Comparison of Sodium Pentobarbital and Carbon Monoxide as euthanasia agents

By Doug Fakkema

       The position on euthanasia methods taken by most mainstream national animal care and control agencies is linked with the most recent AVMA special panel report on euthanasia. The American Veterinary Medical Association (AVMA) has been producing these reports once or twice a decade since 1963. The most recent report, entitled 2000 Report of the AVMA Panel on Euthanasia was published March 1, 2001. There is no legal or ethical requirement to follow AVMA's Panel recommendations on euthanasia. Their recommendations do not carry the weight of law or regulation (except in approximately five states that require euthanasia methods that conform to AVMA's recommendations).

       The American Humane Association (AHA) has recently adopted a position that EBI (euthanasia by injection) is the only acceptable method for animal shelter euthanasia of dogs and cats. AHA no longer approves the use of CO for shelter euthanasia of dogs and cats.

Sodium Pentobarbital

       AVMA considers both sodium pentobarbital and CO as acceptable animal shelter euthanasia methods for dogs and cats. Of the two methods, euthanasia by injection with sodium pentobarbital is considered the preferred method of euthanasia. EBI is a medical procedure (rather than a physical method) and as such is much more humane for the majority of animals. Sodium pentobarbital's mechanism of action is complicated, but brings about a rapid and painless shutdown of the neurotransmitters in the brain resulting in very rapid (+/- 5 seconds) unconsciousness followed, in a few minutes by medical death. Sodium pentobarbital is the method of choice of virtually 100% of veterinarians, nearly 100% of privately funded animal shelters and most, but definitely not all municipally funded shelters.

       Proper training and recommended staff certification is essential to proper delivery of EBI. Like any method, uncaring, thoughtless or poorly trained personnel can abuse EBI and the result is a bad death for the animal.

Carbon Monoxide

       CO is a method of euthanasia that sounds much better than it actually is. Under relatively high concentrations of CO, a non-aquatic mammal will quickly lose consciousness. The concentration should be 6% and time to unconsciousness less than a minute. When delivered properly, carbon monoxide gas does not "sear" the lungs or cause conscious choking or gasping. On the contrary, CO from a bottled and pressurized source is breathed in with little sensation (taste or otherwise). Red blood cells have a high attraction for CO. When breathed in, CO immediately binds itself tightly to the red blood cell. Once bound up with CO, the red blood cell cannot carry its usual passenger, oxygen and the result is a rapid reduction of oxygen to the tissues and brain. Hypoxia (reduced oxygen -- do not confuse with suffocation or asphyxia) sets in within seconds and the animal feels sleepy and soon falls asleep, then unconscious then dies. In theory it sounds good, in actual practice there are problems.

       The number one problem with CO is in the delivery mechanism. Because it is so dangerous to non-target animals, CO must be confined in an airtight box. The box, since it is by necessity airtight, is often hot, smelly, and confining for dogs and cats. It is sometimes overloaded or improperly loaded with incompatible species or animals with respiratory problems. This results in improper absorption (uptake) of the gas and, as a result of less than optimal concentration of gas in the box, a bad death for the animal. This high potential for misuse is most troublesome. In addition, CO, especially home-built boxes are dangerous to shelter workers. A Chattanooga, Tennessee shelter worker passed out and died with his body partially inside the CO box just a few years ago. He was unloading the chamber. Finally, according to the only study ever conducted on CO euthanasia, a high percentage of adult dogs were observed struggling and in an agitated state prior to unconsciousness. Pre-euthanasia tranquilization was recommended by the study to significantly reduce this reaction.

       Although no longer considered acceptable by AVMA or national animal care and control agencies, hotbox CO, where a gasoline engine produces the gas, is still employed by a few agencies.

Generally Recognized CO Minimum Standards

  1. Use only compressed, bottled CO.
  2. Use a commercially manufactured, purpose-built gas chamber.
  3. Chamber is in proper and safe working order and has been inspected by the manufacturer or authorized representative within 12 months.
  4. Chamber door seals are working properly and are free of cracks and tears.
  5. CO cylinders and chamber are located in a well-ventilated environment (preferably outdoors).
  6. Chamber must be equipped with view ports and be well lit inside to permit viewing.
  7. If the chamber is located inside, a CO monitor must be installed with an alarm system, which will sound both in the room with the chamber, and in a separate location.
  8. CO flow rate must be adequate to rapidly achieve a uniform CO concentration of at least 6% after animals are placed in the chamber.
  9. Personnel must be thoroughly trained and understand the proper use, limitations, and safety hazards of CO euthanasia.
  10. . A thoroughly trained individual must observe (by looking through the view port) at least two complete cycles per week and note any irregularities.
  11. Animals euthanized must be healthy (free of major injury and disease which could interfere with the animal's ability to breathe and properly circulate oxygen and CO).
  12. Animals euthanized must be at least 16 weeks of age.
  13. Animals placed together in the chamber must be of the same species and compatible.
  14. Chamber must be thoroughly cleaned between cycles to minimize odors.
  15. Dogs should be tranquilized with acepromazine maleate (.3 mg/lb) at least 10 minutes prior to placement into the chamber. This will "significantly decrease behavioral and physiologic response of dogs…" Chalifoux and Dallaire: 1983.
  16. Animals euthanized must be verified for cardiac standstill (death) prior to disposal of the carcass.]

       Another problem with CO is that the gas does not get into the bloodstream effectively if the animal has respiratory deficiencies due to old or young age, disease or injury. AVMA recommends an EBI backup for such animals. If done properly by the animal care and control agency, this backup is available and personnel are trained in its use. This begs the obvious question: why not use EBI in the first place?

       Much is made of two arguments that imply that CO is a better euthanasia method for animal shelters. The first of the two arguments is that CO is cheaper than lethal injection. It isn't. Numerous cost studies prove that the cost difference is negligible. When done properly, both CO and EBI are nearly equal in cost per animal. What is different is that one person can administer CO euthanasia while EBI properly takes two. The second argument is that CO, since it's a [put the animal in a box, close the door, push a button and] walk away method, is emotionally easier for the employee. This is sheer fantasy typically uttered by people who do not perform euthanasia or who have limited experience with EBI. Those who care for the animals under their protection, always want to do what's best for those animals when and if it comes time to euthanize them.

--Doug Fakkema


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