What are the causes of postoperative shivering?

Causes of Postoperative Shivering

Postoperative shivering is primarily caused by perioperative hypothermia triggering thermoregulatory responses when core temperature falls below 35.5–36°C, but importantly, non-thermoregulatory shivering also occurs in normothermic patients due to pain, acute opioid withdrawal, cytokine release from surgical trauma, and low peripheral temperatures despite maintained core temperature. 1, 2, 3, 4

Primary Thermoregulatory Causes

Hypothermia-Induced Shivering:

• The preoptic region of the hypothalamus triggers shivering when core temperatures fall below the set threshold of 35.5–36°C, representing the body's normal thermoregulatory response 1, 2
• Perioperative hypothermia develops because anesthetic agents inhibit thermoregulation, making this the most common cause of postoperative shivering 4, 5
• Inadequate intraoperative warming and prolonged surgical exposure increase the risk of hypothermia-related shivering 2

Peripheral-Core Temperature Gradient:

• Low peripheral temperature is a significant risk factor for shivering even when core temperature is maintained at ≥36.5°C postoperatively 3
• The temperature gradient between core and peripheral tissues can trigger shivering responses through cutaneous afferent inputs to the hypothalamus 1, 3

Non-Thermoregulatory Causes

Pain-Related Mechanisms:

• Postoperative pain is a recognized cause of non-thermoregulatory shivering that occurs in normothermic patients with cutaneous vasodilation 4, 6
• Approximately 15% of normothermic postoperative patients demonstrate nonthermoregulatory tremor, with 55% of this activity occurring in vasodilated patients 7

Opioid-Related Factors:

• Acute opioid withdrawal, particularly with short-acting narcotics, can trigger shivering 4
• Lower effect-site fentanyl concentrations at extubation are associated with increased shivering risk (OR = 0.66 per unit increase) 3

Inflammatory Response:

• Cytokine release from surgical trauma can cause thermoregulatory-related shivering even in normothermic patients 6
• Infection (respiratory and wound infections) can trigger shivering responses 2
• Sepsis with temperature ≥38.0°C may present with shivering and requires prompt evaluation 2

Patient-Specific Risk Factors

Age and Physiological Factors:

• Younger age is associated with increased shivering risk (OR = 0.98 per year increase, meaning younger patients shiver more) 3
• Patients with higher muscle mass experience more pronounced metabolic effects when shivering occurs 1, 2

Neurological Considerations:

• Altered hypothalamic function following hypoxic-ischemic injury can lower the temperature threshold for shivering 1, 2
• The degree of brain injury influences shivering intensity, with potentially less shivering in more severe brain injury 1, 2

Surgical Factors:

• Longer operation time increases shivering risk (OR = 1.10 per 30-minute increase) 3
• The type and extent of surgical procedure influence cytokine release and subsequent shivering 6

Metabolic Consequences

Oxygen Consumption and Metabolic Demand:

• Shivering doubles the metabolic rate in patients with high muscle mass and nearly triples oxygen consumption 1, 2
• This elevated metabolic expenditure can trigger bioenergetic failure with demand cerebral ischemia and significant mismatch in energy supply and demand 1, 2
• Shivering increases the risk of hypoxemia, induces lactic acidosis, and causes catecholamine release 4

Protective Factors

Pharmacological Modulation:

• Acetaminophen use significantly reduces shivering occurrence (OR = 0.32) in patients with maintained core temperature 3
• Adequate fentanyl concentrations at extubation reduce shivering risk 3

Common Pitfalls

• Assuming normothermia excludes shivering risk: Approximately 27% of normothermic patients still experience shivering, with many demonstrating vasodilation indicating non-thermoregulatory causes 7
• Monitoring only core temperature: Peripheral temperature monitoring is essential as low peripheral temperatures predict shivering even with normal core temperatures 3
• Overlooking pain as a cause: Inadequate postoperative analgesia contributes to non-thermoregulatory shivering and should be addressed 4, 6