Introduction
Rigors are a frequent presentation in prehospital care, particularly in patients with infection or suspected sepsis. Historically, many clinicians were taught to remove blankets from febrile patients in an attempt to “bring the temperature down”. This approach, while well-intentioned, was based on a misunderstanding of fever physiology rather than evidence.
As prehospital clinicians, our practice has evolved. We now recognise that rigors represent a centrally driven thermoregulatory response, and that supportive temperature management can reduce physiological stress rather than worsen fever.
The physiology of rigors
Rigors occur when pyrogenic cytokines released during infection stimulate production of prostaglandin E₂ (PGE₂). PGE₂ acts on the hypothalamus, increasing the body’s temperature set-point.
Once the set-point is raised, the body perceives its current temperature as too low. The response includes:
- Peripheral vasoconstriction
- Intense skeletal muscle contraction (shivering)
- Subjective sensation of feeling cold
Rigors are therefore a heat-generating phase of fever, not a sign that the body is attempting to cool itself.
Challenging traditional practice
Many clinicians will recognise the reflex action of stripping covers from a patient who is febrile. This practice has been passed down through training and culture rather than physiology.
Current understanding makes it clear:
- Fever height is determined by hypothalamic control, not blankets
- External insulation does not raise the temperature set-point
- Removing blankets during rigors increases shivering and metabolic demand
As our understanding has improved, so too has our approach. Rather than reflexively removing blankets, we now appreciate that warming during rigors is supportive, not harmful.
Do blankets worsen fever?
Blankets:
- Do not generate heat
- Do not increase the hypothalamic set-point
- Do not cause fever
Their role is to reduce heat loss. Without insulation, the body compensates by increasing shivering, which significantly raises:
- Oxygen consumption
- Carbon dioxide production
- Lactate levels
- Cardiac workload
In patients with sepsis, respiratory disease or cardiovascular compromise, this additional demand may contribute to deterioration.
Rigors in the context of sepsis
Temperature disturbance is a recognised marker of sepsis in both NICE and JRCALC guidance. Prolonged rigors increase metabolic stress at a time when oxygen delivery may already be impaired.
Providing warmth during rigors:
- Reduces the physiological cost of thermogenesis
- Improves patient comfort and cooperation
- Does not interfere with immune-mediated fever mechanisms
Withholding warmth offers no clinical benefit and may be harmful.
Prehospital temperature management: a phased approach
Phase 1 – Active rigors (cold phase)
Clinical features
- Violent shivering
- Pale, cool skin
- Patient reports feeling cold or freezing
Management
- Provide blankets or other passive insulation
- Avoid active cooling
- Initiate sepsis assessment and management where indicated
- Monitor temperature trends rather than single readings
This approach aligns with JRCALC supportive care principles and NICE guidance on early sepsis management.
Phase 2 – Post-rigor vasodilation (hot phase)
As the hypothalamic set-point falls:
- Shivering ceases
- Vasodilation occurs
- Sweating begins
Management
- Remove excess blankets
- Allow passive heat loss
- Administer antipyretics if appropriate and within scope
This phase is often when patients report feeling “too hot”.
The role of antipyretics
Antipyretics act centrally to reduce the hypothalamic set-point. They:
- Do not immediately cool the patient
- Reduce the likelihood of recurrent rigors
- Complement appropriate thermal support
They should be viewed as an adjunct, not a replacement, for correct temperature management.
Rigors versus seizures
Rigors may be mistaken for seizure activity. Key distinguishing features include preserved or fluctuating consciousness, rhythmic shivering, and resolution with warmth. Recognising this distinction prevents inappropriate treatment and supports timely identification of infection or sepsis.
Conclusion
Rigors indicate that the hypothalamic temperature set-point has been raised. The patient is physiologically cold, even if objectively febrile. Historical practice taught many clinicians to remove blankets; current understanding shows this increases physiological stress without benefit.
Evidence-based practice is clear:
- Warm patients during rigors
- Reduce insulation only once sweating and vasodilation begin
This approach improves comfort, reduces metabolic demand, and aligns with national guidance.
CPD Log Entry (Example)
What?
I reviewed current evidence and guidance on the management of rigors and temperature control in prehospital care. The article explored the physiology of fever, the hypothalamic temperature set-point, and the impact of shivering on metabolic demand, particularly in patients with suspected sepsis.
So what?
This challenged my previous practice of routinely removing blankets from febrile patients. I now understand that during rigors the patient is physiologically cold, and that withholding warmth increases oxygen consumption, cardiac workload and patient distress. This insight reinforces the importance of supportive care as an active clinical intervention.
Now what?
I will provide appropriate insulation to patients experiencing rigors, monitor temperature trends, and adjust warming measures once rigors resolve and sweating begins. I will also share this learning with colleagues to support consistent, evidence-based practice in the prehospital environment.
Estimate time: to read, digest and reflect: 45 minutes
References (Harvard style)
Guyton, A.C. and Hall, J.E. (2021) Textbook of Medical Physiology. 14th edn. Philadelphia: Elsevier.
Kluger, M.J. (1991) ‘Fever: role of pyrogens and cryogens’, Physiological Reviews, 71(1), pp. 93–127.
National Institute for Health and Care Excellence (2016) Sepsis: recognition, diagnosis and early management (NG51). London: NICE.
Resuscitation Council UK (2023) JRCALC Clinical Guidelines. London: Class Publishing.
Sessler, D.I. (2009) ‘Thermoregulation and heat balance’, The Lancet, 373(9672), pp. 1347–1356
