What Do You Know About Hypothermia?


Test your knowledge of the dangers of intraoperative hypothermia and the benefits of warming patients.

No doubt you've heard patients (and maybe even some of your staff) complain about feeling cold during visits to your surgical facility. But chattering teeth and goosebumps are more than just a nuisance for your patients; they're a potential danger. The physiological effects of anesthesia coupled with the relatively cool ambient temperatures in most ORs put nearly all surgical patients at risk for hypothermia — a potentially serious condition that can cause thermal discomfort, increase the risk of infection and blood loss, and lengthen recovery times. Use this quiz and the accompanying evidence-based analysis to brush up on your knowledge of perioperative hypothermia and proven strategies for preventing it.

How and when hypothermia occurs
Normothermia is generally defined as a core body temperature ranging from about 36 ?C to 38 ?C. Hypothermia occurs anytime core body temperature falls below 36 ?C — even a slight dip of 1 ?10th of a degree is considered the beginning of mild hypothermia.1 In a 2008 review of the literature on perioperative thermal care, Andrea Kurz, MD, of the Cleveland Clinic's Department of Outcomes Research found "no widely accepted definition for the term ???mild hypothermia'" but defined it as core temperatures between 34 ?C and 36 ?C.2

A 1- to 3-degree drop in core body temperature is relatively common in surgical procedures.3 Both general and regional anesthesia, plus the patient's increased exposure during surgery, impair the body's natural thermoregulatory control. Add relatively chilly OR temperatures to the mix and you have a recipe for core heat loss.2,4-6 According to anesthesiologists Steven Insler, DO, and Daniel Sessler, MD, of the Cleveland Clinic, intraoperative hypothermia usually develops in 3 phases, starting with "a rapid decrease in core temperature following anesthetic induction, which mostly results from redistribution of heat from the core thermal compartment to the outer shell of the body," followed by "a slower, linear reduction in the core temperature that may last several hours." Finally, "a core temperature plateau is reached, after which core temperature remains virtually unchanged for the remainder of the procedure."7

Although the risk of intraoperative hypothermia may increase with time, the body's core temperature declines most dramatically during the first hour of surgery. Just 30 minutes after induction of general anesthesia, core temperature can fall 0.5 to 1.5 degrees below normal. Therefore, it's important to actively monitor body temperature and take steps to maintain normothermia even during short surgical procedures.1

Hypothermia Quiz

1. Hypothermia is defined as a core temperature of:
a. 36 ?C to 38 ?C
b. less than 36 ?C
c. less than or equal to 35 ?C
d. 32 ?C to 34 ?C

2. It's common for the body's core temperature to drop 1 to 3 degrees below normal during surgery.
a. true
b. false

3. Even mild intraoperative hypothermia has significant adverse implications for the patient.
a. true
b. false

4. Hypothermia is only a problem during surgeries that last 1 hour or longer, because that's when the body's temperature starts to decline most dramatically.
a. true
b. false

5. Both general and regional anesthesia can contribute to perioperative hypothermia.
a. true
b. false

6. Because patients' core temperatures begin to drop after induction of anesthesia, warming patients pre-operatively does little to prevent hypothermia.
a. true
b. false

7. Which of these is not a reliable site for core temperature monitoring during surgery?
a. nasopharynx
b. distal esophagus
c. skin surface
d. tympanic membrane
e. bladder

8. Which of these methods has been shown to help prevent hypothermia in surgical patients?
a. forced-air warming blankets
b. warming intravenous fluids
c. circulating-water garments
d. increasing the OR temperature
e. all of the above

Answers at the end of article.

Serious consequences
For certain patient populations in specific clinical conditions, some studies have shown that intraoperative hypothermia can actually be beneficial in preventing cerebral ischemia and myocardial infarction.4 But these circumstances rarely translate to the outpatient surgery setting. In the vast majority of cases, unintentional intraoperative hypothermia is associated with a variety of adverse events, some quite severe:

  • Increased thermal discomfort.
  • Anesthetic interference. Hypothermia prolongs the duration of the effects of inhaled and intravenous anesthetics and neuromuscular drugs, thereby increasing post-anesthesia recovery times.8
  • Post-operative shivering. Increases patients' metabolic rates above acceptable levels.3
  • Adverse myocardial events. Mild intraoperative hypothermia triples this risk.8,20
  • Increased infection risk. As the body's core temperature declines, so does its antibody- and cell-mediated immune defenses and the availability of oxygen in peripheral wound tissues. Research has shown even mild hypothermia to triple the risk of surgical site infection.3,7,8
  • Blood loss. Mild hypothermia, which impairs coagulation, has been shown to triple the incidence of perioperative blood loss.3

Given the mild to severe complications associated with intraoperative hypothermia, the standard of care is to maintain intraoperative normothermia — that is, core body temperature above 36 ?C.7 Achieving this goal requires a 2-pronged strategy: active monitoring of the patient's core body temperature during the procedure and active warming of the patient perioperatively.

Prevention techniques
When it comes to how and how often you should monitor a patient's core body temperature during surgery, the literature is somewhat mixed. Some studies recommend monitoring during general anesthesia,7 but as the risk of hypothermia is still great when regional anesthesia is used, it's important to monitor temperature in those procedures as well. Other studies only recommend monitoring during procedures lasting 1 hour or more,5 but research has proven that hypothermia is a risk in shorter procedures, too.

In its Standards for Basic Anesthetic Monitoring, the American Society of Anesthesiologists recommends: "During all anesthetics, the patient's oxygenation, ventilation, circulation and temperature shall be continually evaluated." On the matter of temperature monitoring, the ASA elaborates: "Every patient receiving anesthesia shall have temperature monitored when clinically significant changes in body temperature are intended, anticipated or suspected."9 Some patient populations, such as pediatric patients, with their large surface area to body mass ratio, may be more prone to heat loss than others and therefore will require closer monitoring.1 Anesthesia personnel should lead the way in making such determinations.

What's the best way to monitor core body temperature during surgery? It's not so much what you use to monitor it but where you choose to measure it, say Drs. Insler and Sessler. Studies show reliable sites of core temperature monitoring include the tympanic membrane, esophagus, nasopharynx, bladder, rectum and pulmonary artery. The skin surface, however, is not a reliable or accurate indicator of core body temperature.6,7

Although the body's core temperature doesn't begin to drop until after the induction of anesthesia, the prevention of hypothermia begins in the pre-operative period. Many studies have shown that warming patients both prior to and after the induction of anesthesia helps maintain normothermia and prevent the initial redistribution hypothermia associated with anesthesia.6,10 For example, a 1995 French study comparing patients who were actively pre-warmed before surgery and those who were not found that "a single hour of preoperative skin-surface warming reduced the rate at which core hypothermia developed during the first hour of anesthesia." Furthermore, the authors conclude that "pre-operative skin surface warming is particularly helpful during short procedures because redistribution hypothermia is otherwise difficult to treat."11

The patients who fared best in the French study received forced-air warming, while those in the control group were simply covered with a wool blanket before surgery. Many experts have found forced-air warming to be the most effective means of actively warming patients prior to and during surgical procedures,1,12 while others have deemed both resistive and forced-air warming to be comparably effective.13,14 Covering the skin with one layer of drapes, blankets and other insulating materials can reduce heat loss by about 30%, but additional layers do not increase the beneficial effects of this strategy. Most patients require active warming to prevent intraoperative hypothermia.12

In addition to resistive and forced-air warming systems, studies have shown that warming IV fluids above room temperature,15 humidifying and warming inspired gases at 40 ?C,16 and maintaining ambient operating room temperature above 24 ?C17 are also effective strategies for preventing hypothermia. "Any method or combination of methods that maintains core temperature above 36 ?C is adequate," writes Dr. Kurz.12 In the event that warming efforts fail and a patient becomes hypothermic during surgery, forced-air warming is the most effective method of re-warming the patient intraoperatively.6 Temperature monitoring and active warming should continue through to PACU, as post-op hypothermia can occur in as many as 93% of surgical patients.18

On the Web

For references, go to www.outpatientsurgery.net/forms

Warming's many perks
The patient safety benefits of actively warming patients throughout their stay at your facility are fairly obvious. Preventing hypothermia also helps prevent surgical site infections, reduce recovery times and enhance patient satisfaction and comfort. Those benefits have a trickle-down effect for your facility: Shorter patient stays (and happier patients) mean faster turnover times and more efficient surgical throughput.

Answers:1. b; 2. a; 3. a; 4. b; 5. a; 6. b; 7. c; 8. e

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