June 23 (HealthDay News) -- Most general anesthetics -- used to put patients to sleep during surgery -- can increase the discomfort patients feel when they wake up, according to Georgetown University Medical Center researchers.

This finding, the first to scientifically explain anecdotal observations, may lead to increased use of general anesthetics that do not have this side effect or to the development of new anesthetics, the researchers said.

They found that "noxious" anesthesia drugs -- which most general anesthetics are -- activate and then sensitize specific receptors on neurons in the peripheral nervous system. These sensory nerves in the inflammation and pain pathway aren't affected by general anesthesia drugs that target the central nervous system (brain and spinal cord).

"The choice of anesthetic appears to be an important determinant of post-operative pain. We hope these findings are ultimately helpful in providing more comfort to patients," lead investigator Gerard Ahern, an assistant professor in the department of pharmacology, said in a prepared statement.

It was known that general anesthetics cause irritation at the infusion site or in the airways when inhaled and that they can activate pain-sensing nerve cells in the peripheral nervous system, Ahern explained.

But the specific mechanism by which anesthetics affect sensory neurons wasn't known, nor the fact that anesthetics can continue to cause pain and inflammation even as they're used during surgery.

Ahern and colleagues found that the noxious effects of anesthetic drugs are caused by their effect on the nerve cell receptor TRPA1. They also found that nerve-mediated inflammation was greater when pungent (chemical irritants) versus non-pungent inhaled general anesthetics were used on patients.

Both findings suggest that sensory nerve stimulation throughout the body just before and after surgery contributes to the pain felt by patients when they wake up after surgery.

"This is a provocative finding in terms of the clinical setting, because it was not really recognized that use of these drugs results in release of lots of chemicals that recruit immune cells to the nerves, which causes more pain or inflammation," Ahern said.

While some general anesthetics don't activate the TRPA1 receptor, they may not be as effective in other ways.

"This tells us that there is room for improvement in these drugs," Ahern said.

The study was published in the June 23 issue of the journal Proceedings of the National Academy of Sciences.

SOURCE: Georgetown University Medical Center, news release, June 23, 2008