Oklahoma changed its execution protocols twice this year. State officials have five options for lethal injections, including a new three-drug mixture that was used for the first time Tuesday.

Two of the drugs used carry warnings that they can suppress the respiratory system, and the third warns that cardiac trouble can occur with high but non-lethal doses and lists specific steps to take if a patient receives too much of the drug but doesn't die.

MIDAZOLAM (sedative)

Warning labels that accompany packages of midazolam say intravenous use of the drug has been associated with respiratory suppression or respiratory arrest. Monitoring is required in case there is a need to intervene with life-saving medical treatment. Overdoses can result in a slow heart rate.

VECURONIUM BROMIDE (paralytic)

The package labeling warns that means of providing artificial respiration and oxygen therapy should be available when patients are given vercuronium, which is often used to relax muscles for intubation or during surgery. Respiration "insufficiency" is listed as a possible adverse reaction.

POTASSIUM CHLORIDE (stops heart)

The labels include strong warnings that potassium chloride must be given at a slow, controlled rate when administered for the treatment of a potassium deficiency. At higher doses, the drug stops the heart. For non-lethal higher doses, medical literature says to discontinue the infusion immediately and use injections of dextrose and insulin, absorb excess potassium and engage in dialysis. Respiratory paralysis is also possible. Medical literature at the National Institutes of Health says potassium intoxication can cause cardiac arrest and that EKG abnormalities can illustrate trouble.

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Source: National Institutes of Health.