There’s no mystery about why the July 23 execution of Joseph Wood in Arizona took so long. From the anesthesiologist’s point of view, it doesn’t seem surprising that the combination of drugs used — midazolam and hydromorphone — might take nearly two hours to cause death.

Here’s why.

The convicted murderer didn’t receive one component of the usual mixture of drugs used in lethal injection: a muscle relaxant. The traditional cocktail includes a drug such as pancuronium or vecuronium, which paralyzes muscles and stops breathing. After anyone receives a large dose of one of these powerful muscle relaxants, it’s impossible to breathe at all. Death follows within minutes.

But for whatever reason, the Arizona authorities decided not to use a muscle-relaxant drug in Mr. Wood’s case. They used only drugs that produce sedation and depress breathing. Given enough of these medications, death will come in due time. But in the interim, the urge to breathe is a powerful and primitive reflex.

So-called “agonal” breathing, which precedes death, may go on for minutes to hours. The gasping or snoring that eyewitnesses described would be very typical. People who are unconscious after overdoses of heroin try to breathe in a similarly slow, ineffective way, before they finally stop breathing altogether or are rescued by emergency crews.

More about the drugs

Midazolam is a member of a class of drugs called benzodiazepines. The common “benzos” that many people take include Valium, Xanax, and Ativan. What these drugs have in common is that they produce relaxation and sleep. You might take a Xanax pill, for instance, to help you nap during a long flight.

In anesthesiology, we use benzodiazepines for another important reason: Because they produce amnesia. There are stories of people taking a Valium to relax a little before they give an important talk, and the next day panicking because they can’t remember if they actually showed up and gave the talk.

Amnesia can be very helpful in my business. Many of my patients don’t want to remember coming into the operating room and seeing the bright lights and surgical instruments. After I inject one or two milligrams of midazolam into the IV, they’re often smiling and relaxed, and they have no memory later of coming into the operating room at all.  The next thing they know, surgery is over and they’re waking up.

Hydromorphone is a member of a different class of medications: narcotics. These include powerful pain relievers such as morphine, Demerol, heroin, oxycodone, and hydrocodone. These medications, in large doses, will slow or even stop breathing altogether. That’s why the risk of overdose is emphasized so strongly, and why narcotics cause so many accidental overdose deaths.

When you put benzodiazepines and narcotics together, the risk of death by overdose rises sharply. These drugs in combination were implicated in the deaths of actors Heath Ledger and Phillip Seymour Hoffman. There’s no question that if you take enough of these drugs, your breathing will slow and eventually stop if no one steps in to help you.

Why were other executions faster?

The original cocktail designed for lethal injection consisted of sodium pentothal, potassium chloride, and pancuronium. Sodium pentothal produced sleep, potassium chloride stopped the heartbeat, and pancuronium paralyzed the muscles so that the convict was unable to move or breathe. To all appearances, the convict would go to sleep and within a matter of minutes would be pronounced dead.

But opponents of capital punishment argued that there was no guarantee with this recipe that the convict was ever truly unconscious. There could be a chance that the dose of sodium pentothal would be insufficient or would wear off before the other drugs had time to take effect. Being awake and paralyzed during the dying process would truly be cruel and inhumane, they claimed. Of course, there is no way of knowing if this ever occurred, but there is equally no way of knowing that it didn’t.

I’ve written before about the clearly botched execution of Clayton Lockett in April, where the lethal injection drugs worked slowly because they were probably not injected into a vein at all. Unfortunately, he seems to have been conscious at least during the early stages.

In Mr. Wood’s case, the combination of midazolam and hydromorphone appears to have produced sleep and depressed breathing, exactly as predicted. As time went on, and his breathing became slower and less effective, the amount of oxygen in his bloodstream inevitably decreased to dangerous levels, and the amount of toxic carbon dioxide increased. When that happens, the heart eventually starts to beat erratically, and cardiac arrest leading to death is the end result.

How long it will take to die from the effects of midazolam and hydromorphone is impossible to predict, because there are so many variables:  the age and size of the person, how sensitive the person may be to the effects of sedatives, and how much medication was given.

The only thing we can be reasonably sure of is this: Mr. Wood was asleep and unaware during the process of dying.

Since midazolam and hydromorphone don’t paralyze muscles, if Mr. Wood had been awake he would have been able to open his eyes and move around. The fact that he didn’t move or writhe, as Mr. Lockett did, makes a strong case that he was asleep. While Mr. Wood’s slow demise may have been excruciating for witnesses to watch, there appears to be no evidence that he was conscious after the injection took effect.

The cases of Mr. Wood and Mr. Lockett underscore the fact that even with lethal injection, execution may not be fast or painless for observers to watch. As pharmaceutical companies become even more reluctant to provide medications for the purpose of execution, we can expect to see more experiments with different combinations of drugs.

No one should be surprised if these experiments don’t go smoothly.

Karen S. Sibert is an associate professor of anesthesiology, Cedars-Sinai Medical Center, Los Angeles, CA.  She blogs at A Penned Point.