A closely watched execution attempt in Nashville descended into chaos after Tennessee death row inmate Christa Gail Pike survived two separate doses of lethal pentobarbital, according to the Associated Press. The failed procedure immediately raised urgent questions about the drug, its administration and the state’s execution process.

Pentobarbital is a short-acting barbiturate that slows activity throughout the central nervous system, according to medical authorities at the National Institutes of Health. It works by enhancing the activity of GABA, the brain’s primary inhibitory neurotransmitter.

The drug was historically prescribed as a sedative, sleep aid and antiseizure medication. Medical uses have also included sedating patients before anesthesia or surgery and inducing medically managed comas in some patients with severe traumatic brain injury and dangerously elevated intracranial pressure.

According to the NIH and Cleveland Clinic, pentobarbital can be used as a central nervous system depressant to treat severe seizures and induce medically managed comas. However, newer and safer drugs carrying lower overdose risks have largely replaced barbiturates in hospital settings, making pentobarbital uncommon in routine patient care.

Pentobarbital remains a primary agent in veterinary euthanasia and lethal injection protocols in several U.S. states, according to the National Library of Medicine, veterinary drug labeling approved by the FDA and the Death Penalty Information Center. In a lethal injection protocol, it is administered intravenously in quantities typically ranging from 2.5 to 5 grams, far above clinical therapeutic doses.

Medical sources describe three stages expected after the drug enters a vein. Pentobarbital first crosses the blood-brain barrier and is intended to induce surgical anesthesia and unconsciousness within seconds to a few minutes.

As brainstem activity slows, respiratory depression suppresses the body’s ability to breathe automatically, causing respiratory arrest. After breathing stops and oxygen levels fall, the drug’s depressant effects contribute to the heart eventually stopping.

Pike, 50, was convicted in the 1995 murder of classmate Colleen Slemmer. She was slated to become the first woman executed in Tennessee in more than 200 years.

Media witnesses and defense court filings reported that Pike remained conscious and vocal, experienced breathing difficulties and snored loudly for more than 40 minutes after receiving the drug. Witnesses also observed her asking whether her arm was supposed to feel as though it were burning, breathing raggedly and audibly snoring behind closed curtains during a span lasting more than an hour.

According to the Tennessee Department of Correction, officials administered two doses of pentobarbital under the state’s established single drug execution protocol. State officials maintained that staff strictly followed the protocol and that the chemical supply had been verified.

Defense filings described difficulty establishing clear intravenous lines, identifying a potential failure point in the procedure. Despite receiving two doses, Pike continued breathing, occasionally lifted her head and audibly snored for more than 40 minutes after the second dose.

Prison staff ended the procedure at 8:53 p.m. and transferred Pike to a hospital outside the prison. Tennessee Gov. Bill Lee subsequently halted all remaining executions in the state pending an independent investigation.

Court filings and medical literature have raised several possible explanations, although no cause has been confirmed in Pike’s case. One possibility is intravenous infiltration, which occurs when a line slips out of a vein and the drug leaks into surrounding tissue rather than entering the bloodstream.

If pentobarbital enters surrounding muscle and fat, absorption may be too slow for the drug to reach the brain at a concentration sufficient to cause unconsciousness and death. Individual differences in liver function, drug metabolism or previous exposure to barbiturates could also affect the body’s response.

Questions have also been raised about drug quality and potency. Because major pharmaceutical manufacturers restrict the use of their drugs in executions, states may obtain pentobarbital from compounding pharmacies, creating questions about potency, consistency or storage.

As Tennessee officials investigate the failed execution, Pike remains under medical observation, according to reports. Legal challenges are continuing over whether the state may lawfully attempt to execute her a second time.