Fever Pathophysiology
On this page
Direct answer
Fever begins when exogenous pyrogens such as bacterial lipopolysaccharide provoke monocytes and macrophages to release interleukin-1, IL-6 and TNF-alpha; these induce cyclo-oxygenase-2 and prostaglandin E2 at the organum vasculosum of the lamina terminalis, raising the hypothalamic set point. The body then defends the new setting: cutaneous vasoconstriction and piloerection conserve heat while shivering generates it — the chills the patient reports — and defervescence reverses the effector arms with flushing and sweating. Each 1 degree Celsius rise increases basal metabolic rate by about 10-13% and heart rate by roughly 10 beats per minute. Fever differs fundamentally from hyperthermia: the thermostat is reset in fever but overwhelmed in hyperthermia, which is why antipyretics work in fever and are useless in heat stroke.
What you must remember
- Pyrogen cascade: exogenous pyrogen (LPS) to endogenous pyrogens (IL-1, IL-6, TNF-alpha) to COX-2/PGE2 at the preoptic-anterior hypothalamus — set point shifts upward.
- Effector arms rising: vasoconstriction, piloerection, behavioural heat-seeking, shivering (maximum available thermogenesis); falling: vasodilatation and sweating at defervescence.
- Metabolic cost: BMR up 10-13% per degree, heart rate up about 10 beats per minute per degree, insensible water loss and protein catabolism increase.
- Pyrogenic ceiling: fever rarely exceeds 41-42 degrees Celsius because endogenous antipyretics (neuropeptides, cortisol) cap the response; hyperthermia has no such ceiling.
- Fever versus hyperthermia: reset set point versus failed heat dissipation; paracetamol and NSAIDs lower only the former — heat stroke needs physical cooling, malignant hyperthermia needs dantrolene.
- Pattern vocabulary for Indian exams: continuous, remittent, intermittent; tertian fever every 48 hours (Plasmodium vivax), quartan every 72 hours (P. malariae) — a favoured table of PUO discussion.
- Benefit argument: moderate fever lowers plasma iron and zinc, retards bacterial replication and enhances lymphocyte function — the reason routine aggressive antipyresis in comfortable children is discouraged.
Telling fever from hyperthermia at 2 a.m.
A young athlete collapsed after a race: rectal temperature 41.5 degrees, skin dry and hot, confusion. No history of rigors, and the skin vasodilators are shut down rather than constricted — this is hyperthermia, a heat load the body could not dump, not a moved thermostat. Management follows the physiology: paracetamol is worthless because PGE2 is not the problem; evaporative and ice-water cooling, fluids and electrolyte correction are. Contrast a ward patient at 39 degrees with lobar pneumonia: flushed, sweating intermittently, shivering when the set point climbs — here COX-2 inhibition lowers the set point and the patient defervesces. The third member of the trio is malignant hyperthermia after anaesthesia: rigidity, rapid temperature rise, treated with dantrolene, which blocks ryanodine-receptor calcium release from the sarcoplasmic reticulum — muscle physiology erupting into thermoregulation.
Where students slip
The timing of chills is misread: rigor occurs while the set point is rising, when the body is actively generating heat, not at the temperature peak — and a crisis with sweating marks the set point falling, usually after antipyretics. The second slip is the overheated febrile child bundled in blankets by anxious relatives, which fights the physiology by blocking heat loss; Indian teaching emphasises light clothing, fluids and tepid sponging alongside paracetamol. Third, in dengue-endemic areas the drug question has an answer rooted in this pathway: paracetamol is the antipyretic of choice and aspirin is avoided in children with viral fever because of Reye syndrome and bleeding risk — an exam answer with national-programme relevance.
Frequently asked questions
Which prostaglandin mediates fever and where does it act?
Prostaglandin E2, generated by cyclo-oxygenase-2 at the organum vasculosum of the lamina terminalis, raises the hypothalamic set point to produce fever.
How does fever differ from hyperthermia?
Fever is a regulated rise with an elevated hypothalamic set point and intact effectors; hyperthermia is unregulated heat overload with a normal set point, unresponsive to antipyretics.
Why do chills occur during the rising phase of fever?
When the set point rises, vasoconstriction and shivering generate and conserve heat until body temperature reaches the new setting, which the patient experiences as chills.
What happens to metabolic rate and heart rate per degree of fever?
Basal metabolic rate rises about 10-13% and heart rate roughly 10 beats per minute for each 1 degree Celsius rise in core temperature.
Why is dantrolene used in malignant hyperthermia?
Dantrolene blocks ryanodine-receptor calcium release from the sarcoplasmic reticulum, stopping the uncontrolled muscle contraction and heat production that drive the hyperthermia.