San Francisco—Despite forced-air warming, hypothermia is common and often prolonged in patients undergoing noncardiac surgery, researchers have found. The new study showed that patients who experienced temperatures below 35°C had significantly longer hospital stays and greater transfusion requirements.
Perioperative hypothermia is associated with morbid myocardial outcomes, wound infection and coagulopathy—links that have prompted widespread guidelines for maintaining normothermia in surgery patients. Yet even with active warming during surgery, redistribution hypothermia and excessive heat loss combine to make many patients at least transiently hypothermic.
Although studies have documented the incidence of intraoperative hypothermia in a variety of populations, the new work is the first to detail intraoperative temperature patterns in a large general population of noncardiac surgical patients, according to the investigators. It is also the first to evaluate hospital length of stay and transfusion requirements over a broad unselected population.
“There are many randomized trials showing that mild hypothermia worsens outcome,” said Daniel I. Sessler, MD, Michael Cudahy Professor and chair of Outcomes Research at Cleveland Clinic, in Ohio. “Our observational study nonetheless provides novel information. First, the results generalize better. The randomized trials focused on small subsets of high-risk patients, whereas our analysis included a broad array of inpatient operations. Second, most of the randomized trials date to the 1990s, and medical practice has since changed considerably. And third, the randomized outcome trials all compared passive insulation to forced-air warming. In our current analysis, all patients were actively warmed per the current standard of care.”
The investigators extracted data from the most recent visit of 143,157 of the institution’s patients scheduled for noncardiac inpatient surgery between April 1, 2005, and Feb. 15, 2013. Records were included only if the anesthetic lasted at least one hour, forced-air warming was used and core temperature was measured in the esophagus.
Rather than assess final intraoperative temperatures to determine the incidence of hypothermia, the researchers used a time-weighted measurement of core temperature. The primary exposure was integrated area under the curve incorporating both duration and severity of hypothermic exposures into one value. Estimates were obtained for one-degree temperature bands ranging from 34°C to 37°C.
“All previous studies were based on final intraoperative temperature,” Dr. Sessler explained. “But final temperature overestimates temperature in actively warmed patients. It is therefore also important consider the duration and severity of hypothermia. Our exposure index includes both considerations.”
Prolonged Low Body Temperatures
As Dr. Sessler reported at the 2013 annual meeting of the American Society of Anesthesiologists (abstracts 1267, 1272 and 2230), 58,814 patients met all criteria. Approximately one-third of patients had core temperature of 36°C or lower for at least one hour; 8% were below 36°C for more than three hours.
Perhaps most alarming, the researchers said, 5% of patients had core temperatures of 35°C or lower for at least one hour. The incidence of hypothermia was greatest one hour after induction, and then progressively improved.
“Even with forced-air warming, our results indicate that a fair fraction of patients still become hypothermic,” Dr. Sessler said. “In fact, almost 10% of patients were distinctly hypothermic and remained near 35°C at the end of surgery. This is a degree of hypothermia that has been shown to cause major complications in randomized trials.”
Furthermore, the analysis showed that both the duration of hospitalization and red blood cell transfusion requirements increased for patients whose core temperature dropped below 35°C.
Tackling this issue, Dr. Sessler concluded, might involve actively prewarming patients or increasing the efficacy of intraoperative warming systems. “Heat transfer by most any cutaneous warming system is a linear function of surface area,” he said. “Simply increasing the surface covered by forced air, for example, will augment its efficacy.”
Co-investigator Andrea Kurz, MD, professor and chair of general anesthesia at Cleveland Clinic, agreed that proactive measures might help address the issue. “These studies show that hypothermia still exists,” Dr. Kurz said. “There is a lot less [hypothermia] than there was 20 years ago. But despite the fact that most patients are now normothermic at the end of surgery, many still experience periods of intraoperative hypothermia. Even short periods apparently worsen outcomes; we therefore still need to improve this aspect of our practice.”
Steven M. Frank, MD, associate professor of anesthesiology and critical care medicine at Johns Hopkins Medical Institutions, in Baltimore, said that although he was not surprised by the incidence of hypothermia found in the study, he found it hard to believe how rarely the vital sign has been studied. “Ever since the late 1990s, we’ve recognized the importance of maintaining body temperature,” Dr. Frank told Anesthesiology News. “But everybody has been focused on the core temperature at the end of the procedure, and largely ignored what happens during the procedure. We don’t judge the quality of our care by looking at these vital signs just at the end of the case. So why should we not focus on temperature throughout the entire case as well? This is a big issue, and I’m glad Dr. Sessler and his colleagues examined what happens between the beginning and end of a case.