The general consensus is that when a veterinarian euthanizes a cat with sodium pentobarbital the experience is painless for the cat. That’s the whole point of euthanasia. The veterinarians who write the articles on the Cornell University College of Veterinary Medicine website state that “the procedure involves a lethal injection of sodium pentobarbital which according to the AVMA has the ability to induce loss of consciousness and death without causing the pet any pain, distress, anxiety or apprehension”. They refer to the AVMA (American Veterinary Medical Association) but they have endorsed what the AVMA says.

Pentobarbital used to kill prison inmates causing pulmonary oedema
Today, in The Times newspaper Danny Kruger writes that “in the context of execution it has been concluded that barbiturate overdoses result in distressing and painful death, as the person’s lungs fill with fluid and they drown”. Danny is referring to executions of prisoners in the USA.
On the npr website there is an article titled: “Gasping For Air: Autopsies Reveal Troubling Effects of Lethal Injection”. American prisons use a variety of euthanising drugs, one of which is pentobarbital. NPR investigated whether these drugs, including pentobarbital, caused distress in prisoners when they were executed. They found that the drugs caused pulmonary oedema in 84% of the cases. They reviewed more than 200 autopsies obtained from public records.
A federal public defender, Alan Bohnert, who represents Ohio inmates with upcoming execution said that “These autopsy reports show definitively without question that these inmates are developing pulmonary oedema”. The condition results in bloody froth filling both main stem bronchi. In other words, the lungs fill with a frothy liquid which prevents breathing.
Medical experts say that pulmonary oedema is painful both physically and emotionally because it induces a feeling of drowning which causes panic and terror. It’s a bit like waterboarding torture.

Clearly, there is a big fight on between US lawyers and the prison administration in America about the use of pentobarbital and other drugs (as seen in the chart above) used to execute prisoners.
Is the use of pentobarbital painless for cats?
When I saw this article, it prompted a question in my head, namely whether when cats are euthanised with apparently the same drug do they also suffer from pulmonary oedema before dying? The best of my knowledge, cats do not show distress when they are being euthanised with sodium pentobarbital. However, I have not attended the euthanasia of any of my cats. I couldn’t do it but the next time I probably will because Jackson Galaxy says that it is the last duty of a cat caregiver.
Pentobarbital is the top method of animal euthanasia and in some US states legislation dictates that it is the only method according to research by the AVMA (American Veterinary Medical Association).
One study on the use of pentobarbital on rats states that a “growing body of evidence challenges the efficacy and utility of intra-peritoneal pentobarbital. This methodology has been described as inconsistent and may induce pain and stress”. This study was published on May 2020 and they recommend that “a review of the literature is needed to assess the evidence surrounding this killing method, the associated welfare implications, and potential for refinement.”
In another study published in May 2020 they conclude that this method of euthanasia “has important drawbacks including the potential for pain and mis-injection”.
What they’re saying there is that it is possible to position the hypodermic needle in the wrong place but there’s no mention of pulmonary oedema in the study summaries.
My conclusion, though, is that I would like somebody to write a comment on whether cats feel pain when they are euthanised with sodium pentobarbital. Do we know? Are we making assumptions? Does the process need to be reviewed? One lady in one shelter in the US said that carbon monoxide poisoning in a gas chamber is better. She might even be correct for all I know.
SOME MORE ON EUTHANASIA:

Do domestic cats feel betrayed for an instant when you instruct a veterinarian to euthanise them?

Is euthanizing a healthy cat cruel?

Assisted dying of people is good for pets

British government briefly considered ordering all domestic cats to be killed
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Note: sources for news articles are carefully selected but the news is often not independently verified.
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