General Anesthesia

General anesthesia obliterates behavioral adaptive responses and impairs vasoconstriction, and direct peripheral vasodilation effects cause the patient’s core temperature to drop up to 1.6 ° C (2.7 ° F) during the first hour of anesthesia. This drop in core temperature can be explained by redistribution of heat from the body’s core to the periphery. Redistribution occurs because anesthetics inhibit thermoregulatory control and disrupt the tonic vasoconstriction that normally maintains a core-to-peripheral temperature gradient. Anesthesia agents decrease vasoconstriction thresholds by 2 ° C to 4 ° C (3.6 ° F to 7.2 ° F) with the opening of arteriovenous shunts (Matsukawa et al, 1995; Sessler, 2008). Figure 15-1 depicts the decrease in body temperature that occurs when heat is redistributed from the body’s core compartment to the peripheral tissues. Not a clear exchange of heat with the environment, this redistribution is a heat flow from the actual core to the periphery, resulting in decreased core temperatures. Anesthetics inhibit thermoregulation in a dose-dependent manner and inhibit vasoconstriction and shivering approximately three times more than they inhibit sweating (Sessler, 2008).

Figure 15-1 Core-to-peripheral redistribution of heat after anesthesia administration.

(From Sessler DI: Perioperative heat balance, Anesthesiology 92:583, 2000.)

The next phase of the hypothermic action is a slower, linear decrease in core temperature with heat loss exceeding heat production. This phase lasts approximately 2 to 3 hours and depends on the difference between loss and metabolic heat production. During this time, core temperature continues to decrease an additional 1.1 ° C (2 ° F). Approximately 90% of heat loss is through the skin surface, with convection and radiation usually contributing more to the process than evaporation or conduction. Heat from the patient is transferred to the environment in four ways: radiation, convection, conduction, and evaporation (Figure 15-2) (Kurz et al, 1995). Box 15-3 lists heat loss mechanisms that occur in the perioperative setting. If patients are actively warmed during this phase, the heat loss can be effectively limited.

Figure 15-2 Mechanisms of heat loss.

(From Sessler DI: Perioperative heat balance, Anesthesiology 92:583, 2000.)

After 3 to 5 hours of anesthesia, there is a plateau phase that may reflect a steady state of heat loss equaling heat production and is seen in patients who are well insulated. However, if a patient is quite hypothermic, an arrested temperature decline results from activation of thermoregulatory vasoconstriction, which decreases cutaneous heat loss and acts to hold metabolic heat in the body core. Typically this happens when the patient’s core temperature is about 34 ° C (93.2 ° F) (Sessler, 2008).

Regional Anesthesia

Regional anesthesia decreases the vasoconstriction and shivering to a slighter loss, approximately 0.6 ° C (1 ° F), and is dependent on the level of the block. Although the magnitude of response is less, the pattern of thermal impairment is similar to that of general anesthetics. This similar pattern of impairment suggests an alteration in central rather than peripheral control of temperature (Heier and Caldwell, 2006; Sessler, 2008). Core hypothermia during regional anesthesia may not be recognized by the patient. The reason is that thermal perception and behavioral regulation are mainly determined by skin rather than core temperatures. During regional anesthesia, skin temperature may increase even when there is core hypothermia present. The patient often has the perception of warmth, but will have autonomic thermoregulatory responses, including shivering (Sessler, 2008).

Sedatives and analgesics also impair thermoregulatory control to some degree (Sato et al, 2009). If combined with the impairment of regional anesthesia and other factors, such as a preexisting illness, the thermoregulatory impairment would be severe (Sessler, 2008).

Other Risk Factors

Besides anesthesia agents, there are numerous factors that place surgical patients at a higher risk for the development of unplanned hypothermia during surgery. To see a compilation of the risk factors, see Box 15-2. The ambient temperature of the operating room environment determines the rate at which metabolic heat is lost from the skin through radiation, convection, and evaporation. Skin preparation methods may also contribute to hypothermia through evaporative loss (Sessler et al, 1993). There is a greater risk for hypothermia during procedures in which large body surface areas are left exposed, in situations where the peritoneal cavity is opened, and during longer surgical procedures (Roe, 1971). Infusions of cool fluids, blood, and blood products have been shown to increase risk for hypothermia. A unit of refrigerated blood decreases mean body temperature approximately 0.25 ° C (0.45 ° F) in a 70-kg (154-lb) patient (Sessler et al, 1993; Camus et al, 1996; Hasankhani et al, 2007). Use of a pneumatic tourniquet helps prevent hypothermia while inflated; however, when deflated, an abrupt hypothermia occurs. Upon release of the cuff pressure, a redistribution of heat from the core compartment to the peripheral compartment results in a rapid decline in core temperature (Sanders et al, 1996; Akata et al, 1998). The use of cool irrigation solutions into the abdomen, pelvis, or chest cavity enhances heat transfer from the core and decreases body temperature with the increased heat loss (Moore et al, 1997).

Numerous clinical studies have found that hypothermia increases the risk for death in trauma patients. The trauma patient has predisposing factors from injuries and preexisting hypothermia from exposure in the field, blood loss and shock, rapid infusions of cool fluids, removal of clothing, and impaired heat production. Hypothermia depresses ventilatory, renal, and hepatic functions (Moore, 2008). In one large study more than 50% of the trauma patients known to be hypothermic died (Rutherford et al, 1998).

In patients with extensive burns there is a loss of body heat from radiation from the burned tissues and from convection when the tissues are exposed to air currents. Because of the extreme heat loss through the burned tissues, these patients are at high risk for developing perioperative unplanned hypothermia (Corallo et al, 2008; AORN, 2009).

Factors that place patients at an intrinsic risk for developing hypothermia include patient physical status and comorbidities. Endocrine diseases, in particular, cause patients to be more prone to hypothermia (Sessler, 2008). Cardiovascular diseases may cause peripheral vasoconstriction and preemptive hypothermia (Frank, 1995). Thin or small-stature patients with a lack of tissue mass are more likely to become hypothermic. Obese patients generally have low core-to-peripheral temperature gradients and little redistribution hypothermia development. Infants and children cool more quickly because of their high ratio of surface area to weight, which leads to more heat loss through the skin (Kurz et al, 1995). Increased age is considered a predictive risk factor for perioperative unplanned hypothermia. Older adult patients have decreased thermoregulatory efficiency, decreased muscle mass, and changes in vascular tone that inhibit vasoconstriction and decrease heat production (Ayres, 2004).

Thermal Discomfort

Whether negative or positive in nature, memories of thermal comfort after surgery have an impact on overall patient satisfaction with surgical care. Thermal comfort may have more effects on surgical patient outcomes than nurses formally recognize. Even patients with normal core temperatures may experience thermal discomfort because of a low skin temperature that significantly contributes to thermal sensations. The need for thermal comfort is responsible for the initiation of behavioral thermoregulation (Wagner et al, 1996; Sessler, 2008). Thermal comfort measures may be used throughout all phases of perioperative care; however, during both the intraoperative and immediate postoperative phases of care the patient will need more than a thermal comfort intervention. When a patient is unable to relate in a subjective manner while in a surgical environment, thermal comfort measures become thermal protective or prevention measures during the intraoperative and immediate postoperative phase of care. Thermal protection is an appropriate term or concept that reflects the practice of perioperative nurses especially during the intraoperative and postoperative phases of care. Perioperative nurses frequently use preoperative warmth measures both for prevention of hypothermia and to provide comfort.

In a study examining the relative contribution of core and skin temperatures to thermal comfort and autonomic responses in humans, it was noted that thermal comfort was responsible for the initiation of behavioral thermoregulation and that humans are usually able to control both ambient temperature and the level of body-surface insulation. Findings show skin temperature contributed greatly toward subjective thermal comfort, whereas core temperature predominately regulates the autonomic and metabolic responses (Frank et al, 1999).

Of all the complications of hypothermia reported, shivering is the most frequent and probably the most familiar. It is how the body tries to correct hypothermia and usually occurs in the recovery phase following surgery. Even though shivering is the body’s attempt to generate heat, it actually produces little heat, especially when a patient has received anesthesia. Shivering is metabolically valuable to the patient in that it increases oxygen requirements with the increased metabolic rate and carbon dioxide (CO₂) production. Shivering can lead to double or triple the oxygen consumption and carbon dioxide production in postanesthesia patients, although the increases are normally much smaller. Increases in metabolic requirements might predispose patients to further complications if limitations with cardiac or respiratory reserves already exist. Shivering also has been found to increase overall myocardial work and decrease arterial oxygen saturation, mixed venous saturation, and glycogen stores. The stress from shivering frequently results in elevated heart rate, labored breathing, and muscle spasms. Severe shivering may also result in tachycardia, hypertension, and myocardial ischemia, which could lead to cardiac arrest (Sessler, 2008). In addition, shivering increases incisional pain and is often remembered by patients as a fearful experience.