31: Temperature Disturbances

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CHAPTER 31 Temperature Disturbances

James Duke, MD, MBA

1 Describe the processes that contribute to thermoregulation

There are three phases of thermoregulation: afferent sensing, central thermoregulatory integration, and efferent response. Distinct receptors are distributed about the body for sensing warmth and cold. Cold impulses travel centrally via myelinated A-delta fibers, warmth is transmitted by unmyelinated C fibers, and most ascending thermal information travels cephalad via the anterior spinothalamic tract. Thermal information is integrated within the spinal cord and centrally in the hypothalamus. Within a narrow range of temperatures (the interthreshold range) no efferent response is triggered by the hypothalamus. Above and below certain thresholds, efferent mechanisms activate in an attempt to decrease or increase body temperature, respectively. Warm responses include active vasodilation and sweating; cold responses include vasoconstriction, nonshivering thermogenesis, and shivering.

2 Which patients are at risk for hypothermia?

All patients having either general or neuraxial anesthesia or even just sedation are at risk. Patients especially at risk include the elderly, who have reduced autonomic vascular control, and infants, who have a large surface area-to-mass ratio. Patients with burns, spinal cord injuries that involve autonomic dysfunction, and endocrine abnormalities are also at risk.

3 Does hypothermia have an impact on patient outcome?

Mild hypothermia (1° to 3° C) has the following effects:

image Increases incidence of surgical site infections (SSIs). Although this is a multifactorial problem, it has been noted that vasoconstriction secondary to hypothermia decreases blood flow to the wound and bacterial killing by neutrophils.
image Increases the period of hospitalization by increasing the likelihood of SSIs and delayed healing
image Reduces platelet function and impairs activation of the coagulation cascade, increasing blood loss and transfusion requirements (shown by some to also be a cause of increased SSIs)
image Triples the incidence of ventricular tachycardia and morbid cardiac events
image Prolongs the activity of muscle relaxants and anesthetic agents, increasing the likelihood of postoperative weakness and prolonging postoperative recovery.
image The pathophysiologic effects of hypothermia are reviewed in Table 31-1.

TABLE 31-1 The Effects of Hypothermia on Organ Systems

System Effects
Vascular Increases systemic vascular resistance and peripheral hypoperfusion; plasma volume decreases because of cold diuresis
Cardiac Decreases heart rate, contractility, and cardiac output and produces arrhythmias
Pulmonary Increases pulmonary vascular resistance; decreases hypoxic pulmonary vasoconstriction; increases ventilation-perfusion mismatching; depresses ventilatory drive; decreases bronchial muscle tone increasing anatomic dead space; oxyhemoglobin dissociation curve shifts to the left
Renal Decreases renal blood flow and glomerular filtration rate; impaired sodium resorption and diuresis leading to hypovolemia
Hepatic Decreases hepatic blood flow, metabolic and excretory functions
Central nervous system Decreases cerebral blood flow; increases cerebral vascular resistance; oxygen consumption decreases by 7%/°C; evoked potential latencies are increased; MAC is decreased
Hematologic Decreased platelet aggregation and clotting factor activity; increased blood viscosity, impaired immune response
Metabolic Basal metabolic rate decreases; hyperglycemia and insulinopenia; decreased oxygen consumption and CO2 production
Healing Increased wound infections

MAC, Minimum alveolar concentration.

4 Characterize the different stages of hypothermia

Hypothermia is a clinical state of subnormal body temperature in which the body is unable to generate enough heat to maintain a normal temperature.

image Mild hypothermia (32° to 35° C, 90° to 95° F) is associated with mild central nervous system depression (dysarthria, amnesia, ataxia, and apathy), decreased basal metabolic rate, tachycardia, peripheral vasoconstriction, and shivering.
image Moderate hypothermia (27° to 32° C, 80° to 90° F) results in further depression in consciousness, decreased motor activity, mild depression of vital signs, arrhythmias, and cold diuresis.
image In severe hypothermia (<27° C, 80° F) the patient may be comatose and areflexic and have significantly depressed vital signs. Left untreated, such profound hypothermia leads to death.

5 Which perioperative events predispose a patient to hypothermia?

Heat loss is common in all patients during general anesthesia because of peripheral vasodilation, altered thermoregulation, and the inability to generate heat by shivering. General anesthesia widens the interthreshold range; and, since the thermoregulatory response is markedly less robust, body temperature changes passively with existing temperature gradients. Hypothermia is as common during neuraxial anesthesia as general anesthesia and is a result of sympathetic blockade, muscle relaxation, and a lack of afferent sensory input into central thermoregulatory centers.

6 Which physical processes contribute to a patient’s heat loss in the operating room?

image Radiation: The dissipation of heat to cooler surroundings, which accounts for about 60% of a patient’s heat loss, depending on cutaneous blood flow and exposed body surface area.
image Evaporation: The energy required to vaporize liquid from any surface, be it skin, serosa, or mucous membranes. It accounts for 20% of heat loss and is a function of the exposed surface area and the relative humidity.
image Convection: Is responsible for about 15% of heat loss and depends on the air flow over exposed surfaces.
image Conduction: Describes the transfer of heat between adjacent surfaces, accounts for about 5% of the total heat loss, and is a function of the temperature gradient and thermal conductivity.

7 As a practical matter, what is done to a patient in the operating room that increases heat loss?

Cool rooms, cold intravenous and sterile preparation solutions, and exposure of the patient to the environment contribute significantly to hypothermia. Cutaneous heat loss is proportional to the exposed surface area and accounts for 90% of heat loss. One unit of refrigerated blood or 1 L of room-temperature crystalloid decreases the body temperature about 0.25° C. The effects of general anesthesia, regional anesthesia, and neuromuscular blockade have been discussed. Less than 10% of heat loss is through the respiratory tract.

8 Should all patients receive temperature monitoring within the operating room? What are acceptable sites for temperature monitoring?

The American Society of Anesthesiologists’ Standards for Basic Anesthesia Monitoring state: “Every patient receiving anesthesia shall have temperature monitored when clinically significant changes in body temperature are intended, anticipated or suspected.” Sites for monitoring temperature include skin, rectum, esophagus, external auditory canal, nasopharynx, bladder, or pulmonary artery catheter.

9 Review shivering and nonshivering thermogenesis

Shivering is the spontaneous, asynchronous, random contraction of skeletal muscles in an effort to increase the basal metabolic rate. Shivering is modulated through the hypothalamus and can increase the body’s production of heat by up to 300% in young, muscular individuals. It increases oxygen consumption and carbon dioxide production. This effect may be undesirable in the patient with coronary artery disease or pulmonary insufficiency.

Infants younger than 3 months of age cannot shiver and mount a caloric response by nonshivering thermogenesis, which increases metabolic heat production without producing mechanical work. Skeletal muscle and brown fat tissue are the major energy sources for this process.

10 Describe the electrocardiographic manifestations of hypothermia

Mild hypothermia may be associated only with sinus bradycardia. Moderate hypothermia may result in prolonged PR intervals, widened QRS complexes, and a prolonged QT interval. Below 32° C an elevation of the junction of the QRS and ST segments known as the hypothermic hump or Osborne or J wave may be seen. Its size increases with decreasing body temperature; it usually is seen in leads II and V6 and may spread to leads V3 and V4. The J wave is not specific for hypothermia; it also may be seen in hypothalamic lesions and cardiac ischemia. Nodal rhythms are common below 30° C. Below 28° C premature ventricular contractions, atrioventricular blocks, and spontaneous atrial or ventricular fibrillation also occur. Ventricular fibrillation or asystole below 28° C is relatively unresponsive to atropine, countershock, or pacing. Resuscitative efforts should persist until the patient is rewarmed.

KEY POINTS: Temperature Disturbances image

1. Hypothermia is an extremely common event in the operating room because the environment and the effects of anesthetics increase heat loss. Anesthetics also decrease the ability to generate a response to hypothermia (shivering and vasoconstriction).
2. Even mild hypothermia has a negative influence on patient outcome, increasing wound infection rates, delaying healing, increasing blood loss, and increasing cardiac morbidity.
3. The best method to treat hypothermia is use of forced-air warming blankets. Warm all fluids and blood products. Cover all body surfaces possible, including the head, to further reduce heat loss.

11 How does hypothermia affect the actions and metabolism of drugs used in the operative environment?

Drug effects are prolonged by decreased hepatic blood flow and metabolism and decreased renal blood flow and clearance. Protein binding increases as body temperature decreases. The minimum alveolar concentration of inhalational agents is decreased about 5% to 7% per degree centigrade decrease in core temperature. The effects of sedative agents are also decreased. Hypothermia may prolong the duration of neuromuscular blocking agents because of decreased metabolism. Hypothermia delays discharge from the postanesthetic care unit and may prolong the need for mechanical ventilation.

12 Discuss methods of rewarming

image Passive rewarming uses the body’s ability to generate heat if continued heat loss is minimized by covering exposed areas. Because passive rewarming relies on shivering thermogenesis, hypothalamic mechanisms must be intact, and sufficient glycogen stores available.
image Active rewarming is readily performed in the operating room and includes increasing the ambient temperature, administering warmed intravenous fluids, and radiant heat warming. Forced-air warming devices are especially beneficial and superior to circulating water blankets. Airway rewarming is less effective because the heat content of gases is poor. Blood can be drawn from the femoral artery, run through a warming circuit, and returned to the femoral vein; in the extreme, extracorporeal rewarming with cardiopulmonary bypass may be used. A core temperature afterdrop (a secondary decline in core temperature with rewarming) may result from the return of cold blood from the periphery.

13 Define hyperthermia

Hyperthermia is a rise in body temperature of 2° C/hr. Because it is uncommon in the operating room, its cause must be investigated. The usual cause is sepsis or overheating. Hypothalamic lesions and hyperthyroidism are less common. Malignant hyperthermia is a great concern because it is caused by volatile anesthetics or succinylcholine and can be fatal if untreated.

14 Describe the manifestations of hyperthermia

Hyperthermia is a hypermetabolic state associated with increased oxygen consumption, increased minute ventilation, sweating, tachycardia, and vasodilation. An awake patient may experience general malaise, nausea, and light-headedness. With prolonged hyperthermia the patient may develop heat exhaustion or heat stroke. In the anesthetized patient signs and symptoms include tachycardia, hypertension, increased end-tidal carbon dioxide, increased drug metabolism, rhabdomyolysis, oliguria, and hypovolemia. Heart rate increases by 10 beats/min per degree centigrade increase in temperature.

15 What conditions are associated with hyperthermia?

image Malignant hyperthermia, as discussed in Chapter 46
image Hypermetabolic states, including sepsis, thyrotoxicosis, and pheochromocytoma
image Hypothalamic lesions secondary to trauma, anoxia, or tumor
image Neuroleptic malignant syndrome
image Transfusion reaction
image Medications

16 What drugs increase the risk of hyperthermia?

Sympathomimetic drugs, monoamine oxidase inhibitors, cocaine, amphetamines, and tricyclic antidepressants increase the basal metabolic rate and heat production. Anticholinergics and antihistamines may elevate temperature by suppressing sweating.

17 What are the pharmacologic effects of hyperthermia?

Increases in basal metabolic rate and hepatic metabolism decrease the half-life of anesthetic drugs. Anesthetic requirements may be increased.

18 What is the treatment for the hyperthermic patient in the operating room?

Expose skin surfaces and use cooling blankets and cool intravenous fluids. Correctable causes of hyperpyrexia should be ruled out. Consider administering antipyretics, although it is always best to treat the cause, not the symptom.

Website image

http://www.asahq.org/publicationsAndServices/standards/

SUGGESTED READINGS

1. Mauermann W.J., Nenergut E.C. The anesthesiologist’s role in the prevention of surgical site infections. Anesthesiology. 2006;105:413-421.

2. Rajagopalan S., Mascha E., Na J., et al. The effects of mild perioperative hypothermia on blood loss and the transfusion requirement. Anesthesiology. 2008;108:71-77.

3. Sessler D.I. Temperature monitoring and perioperatiave thermoregulation. Anesthesiology. 2008;109:318-338.

Related posts:

58: Congenital Heart Disease 61: High-Risk Obstetrics 46: Malignant Hyperthermia and Other Motor Diseases 5: Electrolytes 76: Electroconvulsive Therapy 41: Acute Respiratory Distress Syndrome (ARDS)