Understanding Rigors During Fever: Physiological Mechanisms and Clinical Implications
Rigors occur during fever because the body's thermoregulatory set-point is elevated, causing the patient to feel cold despite having an elevated core temperature. This physiological response involves muscle contractions (shivering) that generate heat to reach the new higher temperature set-point.
Pathophysiology of Fever and Rigors
Thermoregulatory Mechanism
- The preoptic area (POA) of the hypothalamus serves as the body's thermostat, integrating various thermal signals 1
- During infection or inflammation, pyrogens (like bacteria or viruses) trigger the production of prostaglandin E2 (PGE2), which acts on the POA
- This causes the thermoregulatory set-point to be raised above the normal 37°C (98.6°F)
The Rigor Response
- When the set-point is elevated, the body perceives its current temperature as "too cold" compared to the new higher set-point
- To reach this elevated set-point, the body initiates heat-generating mechanisms:
- Peripheral vasoconstriction (to reduce heat loss)
- Shivering (involuntary muscle contractions to generate heat)
- Behavioral responses (seeking warmth, adding blankets)
The Paradox Explained
- The patient experiences subjective feelings of cold (chills) despite having an objectively elevated body temperature
- Rigors represent the visible manifestation of the body's attempt to rapidly increase core temperature through muscle contractions
- This creates the paradoxical situation where a patient with fever feels cold until their actual body temperature reaches the new elevated set-point
Clinical Implications and Management
Recognition of Rigors
- Rigors often indicate the onset of fever and may signal serious infection
- Characterized by:
- Uncontrollable shivering
- Chattering teeth
- Subjective feeling of cold
- Tachycardia
- Tachypnea
Management Strategies
- The American College of Cardiology recommends providing blankets for comfort during rigors while implementing other fever management strategies 2
- For patients experiencing rigors:
- Protect from further heat loss by moving to a warm environment
- Remove wet clothing if present
- Cover with dry blankets
- Insulate from cold surfaces
- Cover the head and neck to minimize heat loss 2
Treatment Considerations
- Treating the underlying cause of fever is the primary goal
- Antipyretic medications (acetaminophen, NSAIDs) may help lower the thermoregulatory set-point
- The Society for Immunotherapy of Cancer recommends treating rigors promptly to prevent respiratory deterioration, increased oxygen consumption, and patient discomfort 2
Special Clinical Scenarios
Neurogenic Fever
- Fever driven by neurological dysregulation (core temperature >37.5°C) in the absence of infection should be promptly detected and treated 3
- Controlled normothermia targeting 36.0°C to 37.5°C is recommended, especially in patients with traumatic brain injury 3
Infectious vs. Non-infectious Causes
- Rigors are commonly associated with infectious causes of fever, particularly bacteremia
- However, they can also occur in non-infectious conditions like drug reactions, malignant hyperthermia, and neuroleptic malignant syndrome 3
- Procalcitonin levels can help discriminate between infectious and non-infectious causes of fever 3
Monitoring and Complications
- Continuous monitoring of core temperature is important during management of fever and rigors
- Complications of untreated rigors may include:
- Increased oxygen consumption
- Respiratory distress
- Patient discomfort
- Exacerbation of underlying conditions
Conclusion
Rigors during fever represent the body's physiological response to an elevated thermoregulatory set-point. The patient feels cold and shivers despite having an elevated temperature because their actual body temperature has not yet reached the new higher set-point established by pyrogens acting on the hypothalamus. Understanding this mechanism helps explain the paradoxical experience of feeling cold during a fever and guides appropriate clinical management.