said. "We all, every one of us, has a responsibility to reduce our emissions and environmental impact where possible and practicable. The fact is, what I have suggested to reduce IAA emissions is effective, simple, safe and cost-effective.”

The very bad, the bad and the not-as-bad

Some IAAs are worse for the environment than others. For instance, a few persist in the atmosphere for decades. Nitrous oxide has an atmospheric lifetime of 114 years (and is a strong ozone-depleting agent, to boot), compared with nine to 21 years for desflurane, three to six years for isoflurane and one to five years for sevoflurane, according to the review paper published in October.

Mars is "working to decrease" the use of nitrous oxide at its around 2,500 veterinary practices across the globe, having recently announced eliminating the gas for anesthesia at its Linnaeus clinics in Britain, Mosher said.

Desflurane, due to its ability to capture heat in the atmosphere, is a more potent greenhouse gas than nitrous oxide, isoflurane or sevoflurane. Desflurane's use in veterinary medicine is limited, partly due to technical challenges and concerns about its environmental impact, to the point where it is applied more typically for teaching purposes.

"Dose for dose, desflurane is the worst, followed by isoflurane, then sevoflurane. Nitrous oxide is also pretty bad," McMillan said.

A veterinarian using desflurane for an hour at an oxygen flow rate of one liter per minute releases the carbon-dioxide equivalent of driving an average U.K. gasoline-powered car 348 miles, according to a paper published in 2019 by West and Dr. Ronald S. Jones. For nitrous oxide blended with sevoflurane at a dose of equal potency, it's 140 miles; for isoflurane, it's 12 miles; and for sevoflurane alone, it's four miles, the paper said.

Sevoflurane isn't necessarily apt for all situations, though. For one, it is currently more expensive than isoflurane, McMillan said. There are also clinical considerations, West said, who suggested that practitioners may wish to consult an anesthesiologist about their options. For instance, sevoflurane works faster on patients, there are datasheet warnings about a build-up of toxic byproducts if used at low flow rates, and its use may be subject to licensing restrictions in some jurisdictions.

Choosing equipment

Beyond the type of IAA used, there are other aspects of providing anesthesia that affect the procedure's greenhouse gas emissions.

One is the anesthesia delivery system itself. There are two options: non-rebreathing systems or circle rebreathing systems.

Both involve adding a liquified anesthetic agent to a vaporizer, which converts the liquid into a gas that is mixed with a flow of oxygen. The gases are then inhaled by the patient via a tube placed through the mouth into the trachea, or via a face mask.

Non-rebreathing systems are easier to use but create more waste. "Basically, every breath the patient takes is fresh gas direct from the anesthetic machine, and carbon dioxide is flushed away from the patient by the flow of gas so it can't be breathed back in," McMillan explains.

By contrast, circle breathing systems remove the carbon dioxide a patient exhales, using a substance called soda lime, enabling the patient to rebreathe the remaining anesthetic gas. "The amount of gases taken out by the patient is very small, so in theory, you only need to add this amount back into the system to maintain equilibrium," McMillan said.

Circle breathing systems generally are considered less appropriate for small patients, though, because such patients may have difficulty drawing air from them. West recommends circle systems for patients weighing more than 5 to 10 kilograms (11 to 22 pounds), depending on the particular system; studies indicate some systems can be used safely in animals weighing as little as 3 kilograms (6.6 pounds) by using narrower breathing hoses.

McMillan's hypothetical intervention to reduce IAA emissions by 63% involved using circular systems for animals weighing more than 5 kilograms. For animals weighing less, he applied Lack or Humphrey systems, considered to be particularly efficient models of non-rebreathing systems.

McMillan and West acknowledge there might be an upfront cost associated with investing in equipment, plus ongoing costs for soda lime and potentially closer monitoring. But these expenses, they maintain, may be more than offset by the reduced waste.

How low can flow rates go?

Another important variable is the oxygen flow rate used in the anesthesia system. The higher the flow rate, the more anesthetic agent is delivered to the patient. Oxygen flows can vary markedly in veterinary practice, but typically are applied at a rate of at least one liter per minute.

While no standard definitions exist, West considers "lower flow" anesthesia to mean bringing fresh gas flow rates to a minimum of around one liter per minute, and "low flow" anesthesia to mean rates as low as 0.5 liters per minute. McMillan says rates can conceivably go lower still, although the lower the flow, the greater the potential safety risk to patients because they may receive too little oxygen or the anesthetic gas may be insufficient to keep them unconscious. These risks are greater if the veterinarian is not well versed in low or ultra-low anesthesia.

Dr. Lydia Love, a clinical assistant professor of anesthesiology at North Carolina State University College of Veterinary Medicine, uses low flow rates to limit greenhouse gas emissions. She also reuses whatever plastic objects she can, such as endotracheal tubes and breathing circuits that are marketed for single use, to cut down on plastic waste.

"There's a learning curve to using low fresh gas flow rates in anesthesia because most veterinary anesthetists are not really thinking about what oxygen flow they are using, why they are using that, and what the physics of the circuit really are," Love said via email. "Generally, they just use a safe but excessive amount.

"Also," she added, "I doubt most veterinary anesthetists — that is, folks performing anesthesia who are not anesthesiologists — realize that inhalants are greenhouse gases and that those choices matter."

Still, Love warns that reducing flows to as low as 0.5 liters per minute requires greater training, including understanding how long it takes for adjustments in anesthetic concentrations to travel through the equipment and affect patients. "Someone who chooses to use closed or low fresh gas flows during anesthesia should be aware of this fact and be ready to adjust oxygen flow if necessary to make a quick change," she said.

Love also is an anesthesia consultant for the Veterinary Information Network, an online community for the profession and parent of VIN News. She was among a group of anesthesia specialists who recently chimed in on a VIN message board discussion about the potential environmental harm of IAAs.

Another supporter of using lower flow rates is Dr. Nancy Brock, a VIN consultant based in Ontario, Canada. As she sees it, recognizing the environmental harm caused by IAAs and gaining a better understanding of how the agents actually work go hand-in-hand.

"When [veterinarians] try to understand the way low flow functions, they gain insight into the impact of flow rates and thus become more knowledgeable and more in control," Brock said. She's conscious of potential financial benefits, too: "I jokingly point out that what is exiting the exhaust as excess is everybody's raise."

Deep cuts to flow rates need not be necessary to substantially reduce waste: In his hypothetical intervention, McMillan was able to achieve the 63% reduction by applying a conservatively low minimum flow rate of one liter per minute.

Could injectables replace inhalants?

Another way to reduce the environmental impact of IAAs is to replace them with something else.

Injectable chemicals commonly used to anesthetize animals include propofol and alfaxalone. They often are used in combination with IAAs: A small dose of injectable gets the animal to sleep, then a gas keeps it asleep. Using injectables only, in a technique known as "total intravenous anesthesia," involves continuing to administer those drugs to the animal instead of turning on the gas.

Brock, the specialist in Canada, reckons total IV anesthesia is "doable" with adequate training. "Compared to when I began my career in anesthesia in the late '80s, we have way better injectable drugs if we opt to shift to total IV," she said.

Others, including McMillan, are wary of the level of training needed to go that route. Injectables also can be more costly financially and have environmental impacts of their own, stemming from the carbon footprint created by their manufacture, packaging, sterilization, transport, application and disposal.

Another thing to consider is whether injected agents could be toxic, say to fish, when they are excreted in urine or feces and end up in waterways. "Not a huge amount is known about this," McMillan said.

Setting aside total IV anesthesia, practitioners may still deploy injectable drugs as an effective part of the mix. Appropriate patient premedication alone, such as opioids, alpha-2 agonists and acepromazine, will significantly reduce the inhalant requirements for any given procedure, according to West. Several other intravenous adjuncts, including lidocaine and ketamine, and the use of local and regional techniques — in which pain is blocked in the region of the surgery only — can further reduce the amount of inhaled anesthetic used.

Dr. Kris Kruse-Elliott, a veterinary anesthesia specialist and VIN consultant based in Nevada, finds the risks associated with IAA wastage too disturbing to ignore, and is working to raise awareness of the issue.

"I spend a lot of time these days educating folks on anesthesia, and we always have discussions regarding lower flow rates; when and how," she said.

While there may be no one simple, obvious approach, Kruse-Elliott believes that a combination of approaches can be effective. "The learning curve and education needed to move to more total injectable techniques is a barrier, for sure ..." she said. "I feel like there is a happy medium to use IV adjuncts to anesthesia for further reduction of inhalants … and low flows when we can."