Post-Anaesthetic Shivering

Post-anaesthetic shivering is involuntary rhythmic muscle tremor in the early recovery period, from thermoregulatory or non-thermoregulatory causes. It raises oxygen consumption and carbon dioxide production by up to 100% to 300%.

Epidemiology. It occurs in about 20% to 60% after general anaesthesia and up to 55% after neuraxial block. It peaks in the first 30 to 60 minutes.

Causes. Core hypothermia under the thermoregulatory threshold (about 36 C), inhibition of thermoregulation by anaesthetic agents, pain, sympathetic overactivity, volatile agents, and pyrogens (infection, transfusion, drug reaction).

Risk factors. Young age, male sex, long surgery, cold theatre or fluids, major open surgery, neuraxial block and prior shivering.

Pathophysiology.

Hypothalamic thermoregulatory centres trigger vasoconstriction and shivering when core temperature falls under the threshold. Anaesthesia lowers the shivering threshold by about 2 to 3 C.

Spontaneous clonus like tremor can also follow volatile agent washout without hypothermia.

Tremor of the face, jaw, trunk or limbs, piloerection, cold peripheries, tachycardia, hypertension, increased oxygen use and carbon dioxide output, wound pain, interference with monitoring, raised intraocular and intracranial pressure, and lactic acidosis.

Red flags.

• Fever over 38 C with rigors (infection, transfusion or drug reaction).
• Rising end tidal carbon dioxide with rigidity (malignant hyperthermia).
• Cardiac or respiratory compromise, including myocardial ischaemia.
• Hypoxaemia.

Clinical. Measure core temperature, saturation, end tidal carbon dioxide and blood pressure, and grade intensity (0 none to 4 whole body). Exclude hypothermia complications, sepsis, transfusion reaction and malignant hyperthermia.

Differential diagnosis.

• Hypothermia.
• Rigors from infection or bacteraemia.
• Transfusion reaction.
• Malignant hyperthermia.
• Seizure.
• Local anaesthetic toxicity.
• Opioid induced rigidity.

Treat with active warming: forced air warmer set at 38 to 43 C, warmed blankets and warmed IV fluids (37 C). Give oxygen, and treat pain. Use drugs for moderate to severe or distressing shivering.

• Pethidine: 25 mg IV as a single dose, repeated once after 10 to 15 minutes if needed; maximum 50 mg in this indication; monitor sedation and respiratory rate; avoid with monoamine oxidase inhibitors and serotonergic drugs, and in renal impairment (norpethidine accumulation); reduce in older adults.
• Dexmedetomidine: 0.3 to 0.5 microgram/kg IV over 10 minutes; single dose; maximum 0.5 microgram/kg in this use; monitor heart rate and blood pressure; avoid in heart block; reduce in hepatic impairment and older adults.
• Tramadol: 0.5 to 1 mg/kg IV once; maximum 100 mg for this indication (label limit 400 mg daily); monitor nausea, sedation and seizures; avoid with monoamine oxidase inhibitors, epilepsy and serotonergic drugs; reduce interval in renal impairment.
• Ondansetron (prophylaxis, especially after spinal block): 4 mg IV before block; single dose; maximum 8 mg in this use; monitor QTc in at risk patients; avoid in long QT; reduce in severe hepatic impairment.

Prevention. Prewarm for 15 to 30 minutes, maintain theatre at 21 C or more, warm IV fluids over 500 mL, and monitor temperature.

Monitoring. Core temperature every 15 minutes, saturation, shivering score and oxygen need until resolved.

Referral and escalation. Escalate for suspected malignant hyperthermia or transfusion reaction. Cardiology review if ischaemia develops.

Special groups. In cardiac disease and pregnancy aim to prevent shivering. Older adults need lower drug doses and are more prone to hypothermia.