This happened to a friend’s mother. It’s really unbelievably awful:

Imagine this: You’re lying on the operating table, apparently unconscious. The surgeon is cutting. But you’re still awake. Not only that, you’re paralyzed by the drugs the anesthesiologist gave you and can’t speak out.

That horrifying experience happens to between 20,000 and 40,000 Americans every year, leaving many — not surprisingly — severely traumatized.

Now, a study in today’s issue of the New England Journal of Medicine is raising questions about a monitor used by about 60 percent of U.S. operating rooms in an effort to prevent these frightening cases.

The study involved 1,941 patients who underwent operations at the Barnes-Jewish Hospital in St. Louis. Anesthesiologists monitored half the patients with a system called BIS (bispectral index system) that analyzes brain waves so doctors can supposedly tell if a patient isn’t totally unconscious or is starting to wake up. For the other half of the patients in the study, doctors simply paid especially close attention to the dosages of anesthesia.

An equal number in each group — two — turned out to have been awake for at least part of their operations. One 51-year-old patient being monitored by BIS came to during pancreatic surgery and felt “white-hot fire pain” in his abdomen and his “organs and intestines moving around,” the researchers wrote. He remembers “crying and thinking, ‘If someone can see my crying, then someone can help me.’ “

Five other patients who were monitored by BIS might have experienced some awareness as well, compared with just one in the other group, the researchers reported.

Michael S. Avidan of the Washington University School of Medicine in St. Louis, who led the study, said his findings raise questions about whether the pricey brain monitors are worthwhile.

Officials at the company that makes the monitor said other studies have demonstrated its usefulness, especially compared with standard practice. That typically does not involve the kind of close monitoring that patients in the new study received, said Scott Kelley, medical director at Aspect Medical Systems.

Regardless, Avidan and others said, the findings show that more research is needed to better understand why anesthesia fails.

“Even one case of anesthesia awareness is really too many,” said Jeffrey L. Apfelbaum, president of the American Society of Anesthesiologists.