Women and Anesthesia

Women Wake Up Faster From Anesthesia

Most of us take it for granted. When we go under the knife, we assume we won't feel a thing. Thanks to anesthesia, we'll experience the event in ignorant bliss and have no memory of the surgery.

This is not always the case. For every 1,000 surgeries performed, one to three individuals may be somewhat conscious during the procedure, according to an editorial in the March 5th, 2001, issue of The Medical Journal of Australia. Though these anesthetized individuals are "awake," they cannot speak or move - paralyzed by the anesthesia. Some people even recall feeling the surgical incisions and pain.

Anesthesiologists have been puzzled by the fact that women remain more conscious under anesthesia than do men. Results of recent studies have shed some light on this phenomenon.

Sex Matters It appears that women are less sensitive to the effects of anesthesia. In 1999, Dr, Tong J. Gan of Duke University Medical Center in Durham, North Carolina, and a multicenter team discovered that women wake up significantly faster than men following intravenously administered general anesthesia. The investigators used a combination drug regimen of propofol, alfentanil and nitrous oxide. This was the first reported, randomized study to show such a gender effect.

Dr. Gan and colleagues followed 274 surgical patients in order to determine the usefulness of the bispectral index (BIS) in measuring level of consciousness. People who are fully awake have BIS scores between 90 and 100, the investigators say, while scores below 60 usually reflect lack of consciousness. Patients all received the same dose of anesthesia per kilogram of body weight and were then randomly monitored with or without the BIS. The results demonstrated that using the bispectral index helped physicians better control the amount of anesthesia needed.

And, much to the surprise of Dr. Gan's team, the researchers also found that women woke up close to 7 minutes after anesthesia was removed while men didn't open their eyes until approximately 11 minutes after anesthesia was removed. These differences occurred at four study sites and in each treatment group under study.

Much smaller studies had hinted at a gender-dependent response to propofol. For example, Dr. Peter S. Glass and colleagues had shown that women needed significantly greater doses of anesthesia than men in order to lose consciousness. The researchers also had found that women had consistently higher BIS values and were therefore more conscious than men for a given blood propofol concentration.

A just-published report in the March 24th, 2001, issue of the British Medical Journal provides further evidence of this gender difference with a different type of anesthesia - inhaled anesthetics. Dr. Paul S. Myles and his team in Victoria, Australia, showed that women tended to open their eyes close to 2 minutes sooner and obeyed commands almost 3 minutes sooner than men after surgery.

"The new study reinforces that what we found [in 1999] is real," said Dr. Gan in an interview with iVillage, "We had wondered whether this effect [women waking up more quickly than men] was confined to one drug. But the Australian study shows that the brains of women and men interact quite differently with a number of anesthetics."

Dr. Myles and colleagues evaluated 463 adults after they underwent elective surgery. Using interviews, the researchers determined the time to recovery and frequency of complications such as headache, nausea and vomiting after anesthesia. They found that women not only consistently woke up more quickly than men, but also were more prone to side effects.

Why the Difference? Exactly why anesthesia affects women differently, however, still remains a mystery. "We know that there is a clear difference - but we're still trying to understand why and how it happens - what's going on in the brain," Dr. Gan observed.

Some have theorized that the differences in fat distribution and hormone levels between men and women may account for some of the differences in how they respond to anesthesia. And, in fact, study results have shown that fat distribution and hormone levels do play a role in increasing a woman's susceptibility to side effects. But in terms of the disparities in recovery times, Dr. Gan speculates that there is more to the story. He believes that differences between the male and female brain will emerge as the predominant cause of differences in sensitivity.

"But we still do not understand this area very well," said Dr. Gan. "Only more studies examining gender differences will be able answer this question."

Implications So what does this mean for anesthesiologists and their female patients? The results suggest that women are less responsive to anesthesia than men, according to Dr. Gan. This would explain why they wake up sooner and are more conscious after receiving the same dose in relation to their weight as men. It is quite possible that "…women require a larger dose of anesthesia per kilogram of body weight than men," said Dr. Gan.

In addition, knowing that a woman needs a greater dose of anesthesia to lose consciousness may help anesthesiologists reduce the number of cases in which surgical patients remain somewhat "aware" during surgery. And future studies in this area will help to clarify a woman's special anesthetic needs.

"Why should women have three times the incidence of awareness of men?" Dr. Gan queried. "That is completely unacceptable."

Please Note: Information provided here is not a substitute for consultation with a medical professional. The Society for the Advancement of Women's Health Research and iVillage/iVillageHealth make no representation or warranty regarding the content of this information. If you are concerned about your health or that of a child, please consult a health provider immediately and do not wait for a response from our professionals.


Gan TJ, Glass PS, Sigl J, et al. Women emerge from general anesthesia with propofol/alfentanil/nitrous oxide faster than men. Anesthesiology 1999;90(5):1283-1287.

Glass PS, Bloom M, Kearse L, et al. Bispectral analysis measures sedation and memory effects of propofol, midazolam, isoflurane and alfentanil in healthy volunteers. Anesthesiology 1997;86:836-847.

Myles PS, Williams DL, Hendrata M, et al. Patient satisfaction after anaesthesia and surgery: results of a prospective survey of 10,811 patients. British Journal of Anaesthesiology 2000;84(1):6-10.

Leslie K, Myles PS. Awareness during general anaesthesia: is it worth worrying about? The Medical Journal of Australia 2001;174:212-213.

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