Temperature disturbances

1

What is normal core body temperature? What is the definition of hypothermia and hyperthermia?

Normal core body temperature is approximately 37°C. Core body temperature is not constant and fluctuates throughout the day (± 0.5°C), with circadian rhythm, and in women with menstrual cycle. Hypothermia is defined as a core temperature less than 36°C and hyperthermia as a core temperature greater than 38°C.

2

Should all patients under anesthesia receive continuous temperature monitoring?

The American Society of Anesthesiologists' Standards for Basic Anesthetic Monitoring state: “Every patient receiving anesthesia shall have temperature monitored when clinically significant changes in body temperature are intended, anticipated, or suspected.”

In practice, operations lasting less than 30 minutes often do not require continuous temperature monitoring, whereas operations lasting greater than 30 to 60 minutes should have continuous temperature monitoring.

3

What sites can be used to measure core body temperature?

• Distal esophagus

• Nasopharynx

• Tympanic membrane

• Pulmonary artery

4

Fundamentally, what causes hypothermia?

Hypothermia occurs whenever heat loss exceeds heat production. Inhalational and intravenous anesthetic agents cause hypothermia because of increased heat loss (e.g., peripheral vasodilation) and decreased heat production (e.g., impaired shivering).

5

Characterize the different stages of hypothermia in a patient not under anesthesia.

• Mild hypothermia (32°C–36°C) is associated with mild central nervous system depression, decreased basal metabolic rate, tachycardia, peripheral vasoconstriction, and shivering.

• Moderate hypothermia (28°C–32°C) is associated with impaired consciousness, decreased motor activity, dysrhythmias, and cold diuresis. Patients may stop shivering with moderate and severe hypothermia.

• Severe hypothermia (< 28°C) is associated with coma, areflexia, and significantly depressed vital signs. Left untreated, profound hypothermia leads to cardiac arrest.

  See Table 33.1 for an overview of systemic effects resulting from hypothermia.

  Table 33.1

  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; dysrhythmias
  Pulmonary	Increases pulmonary vascular resistance; decreases hypoxic pulmonary vasoconstriction; increases ventilation-perfusion mismatching; depresses ventilatory drive; 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	Altered mental status, lethargy, or coma; decreases cerebral blood flow; increases cerebral vascular resistance; decreases cerebral oxygen consumption by 7%/°C; increases evoked potential latencies; decreases minimum alveolar concentration
  Hematological	Decreased platelet aggregation and clotting factor activity; increased blood viscosity, impaired immune response
  Metabolic	Basal metabolic rate decreases; hyperglycemia; decreased oxygen consumption and CO 2 production
  Wound healing	Increased wound infections

6

What problems can be seen with mild intraoperative hypothermia?

• Surgical site infections . Hypothermia causes vasoconstriction, which decreases blood flow and hence oxygen and antibiotic delivery to the wound. This is particularly true following emergence of anesthesia in the postoperative care unit, as patients under general anesthesia are vasodilated. Hypothermia also has a direct immunosuppressant effect on neutrophils, whereas the corollary is true that fever or hyperthermia activates neutrophils.

• Coagulopathy . Hypothermia reduces platelet function and impairs clotting factor enzyme activity. Hypothermia is associated with increased blood loss and transfusion requirements.

• Adverse cardiac events . Hypothermia itself can cause dysrhythmias and increases the risk of demand ischemia because of hemorrhage and hypotension. Furthermore, shivering and increased systemic vascular resistance following the emergence of anesthesia can precipitate demand ischemia. Shivering increases oxygen consumption, causing a compensatory increased cardiac output and peripheral vasoconstriction increases afterload.

• Decreased drug metabolism. This is most concerning with prolonging neuromuscular blocking agents, causing residual postoperative weakness. Furthermore, hypothermia can impair consciousness and cause delayed emergence.

7

Which patients are at risk for hypothermia?

Although all patients under general anesthesia are at risk for developing hypothermia, some specific patient populations carry a greater risk. One example are patients of extreme age. Elderly patients have reduced autonomic vascular control and newborns have a large surface area-to-body mass ratio. Two other groups with increased risk are patients with burns and spinal cord injuries secondary to autonomic dysfunction.