Surgery patients may be hypothermic during first hour of procedure

http://www.scientificamerican.com/article/hypothermia-found-to-be-common-during-anesthesia-increasing-need-for-transfusion/

Hypothermia is common among patients during the first hour of anesthesia, despite the use of forced air warming to maintain core temperature, a study in nearly 60,000 surgical patients shows

By Anne Harding

NEW YORK (Reuters Health) - Hypothermia is common among patients during the first hour of anesthesia, despite the use of forced air warming to maintain core temperature, a study in nearly 60,000 surgical patients shows.

And the longer patients were hypothermic, the more likely they were to require transfusion, Dr. Daniel Sessler, of the Department of Outcomes Research at the Cleveland Clinic in Ohio, and colleagues found.

"Most patients become hypothermic during the first hour of anesthesia and then temperature slowly returns toward normal, so that by the end of the surgery most patients are normothermic," Dr. Sessler told Reuters Health in a telephone interview. "But the amount of hypothermia that we saw and the number of hypothermic patients was fairly high, and the amount of hypothermia was significantly associated with the need for blood transfusion."

Intraoperative core hypothermia can cause coagulopathy, surgical wound infection, and possibly myocardial complications, Dr. Sessler and his team note in their report, published online January 20 for the February issue of Anesthesiology.

While active warming, usually with forced air, has become the standard of care, the researchers add, a patient's core temperature falls sharply in the first hour of anesthesia, due to core-to-peripheral body heat distribution.

To better understand intraoperative core temperature patterns in patients actively warmed with forced air, Dr. Sessler and his team looked at esophageal temperatures in 58,814 adult patients who underwent noncardiac surgery lasting at least an hour at the Cleveland Clinic between 2005 and 2013.

All patients showed a dip in core temperature during the first hour of surgery, which then increased. On average, the lowest core temperature early in surgery was 35.7 degrees Centigrade. Nearly two-thirds of patients had core temperatures below 36 degrees C within 45 minutes of induction, while temperatures dropped below 35.5 degrees C in 29%.

Almost half of the patients had core temperatures under 36 degrees C for over an hour, 20% had temperatures under 35.5 degrees C for over two hours. Continuous core temperatures were below 36 degrees C for more than an hour in 20% of patients, while they were below 35.5 degrees for more than an hour in 8%.

A total of 4.6% of the patients required transfusion. Estimated mean blood loss was 48 cc for patients in the first quartile based on area under the 37 degree threshold; 50 cc for patients in the second quartile; 100 cc for patients in the third quartile; and 200 for patients in the fourth quartile. When the researchers excluded the 429 patients who received massive transfusion, the association between hypothermia and transfusion needs remained similar.

The researchers also found a link between area-under-the-threshold and hospitalization duration that was significant but not clinically important.

"Active warming is not yet a worldwide standard of care," Dr. Sessler and his team write. "It is thus likely that a substantial fraction of the roughly 240 million patients having noncardiac surgery each year reach core temperatures that increase transfusion requirements and prolong hospitalization. The cost to the healthcare system surely exceeds the now-modest price of active warming."

In order to reduce hypothermia during surgery, "prewarming or at least starting to warm as early as possible during surgery would be helpful," Dr. Sessler said. "One approach you could use is to prewarm patients, say, for a half hour before surgery. That effectively loads the body with heat, and then their core temperature decreases less. Another way might be to combine two different warming systems. The third approach might be to keep the room a little warmer."

Dr. Harriet Hopf, a professor and vice chair of the Department of Anesthesiology at the University of Utah School of Medicine, wrote an editorial accompanying the study. "One limitation of the study is... the complexity of the relationship between duration of surgery, blood loss, fluid administration, and core temperature," Dr. Hopf told Reuters Health by email.

"Although the authors did their best to account for these variables, patients with the greatest exposure to hypothermia also had the longest duration of surgery," she continued. "Since patients generally warm up over time with active warming during surgery, this suggests that blood loss and administration of cold fluids/blood may have been a contributor to the relationship. Nonetheless, the message of the paper is clear: keeping patients normothermic throughout the perioperative period has important benefits-the challenge is to define how best to achieve the goal."

She added: "I would argue that prewarming should be routine, but I would also agree that the value of prewarming hasn't been adequately studied yet. The physiology makes sense and there are a couple of studies that suggest benefit, but little work on how to implement prewarming, which patients benefit, or evaluations of cost/risk vs. benefit. We have used routine prewarming at my institution for several years and are working on an analysis of our data modeled after the analysis in the current study to begin to answer some of those questions."

3M, which makes the Bair Hugger forced air warming device, funded the new study. Dr. Sessler and another author of the study serve on 3M's board, but donate all fees to charity, according to disclosure information included in the article.