Perioperative patient temperature changes are a “hot” topic now. Numerous studies over the past decade have shown a relationship between patient hypothermia and surgical wound infection and length of hospital stay in addition to the things related to patient temperature that have been long known, such as shivering, cardiovascular stress, coagulopathies, and patient satisfaction. The importance of regulation of room temperature for patient normothermia instead of staff comfort has been recognized. Some medical insurance companies audit perioperative patient temperature as an indicator of appropriate care for reimbursement.

Several methods are used to keep patients normothermic in the perioperative period. Some work much better than others. Some provide active warming, but some are passive in nature. We will talk about the different kinds of patient heat loss, as well as the different methods and the efficacy of each.

METHODS OF PERIOPERATIVE HEAT LOSS

Intuitively, it is not hard to understand why patients get cold perioperatively. You are placed in a cold room, told to take off your clothes, and put on a flimsy short-sleeve gown that is open in the back. You may or may not have a blanket. Then you are given intravenous (IV) fluid that is room temperature. You are wheeled down a cold, drafty hallway into a cold room and placed on a cold bed. You are given drugs that negatively alter your body’s thermoregulation. Then after you are unconscious, you are uncovered totally, positioned, washed in room temperature solution, and given more room temperature IV fluid. By the time you are covered up in surgical drapes, your temperature has dropped at least 1°C. The length and location of the operation will affect the degree of hypothermia, as well as the amount of IV fluid. You will lose 0.25°C of body temperature for every liter of room temperature IV fluid given. So it is easily understood why you get cold. But what are the methods of heat loss, speaking in terms of physics?

The four main methods of heat loss in an anesthetized patient are radiation, conduction, convection, and evaporation. You may not remember the differences in these, so we will go over each one.

Radiation

For this form of heat loss, think of heat as light (they both are energy, of course). In this example, the patient emits heat by radiation just like a light bulb emits light into the room. The colder the environment, the more heat is radiated away from the patient. This is actually the main way a patient loses heat.

Conduction

This is when heat energy flows from one molecule to another or one object to another. Contact is needed for heat loss to occur by conduction. The heat energy flows from the warmer to the colder object. The best example for us in the operating room (OR) is when the patient loses heat to the cold IV fluid that was just infused into him or her. The colder IV fluid molecules are in contact with warmer molecules that make up the patient, so heat is lost to the cold IV fluid, resulting in a decrease in overall temperature. Of course, the drop depends on how cold the IV fluid was and how much fluid is given.

Convection

In convection, heat is lost to moving air; it is the OR equivalent of the wind-chill factor. The amount of loss depends on the amount of draft and the temperature of the air.

Evaporation

In Chapter 6 on vaporizers, we mentioned that evaporation of our liquid anesthetic agent takes energy and that energy comes from the ambient temperature of the room where our vaporizer is. The heat of the room is used to cause vaporization of the agent.

The best example of evaporative heat loss is sweating. Why do we sweat when we are hot? It is a way to cool the body by evaporation. It takes energy to vaporize the liquid perspiration on our skin, and that energy comes from the heat of our body. Although our patients might not sweat, they nevertheless do have some heat loss occur through vaporization. The loss is more pronounced if the patient or drapes are wet, with the wetness acting similar to perspiration.

METHODS OF PATIENT WARMING

It does not take a professor of thermodynamics to tell us what we can do to keep a patient’s heat loss to a minimum. Much of it is common sense. The patient should be kept as warm as possible preoperatively, in the holding room, and on the way to the OR. The OR should be as warm as possible while the patient is being anesthetized, positioned, and prepped. The patient should be covered as much as possible during induction, positioning, and prepping. Only after the patient is fully draped should lowering the room temperature be considered.

You can divide methods of warming two ways; there are active and passive methods, or you can think about warming the patient and warming IV fluids as two separate ways. Active methods include forced-air warmers, IV fluid warmers, and the like; passive methods include blankets and other coverings. Passive methods rely on trapping the heat being lost by the patient in the small envelope of air under the coverings.

Forced-Air Warmers

Often generically called “Bair huggers,” after a certain brand, forced-air warmers are a very effective way to either warm a patient or keep a patient warm. A specially designed disposable plastic and paper drape is placed on the patient and attached to the unit that blows warmed air into the drape, dispersing the warm air to all parts of the drape. The air used is sucked into the unit, passing through a bacterial filter, warmed using a heater (with different temperature settings) in the unit, and taken to the drape via a hose. The drapes come in different sizes and shapes to allow for differences in patient size and operative area. Some drapes are designed to go under the patient, around the patient, or over the patient. There are even robe-like drapes that can be used in the holding room to decrease heat loss preoperatively.

There are several advantages to forced-air warmers. As mentioned, they are a very effective means of temperature control in the OR, better than other types in many studies. After the unit has been bought, the cost of using a forced-air warmer is low. The drapes, although only for one-time use, are relatively inexpensive and can be charged to the patient and can go from holding to the OR to recovery with the patient if desired. The units are thermostatically controlled to keep the unit and air from getting too hot. An “ambient” setting on many units will blow room temperature air through the drape if a patient gets too warm during a procedure. Some are designed to allow IV tubing to be warmed also. Using a forced-air warmer can allow the room temperature to be lowered, making gowned personnel more comfortable.

The disadvantages of forced-air warmers themselves are few if they are used properly. The main problem is the risk of patient burns if the warmer is used incorrectly. A forced-air warmer should never be used on a patient without the special warmer drape. As mentioned, the drape disperses the heat evenly so there are no “hot points,” areas where more of the warmer output lands and causes burns. There are case reports of the warmer being used by putting the output hose between the patient’s legs and covering the patient with a blanket (hosing). This concentrates the warmed air, and burns can occur; even though the maximum temperature setting is 43°C, the air coming out of the hose is 3° to 5°C warmer than the set temperature, and it cools as it is dispersed by the special blanket. Other problems can occur from using a unit and hose from different manufacturers (comingling). There is even a website to educate users about the dangers of hosing, called stophosing.com.

In addition to burns from hosing, the hose itself can get warm enough to burn a patient if left on the same site for an extended time. Extremities below a vascular clamp can be burned because there is no blood flow to circulate the warmth. Forced-air warmers add to the noise of an OR (but the benefit to the patient greatly outweighs the noise pollution problem). Forced-air warmers are not magnetic resonance imaging compatible; the blanket is, but the unit is not.

Water Blanket Warmers

These units were used commonly before the advent of forced-air warmers. A specially designed watertight pad is placed under the bedsheets of the OR table. This pad is attached to a warming unit by two tubes, and warm water is circulated through the pad and back to the warming unit. In addition to warming, the unit could be used to cool the patient. These devices are less effective than forced-air warmers.

Conductive Fabric Blankets

This is a relatively new method of patient warming. A special conductive fabric blanket has electricity sent through it. The large electrical resistance of the fabric generates heat. Sensors control how much electricity is sent through the fabric in order to maintain a safe, steady temperature.

These blankets are reusable. The system is ungrounded, so there is no danger of electrical shock to the patient or personnel. One example of a conductive blanket system is called “Hot Dog.” The manufacturer claims there is less incidence of orthopedic wound infection with their device than a standard forced-air warmer because the forced-air warmer creates convection air currents that can spread fomites up into the surgical site.

Radiant Heating Lights

“French fry lights” are often used in pediatric cases. They can be used for adults as well, but the large surface area of infants means the radiant lights are more effective than for adults. The best results are obtained when the patient is uncovered totally. The lights are on a panel or an IV pole, with a measuring stick attached, because if the lights are in too close proximity to the patient burns can occur. Care must be taken because the lights can melt plastic IV bags or burst glass IV bottles or even burn personnel if the bulbs are touched. They are less effective than forced-air warmers.

Heated Anesthesia Circuits

These units were common in ORs 20 years ago but are less common now. They will be seen more frequently in critical care settings. A heated humidifier is attached to the inspiratory limb of the circle circuit, and as the inspiratory flow flows over the heated water, both heating and humidification occur. A temperature feedback probe is placed at the Y piece to regulate the temperature.

Heated humidifiers had many drawbacks for use in anesthesia. One thing was that they were not very effective. Again, forced-air warming is far superior. Second, use of a heated humidifier multiplied the places for and chances of a circuit disconnect. There were more places to become disconnected, and the warmth and moisture of the circuit adapters made a disconnect occur more easily. Third, if a heated humidifier was improperly attached, the circuit could melt, causing a catastrophic loss of positive pressure in the circuit. One of the authors witnessed a STAT call to an OR, where the circuit had been melted apart because the provider had not placed the temperature sensor in the Y piece, so the heater kept on getting warmer and warmer, with no feedback from the sensor, until the plastic circuit melted. You should be glad these devices are no longer in common use.

Heat and Moisture Exchanger

A heat and moisture exchanger (HME) is a small, filter-like attachment to the anesthesia circuit that is supposed to fit between the endotracheal tube or supraglottic device and the Y piece. Its 15-mm fittings allow it to be placed only in that area. It is similar to the filters discussed in the Chapter 7 on the anesthesia circuit and is made of either hydrophobic or hygroscopic material to catch exhaled moisture and allow it to be delivered back to the patient with subsequent inspirations. A common design of hydrophobic HME will contain what looks like a roll of corrugated paper inside it to trap the exhaled moisture. But because it is hydrophobic, the water that is trapped is not absorbed into the paper but stays on the outside of the paper.

Heat and moisture exchangers are designed to have low resistance to breathing. There are different sizes available for adult and pediatric patients. An HME that is too small will be inefficient, but one that is too large (e.g., an adult-sized HME for a child) will increase dead space.

Perhaps they should be called “MEs” because the ability of them to conserve H (heat) is limited. You will not warm up a cold patient by placing an HME. But they do a good job in increasing the moisture content of the inhaled gas between the HME and the patient’s lungs in cases longer than a few minutes.

The HME should be placed distal to any sidestream gas monitoring sample line take off because if the gas sample is taken from the area between the HME and the patient, excess water can be drawn off, interfering with the gas monitor or requiring you to empty or replace the gas monitor water trap frequently. Excess water, sputum, blood, or any other fluid in the circuit can increase resistance and even cause obstruction and increase peak airway pressures.

Blankets

Blankets are low tech, safe, and require no training to use. Although they are better than nothing, they do have limited value. This type of passive heating relies on insulation, trapping the thin layer of warm air next to the patient and keeping it there. Heated blankets may be psychologically appealing, but their efficacy is no better than unheated blankets after the first few minutes. Additional layers of covering are minimally better than a single layer. Blankets are no better than surgical drapes or reflective “space” or “survival” type blankets in the OR setting.

Low Fresh Gas Flow and Coaxial Circuits

As discussed in other chapters, these two modalities may decrease the amount of temperature drop but will not warm a patient.

Warm Intravenous Bags

In this case, we mean using warm IV bags externally like you would use an old-fashioned hot water bottle. We mention this as a warning: do not do this! It may seem like a great idea to heat IV bags in a microwave oven or keep some in a blanket warmer cabinet and then use the warm bags to nestle next to a patient or to use a warm IV bag as an axillary roll for a laterally positioned patient. This is a very easy way to cause burns to the patient.

METHODS OF INTRAVENOUS FLUID WARMING

Warm IV fluid will not warm up a patient who is hypothermic unless large amounts of fluid are being infused. Its main role is to prevent further loss of body temperature from fluid that is room temperature, or in the case of blood products, frankly cold. That being said, we certainly advocate the use of fluid warmers when infusing large amounts of IV fluid or any time cold blood products are given. But their use otherwise is not clear-cut, and many clinicians do not use fluid warming devices for cases not requiring volume resuscitation or transfusion.

There are a few things that influence the effectiveness of IV fluid warmers. One thing is the initial temperature of the fluid. In most cases except for blood, the fluid will be room temperature. Another factor is the flow rate. If you are only infusing 500 mL over an hour to an adult patient, the fluid will probably return to close to room temperature by the time it reaches the patient after leaving the warmer. Yet another factor is the length of the IV tubing from the warmer to the patient. Longer tubing means the fluid will lose more heat as it is being infused regardless of the flow rate. The final thing to consider is the temperature that the warmer is set for.

There are multiple means of warming fluid and blood for IV use. Crystalloid can be kept in blanket-warming cabinets and infused. The patient will not get warmer, but it will decrease the rate of temperature loss. Much of the warmth of the fluid is lost along the tubing from the bag to the patient. Using a microwave oven to warm IV fluid bags has been done, but the fluid can get too warm. There are also cases of people microwaving units of blood, causing erythrocyte lysis and hyperkalemia.

The safest way to warm fluid for IV use is with a dedicated fluid warming system. Several different technologies are used in fluid warmers for ORs, emergency departments, and critical care units. Commonly, the devices are set to deliver IV fluid that is 41°C when the fluid leaves the unit.

Dry Heat Plates

In this method, a flat “cassette” of the same material that IV tubing is made from is placed between two flat plates that are heated. The flow rate is less than on other types, but nevertheless, this is an effective means of warming IV fluid. Care must be taken, as in all IV tubing, to prime the cassette to rid it of all air (Figure 20-1).

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