Clinicians should use bispectral index values to guide propofol induction in patients with morbid obesity, according to Canadian researchers who found that BIS values are better than body weight in this population.
The randomized study, which compared the efficacy of propofol target induction doses calculated using lean body weight (LBW), found the doses based on LBW were consistently lower than those based on the BIS monitoring (Covidien) and resulted in insufficient depth of anesthesia in 60% of cases.
Many anesthesiologists use LBW to determine propofol dosing in bariatric surgery. But Waleed Riad Soliman, MD, PhD, a perioperative fellow in the Department of Anesthesia at Toronto Western Hospital in Ontario, Canada, said his clinical experience suggested this approach was inadequate.
“I observed that patients were not adequately anesthetized and developed tachycardia and hypertension in response to intubation,” Dr. Soliman said.
Dr. Soliman and several colleagues randomized 60 patients undergoing bariatric surgery with a body mass index of at least 40 kg/m2 to receive propofol induction based on their LBW or to be directed according to BIS values. All participants received fentanyl dosed at 3 mcg/kg of LBW and propofol at an infusion rate of 100 mg/kg per hour. In the LBW group, patients received a precalculated propofol dose of 2.6 mg/kg LBW, an approach Dr. Soliman borrowed from a recommendation published in 2011 (Best Pract Res Clin Anaesthesiol 2011;25:27-39).
In the BIS-monitored group, propofol infusion was stopped once BIS dropped to 50. In both groups, anesthesiologists rated the patients’ level of consciousness using the Observer’s Assessment Alertness/Sedation Scale (OAA/S). The study was not blinded.
According to Dr. Soliman, the mean LBW-based precalculated propofol dose was 163.5 mg compared with a mean 225.4 mg propofol required for deep sedation in the BIS group (P<0.05). The mean OAA/S score achieved in those patients with a full propofol dose calculated using LBW was 1 (range 0-3), indicating inadequate sedation. In the BIS group, mean OAA/S scores were 0 (P<0.05 for LBW vs. BIS).
Dr. Soliman said 60% of patients (18 of 30) in the LBW group required additional propofol for induction to achieve deep sedation compared with 3.3% (one of 30) of the BIS patients who required more propofol beyond the dose administered at a BIS value of 50 (P<0.05).
Total induction doses administered in the two groups did not differ significantly; the researchers found a mean 204 mg in the LBW group and 228 mg in the BIS group. There also were no differences in heart rate between the two groups during intubation. However, mean arterial pressure was significantly higher in the LBW group following intubation and through to five minutes after intubation.
Daniel I. Sessler, MD, Michael Cudahy Professor and chair of the Department of Outcomes Research at the Cleveland Clinic, in Ohio, said using total body weight rather than LBW to calculate propofol induction dosing is probably a better strategy in obese patients. Dr. Sessler, who was not involved in the study, pointed to a 1993 publication that supported this view (Anesthesiology 1993;78:657-665).
“The pharmacokinetics of propofol in the obese from those results were clear,” Dr. Sessler said. “Volume of distribution and clearance both increased linearly with body weight, and there was thus no accumulation of the drug. The authors therefore concluded that ‘dosing schemes expressed in mg/kg are the same as those in normal patients.’”
Dr. Soliman’s findings were to be presented at the Canadian Anesthesiologists’ Society’s 2013 annual meeting (abstract 1625814) in Calgary, Alberta. The meeting was canceled because of flooding.