Author: Michael Vlessides
Anesthetic requirements in patients undergoing off-pump coronary artery bypass graft (OPCABG) surgery decrease linearly with simultaneous falls in core body temperature, a study has found.
The researchers concluded that clinicians who do not use sedation monitors to guide the depth of anesthesia may actually be oversedating these patients, potentially leading to discharge delays and adverse respiratory events.
“This study was done to try to improve on enhanced recovery from anesthesia and surgery protocols in patients who are undergoing off-pump cardiac surgery, because our goal for those patients is to get them extubated quickly,” said Kumar Belani, MD, currently a professor of anesthesiology, medicine and pediatrics at the University of Minnesota and Masonic Children’s Hospital, in Minneapolis.
“These patients tend to get cold during surgery, but not many clinicians reduce the anesthetic at lower temperatures,” Dr. Belani said. “So we wanted to see if they have less need for general anesthetic agents when they cool.”
Dr. Belani and his colleagues at the Narayana Institute of Cardiac Sciences in Bengaluru, India, enrolled 40 adults into the prospective observational study. Each patient underwent OPCABG at the institution between November 2014 and October 2015. Participants had a mean age of 51.2 years, mean body mass index of 29.8 kg/m2, and mean left ventricular ejection fraction of 55.4%.
A standardized anesthetic regimen was used in each patient, consisting of propofol, midazolam, fentanyl and pancuronium for induction, followed by isoflurane for maintenance. Patients were intubated with mechanical ventilation.
Standard monitoring—including use of the bispectral index (BIS) and core temperatures—was maintained in all patients, who also were warmed with a convective temperature management system. In each case, isoflurane levels were adjusted to maintain BIS between 40 and 60. All patients were extubated within eight hours postoperatively. Each patient also completed the Brice questionnaire 18 hours after extubation to assess for recall.
In a conversation with Anesthesiology News at the 2019 annual meeting of the International Anesthesia Research Society (abstract D30), Dr. Belani reported that anesthetic requirements as guided by BIS levels between 40 and 60 were found to correlate linearly with core body temperature, with a correlation coefficient of 0.997 (P<0.05).
Indeed, the mean decrease in body temperature at the end of 300 minutes was 2.18° C, with a corresponding mean decrease in end-tidal anesthetic concentration of 0.284%. The reduction in end-tidal anesthetic concentration per degree decrease in temperature was 0.1303%.
“We’d like to let people know that it’s good to use some sort of monitor that measures the sedative effects of general anesthetics,” Dr. Belani said. “Here we used the BIS, and with that we were able to reduce the anesthetic requirement as the temperature dropped. As an added benefit, patients did not have any recall.”
Failing to adjust the anesthetic flow rate in patients with a lower body temperature could have negative effects on them. “You might see side effects from the anesthetics,” Dr. Belani said. “There might be cardiorespiratory depression or be delayed wake-up time. In addition, you might need more inotropes to keep the blood pressure up and the kidneys perfused. You also might see less incidence of emergence agitation and emergence delirium with optimal anesthetic dosing.”
Given these findings, the researchers have implemented regular BIS monitoring at their institution. “We now use it during anesthesia care, and we’ve found that it actually reduces our anesthetic requirements if the patient’s temperature changes,” Dr. Belani said. “So you save money that way, and you also pollute the environment less if you use less inhaled agents. It’s very cost-effective to monitor the brain, especially with factors like cooling, age and certain disease conditions—all of which can have an impact.”
As Hilary P. Grocott, MD, explained, there are many patient factors that are known to influence anesthetic dosing. “For example, the effect of advancing age and hypothermia are well documented to reduce anesthetic requirements,” said Dr. Grocott, a professor of anesthesiology, perioperative and pain medicine and professor of surgery at the University of Manitoba, in Winnipeg.
“However, understanding the precise individual effect is difficult unless some kind of real-time monitoring is used, such as raw and/or processed—i.e., BIS—EEG monitoring,” Dr. Grocott said. “Dr. Belani’s research group has demonstrated this once again, and thus highlights the utility of this widely available though often underutilized cerebral monitor.”