Using Inhaled Anesthetics Differently Could Reduce Global Warming Hazard

Avoiding nitrous oxide (N2O) and unnecessarily high fresh gas flow rates during anesthesia could reduce the environmental impact of inhaled anesthetics, according to a new study.

Researchers at the University of California, San Francisco (UCSF; USA) measured the infrared (IR) absorption cross-sections values for desflurane, sevoflurane, and isoflurane to best determine their atmospheric lifetimes, and used the results to determine their global warming potential (GWP). The total quantity of each anesthetic used in one minimal alveolar concentration (MAC)-hour was then multiplied by the calculated GWP, and expressed as carbon dioxide equivalent (CDE) in grams. Common fresh gas flows and carrier gases--both air/oxygen and nitrous oxide (N2O)/oxygen mixes--were considered in the calculations to allow these examples to represent the common clinical use of inhaled anesthetics.

The results showed that desflurane had a much longer GWP in the atmosphere than the other anesthetics studied, almost 10 years, compared to 1.2 years for sevoflurane, and 3.6 years for isoflurane. Considering the flow rates at which the different anesthetics were given, desflurane had about 26 times the GWP of sevoflurane and 13 times the impact of isoflurane. The carrier gases with which anesthetics are given also affected their GWP impact; in particular, giving N2O as a carrier gas increased the GWP of isoflurane or sevoflurane, but decreased the GWP of desflurane. However, since N2O continues to have an impact over a longer timeframe, the researchers suggest that it may not be an environmentally sound tradeoff to use N2O as a way to reduce the GWP of desflurane. The study was published in the July 2010 issue of Anesthesia and Analgesia.

"Changes people could make in their practice right away could improve the health of the community and the planet,” said Susan Ryan, M.D., a clinical professor of anesthesiology at the UCSF. "After patients inhale them, those anesthetics as well as other gases used to dilute them are usually vented outside the hospital. Some are potent greenhouse gases that can contribute to global warming for decades.”

Inhaled anesthetics are recognized greenhouse gases since they undergo very little metabolic change in the body; the gases exhaled by the patient are almost identical to those administered by the anesthetist, and are usually vented out of the building as medical waste gases, remaining for a long time in the atmosphere. Depending on the types of anesthetic used, an average midsize hospital has an environmental impact comparable to that of 100-1,200 cars per year; using desflurane for one hour is equivalent to 235-470 miles of driving.

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