Oct. 15, 2002 -- That fiery redhead spirit! It seems that redheads require more anesthesia to "go under" than other hair colors do -- about 20% more.

A new study has implications for redheads who require surgery. It also gives researchers a better understanding of how anesthesia works in humans.

Red hair is the first visible human trait, or phenotype, to be linked to anesthesia, says study author Edwin B. Liem, MD, a researcher with the University of Louisville, in a news release.

The link is likely due to an improperly functioning receptor on certain cells that give pigment to people's skin and hair, he explains. Liem says this dysfunction triggers a feedback mechanism that increases release of the hormone that normally stimulates these cells. This hormone also stimulates a brain receptor that increases pain sensitivity.

"In a nutshell, redheads are likely to experience more pain from a given stimulus and therefore require more anesthesia to alleviate that pain," he says.

"The art and science of anesthesiology is choosing the right dose," says Liem. "There is very little difference between the effective dose and the toxic dose of most anesthetics."

In fact, if patients receive too little anesthesia, they can awaken during surgery. If given too much, they can have heart and breathing problems.

Liem's study involved women between ages 19 and 40 who had either natural bright-red hair or dark hair. Each was given a commonly used anesthetic, and their physical response was closely monitored -- especially reflex arm or leg movements in response to painful stimulation.

Significantly more anesthesia was needed to block movement in redheads than in brunettes. People with blond hair are likely similar to brunettes in this respect, Liem says.

"That people with the red hair phenotype require more anesthesia is not only of practical importance, but suggests that genetic characteristics contribute to differences in anesthetic requirements in humans," says Liem.

The findings could help researchers understanding the roles of various systems in producing unconsciousness and altering pain perception. -->