What is in the lethal injection used to execute prisoners?

The lethal injections used to administer the death penalty are probably better described as "three injections which together are lethal" rather than one single shot.

First of all there's a shot of sodium thiopental, which is supposed to anesthetize the victim. Then there's a shot of pancuronium bromide, which causes paralysis. Finally, an injection of potassium chlorate messes up the electrical activity of the heart so that it stops.

There is a lot of controversy over whether this approach to killing someone is really all that humane.

If you don't anesthetize someone enough, and then you paralyze them, you might think they're dying peacefully while they're actually in agony but unable to move.

While there isn't a lot of data, there is some that suggest that at least some victims are aware and conscious but paralyzed when they get their fatal potassium-induced heart attack.

Another possible problem can arise if the first injection (or another) goes into muscle as opposed to a vein. In this case, there can be burning and damage to tissues around the injection site, but not enough actually going to work in the victim. Sometimes the injections have to be done more than once.

It may be of some interest that when animals are "put to sleep" quite a different procedure is followed - usually a fatal overdose of barbiturates that produce deep sleep, coma, and death. This seems like a much safer way to ensure that someone doesn't suffer.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.