January 01, 2015


 Though not always ideal, inhalants can be an effective euthanasia method

Posted Dec. 17, 2014

Of the many agents that can be used to end an animal’s life, inhalants are among the most controversial.

Every year in the United States, carbon dioxide, carbon monoxide, and a combination of other gases and anesthetics are used to euthanize millions of animals in laboratories, on farms, and in animal shelters.

Used properly, these inhalant agents can kill an animal quickly and safely. Research has shown, however, that some, particularly carbon dioxide, have the potential to compromise animal welfare while others, such as carbon monoxide, raise human safety issues.

“We get the impression that inhalants are the problem child of euthanasia methods,” observed Dr. Robert Meyer, head of the Inhaled Agents Working Group for the AVMA Panel on Euthanasia, which wrote the 2013 edition of the AVMA Guidelines for the Euthanasia of Animals. He was speaking at the AVMA animal welfare symposium, titled Humane Endings, in suburban Chicago, Nov. 3-5, 2014.

Dr. Meyer is a professor at the Mississippi State University College of Veterinary Medicine and a diplomate of the American College of Veterinary Anesthesia and Analgesia. He noted that in the current guidelines, inhaled gases continue to be classified as acceptable euthanasia agents when administered under specified conditions. In the 2013 guidelines, efforts were made to clarify the conditions under which these agents can be properly used, Dr. Meyer said.

The guidelines state the “euthanasia process should minimize or eliminate pain, anxiety, and distress prior to loss of consciousness. As loss of consciousness resulting from these mechanisms can occur at different rates, the suitability of a particular agent or method will depend on whether an animal experiences distress prior to loss of consciousness.”

We get the impression that inhalants are the problem child of euthanasia methods."
Dr. Robert Meyer, head,
Inhaled Agents Working Group,
AVMA Panel on Euthanasia

The Panel on Euthanasia recognizes that a total absence of pain and distress during the euthanasia process cannot always be achieved, however. “The Guidelines attempt to balance the ideal of minimal pain and distress with the reality of the many environments in which euthanasia is performed,” according to the document.

Dr. Samuel Cartner, program chair of the Humane Endings symposium, explained that controversies over the use of inhaled euthanasia agents in research settings fall into two general areas. First, some research has shown that carbon dioxide appears to be somewhat more aversive to rodents than other inhalants, such as isoflurane, a halogenated ether used for inhalational anesthesia. Other studies have not shown a clear benefit to using isoflurane as an alternative to CO2, however.

“Making a recommendation to change from CO2 to other inhalants is controversial because ... the increased degree of aversion of rodents to CO2 is very slight. Changing to other inhalants for such a marginal improvement would increase the cost of research and create additional personnel safety issues,” said Dr. Cartner, who is the assistant vice president for animal research services and director of the Animal Resources Program at the University of Alabama-Birmingham.

The other area of controversy in the AVMA guidelines involves the recommended chamber displacement rate of 10 to 30 percent. Many researchers believe a faster displacement rate would be better for animal welfare because unconsciousness and death are quicker.

However, Dr. Cartner explained that the recommended displacement rate is based on scientific evidence from both humans and animals and that with the recommended rate, unconsciousness occurs before pain is experienced as a result of formation and accumulation of CO2 byproducts on mucous membranes.

Dr. Kathleen Pritchett-Corning is a senior clinical veterinarian at Harvard University who oversees a facility with more than 50,000 rodents. There’s a movement in the United States and Europe away from using CO2 to euthanize laboratory rodents, she observed, but the problem is there’s nothing to move toward.

“I’m glad people are doing this work, that people are discovering how rodents interact with CO2 and what it might mean to the animal,” Dr. Pritchett-Corning said. “But there are reasons that it’s still in use. Don’t take my tool away unless you can replace it with something better.”