What is the exact mechanism of shivering in spinal anesthesia?

Mechanism of Shivering in Spinal Anesthesia

Shivering during spinal anesthesia is primarily caused by hypothalamic thermoregulatory disruption combined with redistribution hypothermia, resulting in an involuntary oscillatory activity of skeletal muscles as the body attempts to generate heat.

Pathophysiological Mechanism

Thermoregulatory Disruption

• Spinal anesthesia directly alters afferent thermal input to the hypothalamus, specifically affecting the preoptic region 1
• The temperature threshold for shivering is reduced by approximately 0.5°C during spinal anesthesia 2
• Normal thermoregulatory range (between sweating and vasoconstriction) widens from approximately 0.2°C to 0.9°C during spinal anesthesia 2

Thermal Redistribution

• Spinal anesthesia causes lower limb vasodilation below the block level 3
• This vasodilation leads to redistribution of body heat from central (core) to peripheral compartments 4
• The initial rapid decrease in core temperature results from this redistribution rather than actual heat loss 4

Temperature Regulation Impairment

• Both vasoconstriction and shivering thresholds are synchronously decreased during spinal anesthesia 2
• The hypothalamus normally triggers shivering when core temperatures fall below a set threshold (usually 35.5-36°C) 1
• During spinal anesthesia, this threshold is lowered, but the response is still triggered when core temperature drops sufficiently 4

Metabolic Consequences of Shivering

• Shivering doubles the metabolic rate in patients with high muscle mass 1
• Oxygen consumption can increase by up to 500% during shivering 5
• This increased metabolic demand can be particularly dangerous for patients with:
  • Coronary artery disease
  • Cardiopulmonary insufficiency
  • Anemia
  • Elderly or debilitated status 5

Clinical Implications

• Shivering can trigger bioenergetic failure with demand cerebral ischemia 1
• It creates a significant mismatch in energy supply and demand to the brain 1
• The incidence of shivering during spinal anesthesia can be as high as 56.7% without prophylactic measures 5

Prevention and Management

• Preventative measures should focus on reducing the temperature threshold at which shivering responses are triggered 1
• Skin counterwarming is effective as it can reduce the shivering response threshold by 4°C for each increase in mean skin temperature 1
• Warming of extremities (particularly ears, palms, and soles) can be effective 1
• Pharmacological options include:
  • Low-dose ketamine (0.25 mg/kg) - reduces shivering incidence to 2.5% 6
  • Ondansetron (4 mg) - reduces shivering incidence to 10% 6
  • Intrathecal meperidine (0.2 mg/kg) - reduces shivering incidence to 16.7% 5

Common Pitfalls and Considerations

• Monitoring core temperature is essential but can be challenging during spinal anesthesia
• Esophageal and nasopharyngeal temperatures are most accurate for core temperature monitoring 1
• Axillary temperatures are consistently 1.5-1.9°C below core temperature and should be avoided 1
• Multimodal approaches to shivering prevention are more effective than single interventions 1
• Intraoperative normothermia significantly decreases surgical site infection rates (GoR 1A) 1

By understanding the exact mechanism of shivering during spinal anesthesia, clinicians can implement effective preventive measures to improve patient comfort and reduce associated metabolic and cardiovascular complications.