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

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