Let's find out what you know about the causes of intraoperative hypothermia, why maintaining normothermia is vitally important to your infection control efforts and the best ways to keep patients warm and safe from pre-op to PACU.

1. Intraoperative hypothermia can cause which of the following:

a. increased blood pressure
b. increased risk of wound infection
c. decreased drug metabolism
d. all of the above

d. all of the above

Even mild hypothermia inhibits blood coagulation, which has been shown to significantly increase blood loss during many types of surgery. Hypothermia increases blood pressure, heart rate and plasma catecholamine concentrations, which in turn triples the risk of heart attacks. Drug metabolism is also reduced in hypothermic patients, meaning the effects of anesthetics linger longer, thus prolonging post-op recovery times. Finally, hypothermia increases wound infection risks by impairing immune function, triggering thermoregulatory vasoconstriction and decreasing oxygen delivery to the surgical site. In fact, mild hypothermia triples wound infection risks in patients who undergo colon surgery.

2. Maintaining intraoperative normothermia is ______________.

a. optional
b. required by community and professional society standards
c. only marginally supported by research
d. difficult and expensive

b. required by community and professional society standards

The Surgical Care Improvement Project and the Physician Quality Reporting Initiative require that you employ active warming to maintain normothermia in all patients — regardless of age — who are undergoing surgery lasting 60 minutes or longer under general or neuraxial anesthesia.

According to the guidelines, a body temperature equal to or greater than 36 ?C must be recorded within the 30 minutes immediately before or the 15 minutes immediately after anesthesia end time. Hypothermia can still occur in patients during cases lasting less than 60 minutes. While these patients aren't considered high risk, they can and will feel cold, so warming as a comfort measure is still recommended during shorter procedures.

3. Normal body temperature is usually:

a. within the interthreshold range
b. above the sweating threshold
c. below the vasoconstriction threshold
d. between the vasoconstriction and shivering thresholds

a. within the interthreshold range

A patient's normal body temperature sits within the interthreshold range, defined as core temperatures that do not trigger autonomic thermoregulatory responses. Autonomic responses are about 80% determined by core temperature; in contrast, about half of behavioral responses are driven by skin temperature. Skin temperature therefore contributes more to thermal comfort than to vasoconstriction, sweating and shivering.

Shivering, which occurs when temperatures dip below the shivering threshold, is the body's most intense defense against a drop in temperature. But it's hard for the body to maintain shivering for long periods, and it's an inefficient and metabolically costly response. Sweating, on the other hand, occurs when temperatures push past the upper limit of the interthreshold range. Sweating is effective and can dissipate 10 times the metabolic rate, even when ambient temperature exceeds core temperature.

4. Typical surgical concentrations of volatile general anesthetics increase the normal interthreshold range from approximately 0.2 ?C to __ ?C?

a. no change
b. 1 ?C
c. 2 ?C
d. 4 ?C

d. 4 ?C

General anesthetics inhibit the body's normal autonomic responses to thermal perturbations through a slight increase in the sweating threshold accompanied by marked and synchronous reductions in the vasoconstriction and shivering thresholds. The consequence is that the interthreshold range — normally a few tenths of a degree Celsius — increases to about 4 ?C. Patients given volatile anesthesia are therefore poikilothermic over a wide range of core temperatures. Patients given neuraxial anesthesia are also at risk. Neuraxial anesthesia increases the interthreshold range, impairs peripheral thermoregulatory defenses and blunts behavioral responses (patients feel warm even when hypothermic).

5. Which of the following is the most reliable core-temperature monitoring site:

a. rectum
b. axilla
c. esophagus
d. forehead skin

c. esophagus

Measuring the body's core temperature to assess the onset of intraoperative hypothermia is best done at the tympanic membrane, pulmonary artery, distal esophagus or nasopharynx. Among these, the distal esophagus is the easiest to use and most resistant to artifact. Near-core sites include the mouth, axilla and bladder.

The secondary sites, however, carry accuracy limitations. Oral temperatures, for example, can be inaccurate in patients who regularly breathe through their mouth or who recently drank cold or hot liquids. Temperatures measured on the skin's surface are approximately 2 ?C lower than core, and the temperature difference between skin and core varies both among patients and within a given patient over time.

While the level of temperature accuracy needed to be clinically acceptable is up for debate, the difference between the thermometer and site used to measure core temperature and the actual core temperature should not exceed 0.5 ?C — the smallest temperature differential associated with complications caused by hypothermia.

6. Initial hypothermia following induction of anesthesia results primarily from:

a. ventilation with cold, dry respiratory gases
b. redistribution of heat within the body
c. undressing patients in a cold environment
d. anesthetic-induced reduction in metabolic rate

b. redistribution of heat within the body

Intraoperative hypothermia can occur when anesthesia impedes the body's normal ability to regulate its core temperature while it's exposed to a cold environment, as is often the case in ORs kept chilly for the comfort of the surgeons and staff. Hypothermia onset is characterized by a rapid decrease in core temperature followed by a slower more gradual decline.

The rapid initial drop in temperature of 1 ?C to 1.5 ?C in the first hour of general anesthesia occurs when the anesthetics cause vasodilation, letting the body's core heat flow to the extremities. The more gradual, straight-line loss of heat during longer procedures occurs because the body's core temperature plateaus and remains constant after 3 to 4 hours of anesthesia. Subsequent heat loss is therefore caused by the body's inability to metabolically produce heat that exceeds gradual heat loss.

7. Which of the following is true about fluid warming?

a. fluid warming prevents fluid-induced cooling, but does not warm patients
b. each liter of IV fluid at ambient temperature decreases mean-body temperature by 0.5 ?C
c. fluid warming should be used in most patients
d. fluid warming is a reasonable alternative to active surface warming

a. prevents fluid-induced cooling, but does not warm patients

While warmed fluids administered during surgery do not raise a patient's body temperature, the practice is very effective in preventing the significant heat loss that occurs when room temperature IV fluids or refrigerated blood are administered. A patient's core body temperature drops by 0.25 ?C for every 1 unit of refrigerated blood or 1 liter of IV solution given. To avoid these incremental temperature drops that can add up to significant and potentially dangerous lows, use fluid warmers when you expect to deliver large amounts of fluid during a case.

Fluid warming, though, is always a second-line approach and should be added to effective surface warming methods during surgery when high volumes of fluid are being given (cases involving the administration of 1 to 2 liters of fluid per hour, for example).

8. Which of the following is most effective in preserving intraoperative normothermia:

a. ventilation with warm, humidified gases
b. circulating water mattress
c. increasing ambient OR temperature by 4 ?C
d. upper-body forced-air warming

d. upper-body forced-air warming

The ambient temperature in the OR impacts how quickly a patient's body heat is lost through convection from the skin and evaporation at surgical sites. Increasing the ambient temperature in your ORs, however, isn't a practical way to warm patients since you'd have to keep thermostats set at 73 ?F or higher to maintain normothermia. Warmed cotton blankets reduce heat loss by approximately 30%, which is a clinically important amount, but rarely enough to avoid hypothermia in surgical patients.

Mattresses that circulate warmed water underneath patients are nearly ineffective because little heat escapes from a patient's back that's pressed firmly against the operating surface. These devices are more effective when positioned over patients. Newer circulating water garments take advantage of this approach and can eliminate metabolic heat loss, thus increasing mean body temperature by about 1 ?C per hour. Forced-air warming systems also eliminate metabolic heat loss, and therefore also increase mean body temperature by roughly 1 ?C per hour.

I recommend applying effective surface warmers to all patients, because even during short cases, the devices will increase mean-body temperature and improve patient comfort.