Patients enter pre-op mildly to moderately dehydrated. They've had no recent food to metabolize. We strip them of their insulating layers and replace their clothes with a thin gown that flaps open in the back. We drip cold fluid through their veins, lay them on cold surgical tables in breezy rooms, put their brains to sleep and breathe dry air into their lungs. We expose their skin, paint volatile fluids onto the surgical site and expose their warm cavities to room air. These factors are enough to overwhelm normal thermoregulation, and they explain why all patients undergoing any surgical procedure face the risk of hypothermia.

Inducing hypothermia can be a deliberate medical maneuver in special surgical circumstances, but all too often the complication is insidious and unintentional. Read on to learn about the science behind this common surgical complication.

Physics of heat transfer
Heat transfers between objects by four processes: radiation, conduction, convection and evaporation. Let's quickly review each.

• Radiation. Radiation is the transfer of heat through electromagnetic frequency. It requires no contact between atoms; the heat simply transfers, or radiates, across the gradient from a warm body to a colder environment. Body surface area affects the importance of radiation as a mechanism for losing body heat, as does temperature gradient.
• Conduction. Conduction is the molecular transfer of heat through direct contact. Lay a patient's warm body on a cold operating table and the surface molecules will transfer heat from the patient to the table. Conduction occurs even without two solid surfaces coming into contact, because air itself contains enough molecules to transfer heat by this mechanism. Wetness, too, plays a factor, even in still air. Wet skin will conduct heat 25 times faster than dry skin because the greater density of water means a greater molecular exchange of energy; there are simply more molecules in water to do the work of heat transfer. Swim in a cold ocean and you'll lose heat very quickly.
• Convection. Convection is a special kind of conduction, occurring when one of the objects is in motion. When the air is moving against our skin, we can't maintain a microclimate of warmth because new, cold air molecules quickly replace the air we warm. Put on a wet suit before you slip into that cold ocean and you'll stay warmer because once the surface next to your skin warms up, it won't circulate with the cold water.
• Evaporation. Evaporative heat loss occurs when a liquid changes phase to a gas. On a hot day this process is useful, since our sweat evaporates to keep us cool. In the operating room our patients don't normally sweat, but they still lose heat due to evaporation as they moisten the air they breathe and the air touching their skin. This type of evaporative heat exchange is called insensible heat loss.

Clinical relevance
Our bodies constantly produce heat as a byproduct of cellular metabolism, which must be managed properly in order to maintain the narrow operating temperature we require. Our temperature rarely varies more than about 0.5'C from its normal 37'C. To cool things down when we're heating up, our blood vessels dilate and our skin begins to sweat. To conserve heat when we're getting too cool, we produce catecholamines, constrict our blood vessels, increase skeletal muscle tone, and produce more heat via shivering and non-shivering thermogenesis. In the course of our normal daily lives, heat gains equal heat losses, so our autoregulation keeps us well within our homeothermic limits.

Autoregulation begins with thermal inputs arriving at the central nervous system from the surface of skin, from deep tissue and from lower neural centers. The spinal cord initiates some thermoregulatory responses and passes the inputs along to the brain for more processing. Changes in temperature perceived by skin sensors tend to elicit behavioral responses, whereas changes noted in core temperature evoke autonomic responses.

There are two major reasons that anesthesia puts surgical patients at greater risk of hypothermia. The first is that hypothermia changes the threshold temperatures at which the hypothalamus will elicit autonomic thermogenesis. Under anesthesia, thermoregulatory vasoconstriction (which normally occurs near 37'C) doesn't occur until core temperature reaches about 34'C. Sedation alone depresses this normal response to hypothermia, and general anesthesia depresses it further.

The second major effect of anesthesia on heat loss is the redistribution of blood flow from the warm core to the cooler periphery. Regional anesthesia can produce large temperature drops due to pronounced vasodilatation and also from pronounced muscle relaxation, which results in decreased thermogenesis.

So is perioperative hypothermia enough of a concern to worry about? The consequences of hypothermia are in fact mixed; there can actually be some benefits. Cerebral metabolic rate decreases, which can offer brain protection. Improving the brain's tolerance to ischemia can offer some real benefit in specialized circumstances, such as surgery requiring cardiopulmonary bypass.

But more commonly the negative consequences outweigh the benefits. Even mild hypothermia (35'C) is associated with increased mortality rates. Hemoglobin becomes more reluctant to release oxygen to the tissues. The pH balance becomes more alkalotic. Blood sugar rises, increasing the risk of surgical wound infection. Cardiac arrhythmias are more common. Blood flow is shunted away from skeletal muscle and the extremities. The blood also becomes more viscous and harder to circulate. Coagulation is impaired. Potassium levels increase. Renal and hepatic functions become impaired. Ezymatic reactions decrease, which can substantially increase the duration of some drugs.

To some patients, these perturbations present unacceptable risk of morbidity, so unless hypothermia is a known benefit to a particular patient having a particular procedure, we should strive to maintain patient temperatures above 36'C.

Prevention versus treatment
The best way to treat hypothermia is to prevent it in the first place. The greatest loss in temperature typically comes in the first hour of anesthesia because blood is shunted from the body's core to the periphery and because anesthesia lowers the set point at which the body will respond to hypothermia. Under anesthesia, the body cools down quickly and also loses its ability to adequately respond to that cooling.

Redistribution hypothermia, when heat is transferred from the core to the periphery via vasodilatation, is difficult to treat but can easily be prevented by cutaneous warming before induction of anesthesia. Prevention of hypothermia should begin even before the start of anesthesia.

Strategies such as keeping the patient covered, applying warm air convection via a forced-air warming blanket, keeping room temperature warm (30'C), humidifying ventilatory gases and warming intravenous fluids can make all the difference. These strategies should begin before surgery and continue through recovery.

Patients' temperatures should be monitored as soon as they get admitted. A pre-op patient that complains of feeling cold should be considered hypothermic regardless of his measured temperature and should be offered a warmed blanket or a heating device as soon as he changes clothes. Warming should continue as needed throughout the surgical experience, and measures should be taken to ensure a core temperature of 36'C by discharge.

Although injury is rare when using warming devices, burns do occasionally occur (see "Proper Use of Patient Warming Devices," December, page 74). A cautious approach to the use of these devices is mandatory. Blowing warm air on the patient directly from the hose of a forced air warmer without attaching the disposable blanket is a known cause of burns. Mattress warmers cause burns when their heating elements fracture. Fluid-circulating pads can cause burns from pressure points against the circulating fluid. Always follow the manufacturers' instructions for your devices and be alert for device failure.

The cost of preventing hypothermia can certainly add up. Portable forced air blowers and blanket warming cabinets cost thousands of dollars and the cumulative expense of disposable warming blankets can be significant. While some warming techniques may, in the short term, seem like an extravagant expense, the costs pale in comparison to the cost demonstrated for the cumulative outcome of even mild hypothermia. Meta-analysis has demonstrated an increased per-case cost of $2,500 to $7,000 due to such things as increased length of stay in the PACU, ICU or medical floor, increased use of blood products, increased need for post-operative mechanical ventilation and increased cardiac problems.

Managing the risk
Hypothermia is a known and manageable risk to those who venture outdoors, even in mild climates. Without thinking about it, we put on extra layers and work a little harder to stay warm. Surgical patients face this same risk in the mild climate of the operating room, but rely on vigilant caregivers to manage the risk for them. With a little forethought, keeping our patients normothermic can be just as intuitive as staying warm outdoors.