In 33 North Carolina executions, the average death time was 10 to 14 minutes, depending on the combination of drugs used, the authors report. Calculating each inmate's actual dose, based on his or her weight, they concluded that some did not receive enough.

"The person would feel either asphyxiation or the burning sensation associated with the potassium," said Dr. Leonidas Koniaris, a surgeon and co-author at the University of Miami. "The potassium would cause extreme discomfort, something like being put on fire."

Even the final drug did not always prove fatal as intended. At least one California inmate required a second dose, and the California warden has said additional doses were used in two other executions, the study reports.

Death penalty proponents complained the report's conclusions were based on scant scientific evidence.

"It's more like political science than medical science," said Mike Rushford, president of Criminal Justice Legal Foundation in Sacramento.

Steve Stewart, prosecuting attorney in Clark County, Indiana, where an execution is scheduled for May 4, said the simple solution seemed to be to give a higher dose of the anesthetic, which probably would not satisfy opponents who see all methods as barbaric.

"It doesn't matter a whole lot to me that someone may have felt some pain before they were administered poison as a method of execution," he said.

Dr. Mark Heath, an anesthesiologist at Columbia University Medical Center who has studied lethal injection cases, took issue with some of the paper's conclusions, but said it generally showed that concerns about lethal injection in its current form "are well-justified."

Editors said they sent the manuscript to three independent medical experts for review - an anesthesiologist, a forensic pathologist and someone in charge of a critical care unit, plus a lawyer.

"We were satisfied" with the science, said Dr. Virginia Barbour, a British physician who is managing editor of the journal, published by the nonprofit group Public Library of Science. "The difficulty of a paper like this is that there is very poor evidence for all the kinds of protocols used" in lethal injections, but the authors did a good job analyzing what there is, she said.

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