Obese more likely to ‘wake up’ mid-surgery

The woman regained consciousness just as surgery was starting.

She could see a canopy of lights overhead, and feel something pushed inside her mouth. Then, a tugging, searing pain, as if animals, as she would later describe it, were biting into her flesh.

She tried to yell out, but couldn’t speak or move.

The woman, who was morbidly obese, was part of the largest audit of “accidental awareness during general anesthesia” published three years ago, a U.K.-based review that found a disproportionate number of people who report experiencing the terrifying phenomenon are obese.

New Canadian research may help explain why. The study found a standard formula used to calculate the amount of propofol, a widely used anesthetic, needed to induce unconsciousness can result in serious under-dosing for people with morbid obesity, which is defined as a body mass index of 40 or more and is the fastest growing weight class in the country.

FORMULA USED TO DETERMINE AMOUNT OF ANESTHESIA NEEDED LEADING TO INSUFFICIENT DOSES, STUDY FINDS

Anesthesiologists usually base propofol induction doses in the obese on lean body weight — the person’s body weight, minus the fat.

The worry is that using total body weight can lead to serious overdoses in the extremely obese, resulting, among other serious complications, in a rapid drop in blood pressure and decreased blood flow to the heart and brain.

However, the new study finds using lean body weight to calculate induction doses for morbidly obese people resulted in an “insufficient” depth of anesthesia in 60 per cent of cases, compared to using brain waves to guide dosing.

A leading cause of accidental awareness is failure to deliver sufficient anesthetic to the body.

For the new study, researchers randomly assigned 60 patients undergoing bariatric, or stomach-shrinking surgery, to one of two groups.

In the first group, doctors dosed propofol based on lean body weight.

In the second, they used a special device known as a BIS, or bispectral index monitor, which measures brain activity to determine how deeply a patient is anesthetized.

The monitor translates the information into a single number, from 100 (meaning the person is wide awake) to zero (no brain electrical activity.)

The propofol infusion was stopped once the number dropped to 50.

In both groups, doctors used the trapezius-squeezing test — squeezing the trapezius muscle located between the neck and shoulder — to assess the depth of anesthesia.

In the first group, 18 out of 30 people were still “responsive” after the initial dose of propofol and required additional doses before reaching a sufficient level of sedation.

In the brain-monitored group, all but one of the patients was “unresponsive” at the target of a BIS of 50.

None of the patients in either group reported awareness. The phenomenon itself is exceedingly rare, occurring in an estimated one in 19,000 general anesthetics.

However, propofol is highly fat soluble, meaning it gets stored in fat, making it harder to reach sufficient levels in specific target receptors in the brain to sedate people for surgery.

“Using the lean body weight formula probably isn’t the best method to dose your propofol in the morbidly obese patient, because it results in under-dosing,” said Dr. Jean Wong, an anesthesiologist at Toronto Western Hospital and senior author of the new study, published in the Canadian Journal of Anesthesia.

Others have argued that dosing based closer to total body weight, which takes fat mass into account, is more optimal, despite the increased cardiovascular risks.

Dr. Jaideep Pandit, a consultant anesthetist at Oxford University Hospitals who led the largest-ever prospective study on accidental awareness in the world, said the results of the new Canadian study show “that the true dose of propofol required is certainly higher than LBW (lean body weight) but a little lower than TBW (total body weight.)”

University of Manitoba anesthesiologist Dr. John Friesen doesn’t believe awareness during anesthesia is a major issue in Canada. “I have been practising for over 25 years — so over 25,000 anesthetics — and so far have not had one patient mention awareness as a problem,” he said in an email. “Having said that, obesity is becoming more common and more severe all the time,” he said. “No one really knows how to adjust the doses of drugs for large patients.”

Friesen said the problem goes beyond accidental awareness. “The problem affects all drugs,” he said. “Are bacterial infections adequately treated if obese patients are given too little antibiotic? ... No one knows.”