Alternative Induction Techniques Can Reduce Redistribution Hypothermia

Cold hands, warm heart, as the saying goes. But when it comes to anesthesia, the inverse may be true. Standard induction techniques cause vasodilation, which transfers heat energy away from the core to the periphery in a process known as redistribution hypothermia. According to a new study, however, changing how anesthetic inductions are performed can reduce the degree of redistribution hypothermia and keep patients warmer.

Results of a randomized, prospective, s ingle-blind study have shown that the average core temperature for patients induced with propofol (2.2 mg/kg) is significantly lower compared with inhalation anesthetic techniques involving combinations of sevoflurane (Ultane, AbbVie), oxygen and nitrous oxide. Administration of phenylephrine (160 mcg) before propofol induction also was found to be effective in reducing redistribution hypothermia. These techniques may help reduce adverse complications associated with hypothermia, the authors noted, and can be employed in all anesthetizing locations.

“Although this is a process study, the public health significance of these findings is potentially very significant,” said Jonathan V. Roth, MD, an anesthesiologist at Einstein Medical Center Philadelphia. “These results clearly show that changing induction techniques keeps patients warmer, and we know that warmer patients tend to have fewer complications.

“Logic would suggest that alternative induction techniques might lead to fewer complications,” Dr. Roth said, “but this needs to be proven by an outcome study.”

As Dr. Roth reported, the complications of perioperative hypothermia have been known for years, but relatively little attention has been paid to preventing redistribution hypothermia. Although virtually all anesthetic induction techniques cause some degree of vasodilation, he explained, inhalation anesthetics are lesser vasodilators, leading investigators to hypothesize that they should reduce the degree of intraoperative hypothermia.

“We also thought that administering a vasoconstrictor to counter the propofol-induced vasodilation would result in less vasodilation while keeping patients warmer,” he added.

Between August 2014 and October 2015, Dr. Roth and his colleagues enrolled 200 afebrile patients, aged 18 to 55 years, who were randomly assigned to one of four groups:

• Group A patients were induced with 8% sevoflurane in 100% oxygen.
• Group B patients were induced with 8% sevoflurane in 50% oxygen and 50% nitrous oxide.
• Group C patients were induced with 2.2 mg/kg of propofol.
• Group D patients were induced with 2.2 mg/kg of propofol immediately preceded by 160 mcg of phenylephrine, which acts as a vasoconstrictor.

Patients were positioned supine or in the lithotomy position and maintained with sevoflurane in 50% nitrous oxide and 50% oxygen in addition to being administered an opioid narcotic, the authors noted. Core temperatures were recorded every 15 minutes after induction.

Significant Reduction in Intraoperative Hypothermia

As Dr. Roth reported at the International Anesthesia Research Society 2017 annual meeting (abstract 1135), when the three alternative anesthetic induction techniques (Groups A, B and D) were compared with standard IV propofol (Group C) administration, the mean temperatures were significantly higher at all time points (P<0.0001; Table). Moreover, in patients receiving standard IV propofol (Group C), 60% had at least one temperature below 36.0° C in the first hour, whereas only 16% did so in Groups A, B and D (P<0.0001). The average lowest temperature in patients receiving alternative inductions also was higher, the authors noted.

“People who received inhalation inductions [Groups A and B] were approximately 0.5 degree warmer at every time point, and people who received phenylephrine before propofol [Group D] were 0.4 degree higher,” said Dr. Roth, who noted that the lowest temperature achieved by any single patient also was lowest in the control group.

Although this study “clearly demonstrates” that one of the three alternative induction techniques results in a clinically and statistically significant reduction in the degree of intraoperative hypothermia, according to the authors, additional research is needed.

“The major limitation of this study is that it is a process study and not an outcome study,” Dr. Roth acknowledged. “Although we can infer that changing induction technique will result in the reduction of adverse complications associated with perioperative hypothermia, definitive outcome studies are now justified.”

The moderator of the session, Edward C. Nemergut, MD, professor of anesthesiology and neurological surgery at the University of Virginia, in Charlottesville, asked whether the researchers have plans to continue this work.

“I’m close to retirement, so I won’t do it myself,” said Dr. Roth, “but it would be nice to combine inhalation anesthesia with other warming techniques. Pre-warming of patients with forced air-warming devices 30 minutes before they enter the operating room, for example, has been used to help prevent perioperative hypothermia. It would be interesting to combine this technique with inhalation induction or phenylephrine-propofol induction to see where they end up.

“Although we don’t have the data yet, I’m predicting these patients would be very thermally stable,” Dr. Roth said. “I would love to see that study.”