The theory: This is how Christa Pike survived two fatal doses

When US prisoner Christa Pike, 50, was to be executed with the drug pentobarbital, at least seven needles were used in an attempt to find a suitable vein in her arm, according to her lawyers.

Finally, the medical team set up intravenous lines where the preparation could be administered.

According to the lawyers, Pike suffered blisters and burns around the entrances after the injection, which may indicate that some of the drug leaked into the skin tissue around the injection sites instead of reaching the bloodstream, several medical experts told the New York Times.

This may, say the experts, have been the reason why Christa Pike survived the execution - despite two lethal doses of pentobarbital.

Small veins may have contributed

According to anesthesiologist Joel Zivot, who was previously hired as a medical expert by Pike's lawyers, Pike's small and possibly fragile veins may have made her particularly vulnerable to leakage, the BBC reports.

According to the lawyers, medical staff have previously had difficulty taking blood samples from Pike because of her veins. They also reportedly warned that Pike suffers from a blood disorder that could further complicate the process.

Experts state that it can also be a fault with the medicine itself, for example that it was incorrectly prepared or passed the expiry date.

According to witnesses, after the first dose, Christa Pike complained of a burning pain in her arm. Anesthesiologist Mark Heath says the pain, along with the burns, should have been a clear warning sign that the inlet was leaking.

The Tennessee Department of Corrections has stated that procedures for the execution process were followed to the letter.

Risk of permanent brain damage

After the failed execution attempts on September 30, Christa Pike was taken to hospital for life-saving care. A week later she was reported to be conscious and able to speak.

"The prognosis is still unclear, but we expect a long recovery period," the lawyers write in a statement.

According to experts, prolonged exposure to a non-lethal dose of pentobarbital carries a risk of permanent brain damage and serious complications.

Christa Pike was sentenced to death in 1996 for torturing a classmate to death together with her then-boyfriend the year before.

She would have become the first woman in over 200 years to be executed in the state of Tennessee.