My anesthesia instructors always taught me that it’s easier to retain heat than it is to regain heat in the OR. Perioperative hypothermia is a common, and often preventable, complication of anesthesia. However, preventing it requires understanding its causes. It requires acting even before the anesthetic begins.
I was recently reminded of perioperative hypothermia when the air conditioner unit at one of the surgicenters where I work malfunctioned, forcing us to run it on full chill mode for a few days until fixed. OR temp was about 13°C (55°F) As the garb my partner is wearing in this photo shows, it was hard to keep us warm, let alone the patients. But we did it by paying attention to a few details.
Definition
Mean core temperature in healthy humans is 36.5°C–37.3°C (97.7°-99.1°F). Perioperative hypothermia is defined as a core body temperature below 36°C (96.8°F). Perioperative hypothermia can cause:
- cardiovascular complications
- delayed wound healing
- altered states of consciousness
- increased infection rate
- increased blood loss
- prolongation of drug effect.
Yet despite the importance of temperature control in the OR, some studies report the incidence of perioperative hypothermia as high as 80% in some studies.
Causes of Perioperative Hypothermia
Thermoregulation depends on the balance between heat production (metabolism and basal metabolic rate) and heat loss. If core temperature drops, then the body maintains normothermia by using behavior (donning extra layers, moving to a warmer location), increased activity, vasoconstriction, and shivering. However, hypothermia will develop if heat loss is greater than heat production.
Anesthesia disrupts normal thermoregulation by:
- changing the brain’s thermal regulatory set points
- decreasing metabolic rate,
- altering the distribution of blood flow.
Thermoregulatory Set Points
The brain maintains a mean core temperature in a healthy patient between 36.5°C–37.3°C (97.7°-99.1°F). No thermoregulatory effects trigger if the temperature stays between those set point temperatures. Temperatures above and below those set points trigger a response. Surpassing the upper threshold triggers vasodilation and sweating to reduce core temperature. Sinking below the lower threshold triggers vasoconstriction and shivering to retain heat and to increase heat production.
Anesthesia resets those thresholds by 2-3°C, making perioperative hypothermia more likely.
Vasoconstriction and shivering thresholds are often reduced even further in elderly patients.
Metabolic Rate
Metabolic rate decreases anywhere from 15-40% under anesthesia. Not only does this effect temperature regulation, but it also slows drug metabolism.
Blood Flow Redistribution
The body needs to maintain a stable core temperature for optimal organ function. In a cold environment, the body retains heat centrally by vasoconstricting peripheral blood flow. Anesthesia decreases the threshold for vasoconstriction.
Intraoperative vasodilation is caused by both:
- the direct effect of anesthetic agents
- the indirect consequences of a lowered vasoconstriction threshold.
The result is core cooling as cooler blood in the peripheral flows centrally into the core.
A patient who is cold in preop is much more likely to develop perioperative hypothermia. It’s common to see core temperature immediately drop a degree or two soon after induction of anesthesia because cooler peripheral blood is redistributing to the core. Pre-warming the patient will decrease or eliminate this drop in temperature.
Rewarming Hypotension
Blood flow redistribution is also responsible for the phenomenon of rewarming hypotension. Cold patients enter the recovery room with baseline vasoconstriction. However, that vasoconstriction will sometimes mask hypovolemia. As the hypovolemic patient rewarms, the patient will vasodilate, potentially causing hypotension.
In a severely hypothermic patient, such as from cold water emersion, rapid rewarming can precipitate cardiovascular collapse and possible cardiac arrest for the same reason.
Perioperative Heat loss
As we have seen, heat production decreases and blood flow distribution of that heat changes in ways that cools core temperature. At the same time, heat loss increases in the OR setting. There are 4 major ways to lose heat: radiation, conduction, convection and evaporation.
Radiation (40%)
Radiation is the emission of infrared electromagnetic energy. Heat loss is proportional to the temperature difference by a power of 4. The typical OR is purposefully kept quite cool to accommodate surgeons wearing insulating OR gowns under hot OR lights. My surgeons complain bitterly if the OR temperature goes above 18°C (65°F) and would love me if I let them lower it below that.
Any uncovered part of the patient’s body will radiate heat. Patients with high surface to volume ratios, exposed to cold air and/or prep solution for prolonged periods of time, will develop perioperative hypothermia.
Convection (30%)
Convection is the direct transfer of heat to moving currents of air or water surrounding the body. It’s proportional to air velocity.
Laminar air flow in the operating room uses positive pressure air currents through filtration units to direct air streams away from the operative field in order to create an ultraclean zone around the operative site. Guidelines recommend an OR air exchange rate of at least 20 air exchanges per hour. Laminar flow in the OR can significantly increase heat loss in the uncovered patient.
Evaporation (25%)
Evaporation is the vaporization of water through both insensible losses and sweat. Sweating is the obvious example in a warm environment. In a cold OR, evaporation of moisture from open body cavities and exposed mucous membranes, from skin prep solutions, and from breathing unhumidified gases easily occurs.
Conduction (5%)
Conduction is the direct transfer of heat to an adjacent, cooler object. The use of non-warmed IV fluids as well as contact with a cold OR mattress promote conductive heat loss.
Body Surface Area
The surface to volume ratio effects all 4 heat loss mechanisms. The higher the surface to volume ratio, the more heat can potentially be lost. Infants and young children are much more likely to get hypothermic in a cold environment because of significantly higher surface to volume ratio (toddlers 2X and infant 3X the adult ratio) and little muscle mass or fat insulation. The infant and small child can get severely hypothermic — even in environments that adults would tolerate quite well.
Older, frailer, and skinnier adults are also more at risk of perioperative hypothermia.
Preventing Perioperative Hypothermia
Before Surgery
- Identify patients who are high risk of perioperative hypothermia.
- Keep patients warm while waiting in preop. Offer blankets or use forced air warming devices. Warming the patient prevents significant temperature drop post induction of anesthesia.
Intraoperative
- Consider maintaining an OR temperature of 21°C (69°F) for patients at high risk of hypothermia. This is neutral thermal environment. Surgeons are often agreeable to raise the OR temperatures for high-risk patients. They are often less enthusiastic about doing so for routine patients. A good strategy is to raise room temperature for induction and then reduce it to a temperature comfortable for surgeons after the patient is prepped and draped.
- Warm the bed
- Keep the patient covered with blankets as best you can. In a cold room use multiple blankets if available. Expose only the areas that are needed for prep.
- Prep and drape quickly to minimize exposure!!!
- Use of forced air warming if available. Do NOT direct forced air warming directly against the patient’s skin as burns could result
- Cover the head. About 7-10% of heat loss occurs through the exposed head.
- When multiple liters of IV fluid will be needed then consider using a fluid warmer. Prewarmed fluids used within 30 minutes of removal from the warmer are as effective.
- Monitor patient temperature at least every 30 minutes. Titrate active warming as appropriate. Be aware that use of high heat forced air warming on a well covered patient can sometimes cause hyperthermia. Warm, but don’t cook your patient.
Postoperative
- Continue to monitor temperature (for both hypo and hyperthermia)
- Use forced air warming if the patient is hypothermic to actively rewarm. Warm blankets provide comfort but little active rewarming.
Perioperative hypothermia Keeping our patients warm is important to optimizing their safety and their surgical recovery. Prewarm them. Minimize exposure. Prep and drape quickly. And actively rewarm them if their temperatures are dropping.
Further Reading:
- Bindu, Barkha; Bindra, Ashish; Rath, Girija. Temperature management under general anesthesia: Compulsion or option. Journal of Anaesthesiology Clinical Pharmacology 33(3):p 306-316, Jul–Sep 2017. | DOI: 10.4103/joacp.JOACP_334_16
- Riley, C; Andrzejowski, J., Inadvertent Perioperative Hypothermia. BJA Education, 18(8): 227e233 (2018), doi: 10.1016/j.bjae.2018.05.003.
