The anaesthesiologist who told a court that a new two-drug protocol used in an execution in Ohio would cause the inmate “agony and horror”, has expressed anger the state pressed ahead with the experiment despite his warnings.

David Waisel, associate professor of anaesthesia at Harvard medical school, who acted as expert witness for Dennis McGuire's defence attorneys, said he was angry when he learned Ohio had gone ahead with the execution last Thursday using a previously untested combination of midazolam and hydromorphone.

Eyewitness accounts from inside the death chamber suggest his predictions turned out to be accurate.

“Initially I was angry, because I told them this would happen. Now I'm very sad about this. I'm also horrified and aghast. This was all totally unnecessary,” he said.

McGuire was executed for the 1989 rape and murder of 22-year-old Joy Stewart, who was about 30 weeks pregnant. Her unborn child also died. Before the execution, members of Stewart's family said in a statement that the manner of McGuire’s death would be more humane than the brutal way in which she died.

On 6 January, Waisel presented Judge Gregory Frost, sitting in the US district court for the southern district of Ohio, with a nine-page declaration in which he spelled out his prediction of what would happen were the state to proceed with a previously untested use of the two drugs. His testimony turned out to foreshadow the eyewitness accounts that reported McGuire gasping for breath for at least 10 minutes, snorting loudly, clenching his fists and trying to sit up from the gurney.

Waisel warned the court its proposed dose of midazolam was too low to induce anaesthesia. He also said the two drugs would take several minutes to kick in, leaving McGuire aware of what was happening while at the same time suppressing his breathing and giving him the sensation that he was suffocating, an experience known as “air hunger”.

In his declaration, Waisel writes: “In light of the insufficient dose of midazolam, it is substantially likely that McGuire will be aware of this agony and horror." He added that the inmate was at “substantial, palpable, objectively intolerable risk of experiencing the agony and horrifying sensation of unrelenting air hunger during the midazolam/hydromorphone execution under Ohio's execution protocol. It is substantially likely that McGuire will remain awake and actively conscious for up to five minutes, during which he will increasingly experience air hunger as the drugs suppress his ability to breathe.”

The same court was told by the state that the two drugs would take effect within a couple of minutes and that there would be no distress or struggle to breathe on the part of the inmate. One Ohio official, addressing Frost, said: “You're not entitled to a pain-free execution.”

Waisel told the Guardian that, were initial reports from eyewitnesses accurate, the only mistake he made in his legal declaration was to underplay the length of time it would take for McGuire to lose consciousness. “I told them that he was going to suffer the horror of suffocation for five minutes — in fact, it appears to have been much longer than that.”

A minute-by-minute timeline of the execution recorded by the department of correction shows that the first syringe of midazolam and hydromorphone was injected into McGuire at 10.27am on Thursday, and the second syringe of the two-drug cocktail a minute later. At 10.42am, the medical team asked for five more minutes to be certain of death, and it was not until 10.52am – 25 minutes after the start of the execution – that the timeline records “no heart sounds, no lung sounds”.

Jonathan Groner, a professor of clinical surgery at Ohio state college of medicine, said there were several disturbing issues thrown up by the department of correction's record. The actual cause of death would have been a massive overdose of hydromorphone, which is an opioid.

“This was a death by narcotic overdose. When heroin addicts die they die unpleasantly — and that's what happened here.”

He said it was also highly puzzling that the officials present at the execution had felt it necessary to ask for five more minutes to ascertain death. “Proving death should not be difficult – there should be no heartbeat or pulse — so that leaves a question of why this took so long.”

Waisel's declaration provides possible clues as to why McGuire appeared to endure such distress over such a prolonged period. Under Ohio's new experimental protocol, the inmate is injected with 10 mg of midazolam.

The drug is a benzodiazepine, in the same family as valium, and only induces anaesthesia if it is given in very high does of around 35-40mg. In addition, McGuire, at 5ft 10ins tall, was severely obese, weighing 253lbs (115kg), which would have further reduced the potency of the drug given his body mass.