In a study that could end the unforgettable chill that goes with surgery, doctors at the Johns Hopkins School of Medicine found that a heated blanket can reduce cardiac complications by more than half.
Now, they are ready to issue this simple advice: Keep the patient warm.
"There is a reason why people live at 98.6 degrees," said Dr. Steven M. Frank, lead author of the study that appears in today's Journal of the American Medical Association. "We've been focusing on heart rate and blood pressure for 100 years but only recently on body temperature."
Patients who were covered with a heated blanket had fewer heart attacks, chest pains and abnormal heart rhythms as a consequence of surgery than did patients who had nothing to counteract the chilly operating-room air or the cooling effects of anesthesia.
Forty years ago, studies showed that heart attacks occurred twice as often in the winter as in summer months. "You've heard of people walking out into the cold, shoveling snow and dropping dead," said Frank, an anesthesiologist. While physical strain may be partly to blame, he said, so too may be the exposure to cold.
"For years, people have been developing hypothermia [lowered body temperature] from surgery," he said. "Only recently has equipment been available to prevent hypothermia."
How it works
The Hopkins team experimented with a blanket that resembles an inflatable mattress, puffed with heated air fed by a mechanical blower that sits on the floor. The blanket's underside is covered with tiny holes that allow streams of warm air to leak onto the patient's body.
Hypothermia appears to raise levels of stress hormones in the body, Frank said. Under normal circumstances, these hormones work to maintain proper blood pressure and cardiac output, but an oversupply puts excessive demands on the heart and vessels.
In the recent study, Frank and colleagues followed 300 patients who underwent major operations, including surgery for pancreatic, liver and lung cancer and procedures to correct clogged arteries of the neck and leg. The study excluded heart bypass surgery, in which a lowered body temperature is known to benefit patients.
Half of the patients received normal treatment, meaning only minimal protection from chilly conditions of the operating room. The other half were covered with the heated blankets, which have been available for about a decade but never subjected to scientific scrutiny until now.
The study found that heart attacks, cardiac arrest and unstable angina (severe chest pains) occurred in 1.4 percent of the warmed patients, compared with 6.3 percent of the others. Under normal operating room conditions, a patient's body temperature drops about 2 degrees, Frank said.
Rapid heartbeats occurred in 2 percent of the patients given blankets, compared with 8 percent of the others.
"It is well recognized that cardiac complications are the most common cause of mortality in surgical patients," Frank said.
Although the blankets reduced this risk significantly, none of the cardiac complications detected in either group was fatal.
Two patients in each group died, all several days after surgery and none from complications related to temperature management, he said.
The warming devices, called "forced-air warming systems," have been available since the late 1980s and are made by several companies. They are used in about a third of all operating rooms at Hopkins and perhaps 10 percent to 30 percent elsewhere, said Frank.
Companies provide the air blowers free of charge to encourage use of their disposable blankets, which cost $12 to $15 each.
There is a long history of patients being rolled into cold operating rooms and waking from anesthesia feeling chilled to the core.
Surgeons have long preferred cool working conditions to keep them comfortable and alert, especially since the 1980s when concerns about AIDS brought in new gowns that are impermeable to blood and hotter to wear.
Anesthesia also turns off the body's mechanism of thermal control, which keeps internal temperature at 98.6 degrees despite changing weather.
More than just cold
Add to this the other indignities to which patients are subjected.
"The first thing that happens to you is they take away your clothes," said Frank. "Then, they paint you with Betadine," a disinfectant that makes the skin feel cool.
Doctors use two measures to counteract the chilling effect of surgery. Transfused blood circulates through a warming machine before it trickles into the body, and a mechanical device traps the heat carried by a patient's exhalations.
In an editorial accompanying the Hopkins article, Dr. Frederick W. Cheney of the University of Washington School of Medicine said the study presents compelling evidence that patients should be kept warm.
"When one adds the fact that patients who maintained normothermia [normal temperature] during surgery are more comfortable in the immediate post-operative period," he wrote, "it is clear that the costs and risks are low and the potential benefits are high. "
Cheney said it will be interesting to see if the study triggers a major shift toward keeping patients warm during surgery. But he cautioned that this would make it impossible to conduct further studies comparing surgical complications among warm and cool patients.
Pub Date: 4/09/97