The woman, who was morbidly obese, was part of the largest-ever 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, defined as a body mass index of 40 or more, and the fastest growing weight class in the country. 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.
The rue dose of propofol required is certainly higher than the lean body weight but a littler lower than total 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.
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