By Kris Ellis

Unplanned hypothermia (a core temperature of less than 36 degrees C) can negatively impact patients in many ways. Even mild hypothermia may contribute to complications such as surgical site infection (SSI), altered drug metabolism, impaired blood clotting, cardiovascular ischemia, prolonged recovery following surgery, and shivering.1

The American Society of Anesthesiologists (ASA) says active patient warming is associated with normalizing patient temperature.2 “The literature supports the use of forced air warming devices for normalizing patient temperature and reducing shivering,” notes ASA. “In addition, the literature suggests that forced-air warming is associated with reduced time in recovery. The Consultants and ASA members agree that both the perioperative maintenance of normothermia and the use of forced-air warming reduce shivering and improve patient comfort and satisfaction.” ASA recommends that normothermia should be a goal during emergence and recovery, and that when available, forced-air warming systems should be used for treating hypothermia.

The American Society of PeriAnesthesia Nurses (ASPAN)’s Clinical Guideline for the Prevention of Unplanned Perioperative Hypothermia specifies that a preoperative patient management assessment should include:

  • Identification of a patient’s risk factors for unplanned perioperative hypothermia 
  • Measurement of patient temperature on admission 
  • Determining patient’s thermal comfort level (ask the patients if they are cold) 
  • Assessment of other signs and symptoms of hypothermia (shivering, piloerection, and/or cold extremities)3

ASPAN notes that interventions may include:

  • Instituting preventive warming measures for patients who are normothermic. A variety of measures may be used, unless contraindicated. Passive insulation can include warmed blankets, socks, head covering, limited skin exposure, circulating water mattresses, and an increase in ambient temperature (a minimum of 20°C-24°C or 68°F-75°F) 
  • Instituting active warming measures for patients who are hypothermic. Active warming is the application of a forced-air convection warming system. Appropriate passive insulation may also be applied, as well as increased ambient room temperature. Warmed IV fluids may also be considered4 During the intraoperative period, ASPAN recommends that a patient’s temperature should be monitored. 

Recommended warming methods include:

  • Application of appropriate passive insulation, such as warm blankets, socks, head covering, limited skin exposure, and circulating water mattress 
  • Increasing ambient room temperature. AORN (Association of periOperative Registered Nurses) Practice Guidelines for ambient room temperature should be followed 
  • Institution of active warming: Apply forced-air warming system 
  • Warming IV and irrigation fluids
  • Humidify and warm anesthetic gases5

ASPAN notes that the patient’s core temperature should be maintained at 36 degrees C (96.8 degrees F) or above during the intraoperative phase unless hypothermia is indicated. Temperature should also be assessed on admission to the PACU. If normothermic, preventative warming measures should be instituted, such as applying appropriate passive insulation. If hypothermic, active warming measures such as those detailed above should be instituted. The patient’s temperature and thermal comfort level should be assessed every 30 minutes until normothermia is reached.

“The surgery center is really no different than the acute care environment when it comes to hypothermia,” says Judson Boothe, associate marketing director, surgical solutions, Kimberly-Clark Health Care. “Any surgery that involves the use of an anesthetic has a risk of hypothermia.

Hypothermia often results in slower processing of anesthetics, and therefore delayed recovery as patients come out of the anesthesia. There can also be excess bleeding complications related to even mild hypothermia; the more severe the surgery, the more likely bleeding complications are.”

Boothe also points out that thermal stability is linked to overall patient stability. “Cold patients tend to take longer to stabilize and therefore in a surgery center situation this often creates a backlog as you’re trying to recover patients after surgery,” he says. “Many times there’s a backlog that can block up the OR because of throughput. There are a number of consequences that surgery centers should be aware of, and these can translate into hidden costs. There are costs to managing all those complications that can come from cold patients that are often assumed to be unavoidable. In fact, with a good warming program, they are avoidable, and can be documented and captured.”

Maintaining normothermia with forced-air warming has been shown to reduce the risk of complications and costs associated with unintended hypothermia, explains Troy Bergstrom, marketing communications manager, Arizant Healthcare Inc. “The good news is, maintaining normothermia is simple and cost-effective,” he says. “Through the use of forced-air warming gowns and blankets, along with warmed IV fluids, both the patient and surgical facility can benefit. Studies have suggested that maintaining normothermia may yield positive results such as a reduction in the rate of postoperative wound infections, decreased ICU time, shortened length of hospital stay, and lower mortality rates. In addition, maintaining normothermia can reduce per patient hospitalization costs by $2,500 to $7,000 across a variety of surgical procedures.

“Inadvertent hypothermia can be triggered by the induction of anesthesia, and research shows that in the first 60 minutes of anesthesia, unwarmed surgical patients can lose up to 1.6 degrees Celsius,” Bergstrom adds. “Warming patients before surgery can reduce this core temperature drop by banking heat, and hypothermia is easier to prevent than treat. By prewarming patients, then following with intraoperative warming and post-op warming, facilities can prevent hypothermia before it has a chance to begin.”

Boothe speculates that pre-warming might be the most underappreciated technique for keeping patients warm, particularly in a surgery center setting. “It’s been proven that a patient can be ‘pre-heated’ if you will, before surgery, and by doing so you might in fact prevent any hypothermic time during surgery. Particularly for the short-duration procedures, that can be very effective, and it’s easily executable with any number of technologies. Pre-warming could be a very effective solution for most surgery centers if done well.”

Boothe also reiterates the connection between hypothermia and SSI. “Most protocols that are aimed at reducing the incidence of surgical site infections will include maintenance of normothermia as a primary prevention measure.

For example, the Institute for Healthcare Improvement (IHI)’s 100,000 Lives Campaign calls for four mechanisms that should be incorporated into any SSI prevention routine, and those are antibiotics, proper treatment of the surgical site in terms of hair removal or cleaning, glucose control, and managing hypothermia.

It makes sense — your body reacts to an invasion of pathogens by spiking a fever if you’re healthy, so if your immune system is cold and suppressed, you would expect that it doesn’t work as well, which is in fact the case.”

In addition to proper use of prophylactic antibiotics and good surgical technique, IHI notes that other factors under the control of the operative team can have a significant impact on the risk of SSI. These include preventing hypothermia during procedures. IHI’s changes for improvement in this area include:

  • Limiting heat loss in patients prior to operative procedures; keep temperature at greater than 36 degrees C 
  • Standardizing the use of warming devices (such as warming blankets, hot air blankets, IV fluid heaters, and filter heater hydrator for laparoscopic procedures) to ensure patient temperature of greater than 36 degrees C perioperatively 
  • Standardizing an intraoperative temperature monitoring method 
  • Providing devices and protocol for the consistent measurement of patient temperature 
  • Designating responsibility and accountability for thermoregulation 
  • Assuring engineering controls that allow surgical staff to control room temperature 
  • Increasing ambient room temperature in the OR. (Note: humidity should be increased if ambient room temperature is increased in order to prevent dry eyes and/or skin among staff members) 
  • Consider increasing temperature when the OR is not in use (i.e., after last patient of the day and before first patient of the day) to make sure that the equipment is warm even if the temperature is decreased during OR use 
  • Providing surgical staff with cooling gear/devices6

Product Evaluation

“When evaluating a patient warming system, it’s important to find a system that can warm safely and effectively while demonstrating the flexibility to meet the changing needs of the customer,” Bergstrom contends. “For example, Arizant’s Bair Hugger line of forced-air warming blankets offers 23 different models covering many surgical procedures, and the Bair Paws system uses one gown throughout the perioperative process. Our Ranger fluid warming system is designed to maintain safe temperatures across a wide range of flow rates. Today, forced-air warming gowns and blankets are available in styles that meet the needs of virtually every patient or warming/surgical situation. It’s an inexpensive tool to prevent the costs associated with unintended hypothermia.”

Boothe notes that the Kimberly-Clark Patient Warming System is a ‘high horsepower’ warming solution. “It’s a very active system that clings to the patient via hydrogel, which provides a high level of energy directly to the patient’s skin,” he says. “It’s absorbed by the patient and creates a very high rate of warming compared to other surface warmers.

“Surface warming is driven by the amount of heat provided and the ability of the body to absorb it,” Boothe continues. “Inefficient systems such as forced air require large amounts of surface area to be effective. Given a large amount of surface area, say 60 percent to 80 percent of the body surface, a forced-air system can be very effective. If limited by the surgery site, like some plastic surgery procedures or large orthopedic surgery, where there are very large surgical sites, forced-air warming becomes ineffective because you don’t have enough surface area. That’s when systems like Kimberly-Clark’s become quite useful, because they require less than 20 percent of the surface area to actually warm the patient.”

Boothe emphasizes that the need and solution must correspond in terms of warming systems. “All of them can work given the right conditions. For a facility that’s focused on warming its patients as a clinical objective, they’re probably going to have multiple warming solutions in place so they can match up their needs with the right solution. There’s not one solution that addresses all needs, and no one company sells everything you need yet.”

Patient warming gowns are on the cutting edge of warming technology, Bergstrom adds. “The Bair Paws system from Arizant Healthcare allows one single-use gown to be used throughout the perioperative process — it pre-warms in pre-op, offers clinical warming in the OR, and provides both clinical and comfort warming in post-op.”

Fluid and Blanket Warming

Warming injection and irrigation fluids, as well as blankets, can be useful components of a facility’s efforts. One study explored the effects of warmed IV fluids compared to room-temperature fluids when used during ambulatory gynecological surgery.7 In this instance, 38 adult patients undergoing elective outpatient surgery less than 30 minutes in duration were randomized into two groups: fluid warming at 42 degrees C, or a control group (room temperature fluids at approximately 21 degrees C).

Results showed that more patients had a final core temperature of less than 35.5 degrees C in the control group, compared with the warm fluid group (35 percent vs. 0 percent). The authors concluded that fluid warming, in conjunction with standard heat conservation measures, was effective in maintaining normothermia during outpatient gynecological surgery.

Enthermics Medical manufacturers blanket and fluid warmers that incorporate its Warm- Right Concept of warming. This means the warmers are designed to match the right temperature to the right product. The Enthermics warmers are designed to allow temperature stabilization for blankets, irrigation fluids, and injection fluids at three different temperatures in three different chambers.

“The fluid warmers are sold as one device that can be set two ways,” says Mark Suszkowski, vice president, sales and marketing, Enthermics Medical. “When you turn on one of our fluid warmers, you push either the injection button or the irrigation button. If you push injection, you get a temperature range of 90 degrees F to 110 degrees F. If you push irrigation, you get a range of 90 degrees F to 150 degrees F.

“When you’re warming injection bags for infusion, those are taken out of the chamber, hung on an IV pole, and directly injected into the patient,” Suszkowski continues. “That’s different then irrigation warming, and that’s why it only goes up to 110 degrees F, and it’s more accurate. We’re labeled to do both of those things, whereas most manufacturers are not.”

Suszkowski points out that fluid manufacturers have their own standards concerning the time and temperature that fluid products can be placed into machines. “The companies that manufacture fluid bags recommend a limit of 104 degrees F for bags for injection. Ours goes to 110 degrees F, but you have to follow what the fluid bag manufacturers recommend.” Suszkowski also notes that injection fluids and irrigation fluids cannot be warmed together unless they are warmed in the injection temperature range, which is the lower range. “You also cannot warm any fluids in blanket warmers,” he says.

Temperature stabilization is integral to Enthermics’ WarmRight concept, particularly for fluids. It may take several hours for fluids to warm to the set-point temperatures, at which point they are considered stabilized. Sometimes fluids are placed in a warmed chamber that is too warm, and then removed when they are thought to be at about the right temperature. This is not temperature stabilization at a determined setpoint temperature, Enthermics contends, and is an inaccurate warming technique.

While fluid warmers are Class I medical devices, blanket warmers are not medical devices, and therefore they’re not subject to FDA regulation, Suszkowski explains. “You cannot warm fluids in a blanket warmer under any conditions, because it’s not labeled that way, and blanket warmers are not nearly as accurate as fluid warmers. Warming cotton is much different than warming water; it takes a lot more heat to warm water, so the machines are designed a lot differently.

“We built three separate [warming] chambers — the two fluid chambers that are actually in any fluid warmer, and the blanket chamber,” Suszkowski continues. “We determined that the blankets should really go to about 200 degrees F, because that’s a better temperature; blankets don’t hold heat very well, so they cool down very quickly after you take them out of the warmer. 200 degrees F is near the boiling point, but blankets and water are very different. Water has a very high heat capacity; it takes a long time to boil, because it absorbs a lot of heat. The heat capacity of water is miniscule compared to water — it’s at least a hundred times lower. Cotton can be warmed at 200 degrees and it won’t burn the skin; it will just feel warm. In addition to temperature, heat capacity must also be considered; you need to know both.”

Since warm blankets hold very little heat, they don’t really affect a patient’s core temperature, Suszkowski states. “What blankets do, however, is comfort the patients and make them feel better; there’s a major psychological impact in giving patients the warm blankets. The patient will feel warmer for a time, but they won’t change in terms of core temperature.”


1. Wagner DV. Unplanned perioperative hypothermia. AORN J. 2006 Feb;83(2):470, 473-6. Review. 

2. American Society of Anesthesiologists Task Force on Postanesthetic Care. Practice guidelines for postanesthetic care: a report by the American Society of Anesthesiologists Task Force on Postanesthetic Care. Anesthesiology. 2002 Mar;96(3):742-52.

3. American Society of PeriAnesthesia Nurses. Clinical Guideline for the Prevention of Unplanned Perioperative Hypothermia. http://  

4.   Ibid.

5. Ibid.

6. The Institute for Healthcare Improvement. Maintain normothermia perioperatively.

7. Smith CE, et al. Warming intravenous fluids reduces perioperative hypothermia in women undergoing ambulatory gynecological surgery. Anesth Analg. 1998 Jul;87(1):37-41.


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