SN2 SHORT NOTE 5M ★ High-yield

Define fever. Describe its mechanism, phases and effects.

Physiology · 1st Professional — Pre-Clinical

Definitions

Fever (pyrexia) = a regulated rise in body temperature due to an upward resetting of the hypothalamic set point.Hyperthermia = a rise in temperature with a normal set point, because heat loss is overwhelmed.

Mechanism

Exogenous pyrogen — bacterial endotoxin (lipopolysaccharide) → Macrophages and monocytes activated → Endogenous pyrogens — IL-1, IL-6, TNF-α, interferon → Act on the OVLT near the anterior hypothalamus → ↑ Prostaglandin E2 → set point raised to, say, 39 °C

Phases of Fever

Phase	What the body is doing	Patient feels
Chill (rising)	Set point is above actual temperature → vasoconstriction, shivering, piloerection, curling up	Cold, shivering despite a rising temperature
Fastigium (plateau)	Actual temperature has reached the set point	Hot, flushed, comfortable
Defervescence (falling)	Set point returns to normal → vasodilatation, sweating	Hot, sweating (crisis or lysis)

The paradox of feeling cold while the temperature rises is explained entirely by the shift in set point

Effects of Fever

Beneficial	Harmful
Impairs bacterial and viral replication	↑ BMR by 13% per °C → catabolism, weight loss
↑ Neutrophil and T-cell function	Tachycardia — 10 beats/min per °C
↑ Interferon activity	Febrile convulsions in children 6 months to 5 years
↓ Serum iron, depriving bacteria	Dehydration; delirium; risk in cardiac disease

Relative bradycardia — a pulse slower than expected for the fever; seen in typhoid

Applied Aspects

Antipyretics (paracetamol, aspirin, NSAIDs) inhibit cyclo-oxygenase → ↓ PGE2 → reset the thermostat. They have no effect on hyperthermia, where the set point is already normal
Steroids also suppress fever, by inhibiting pyrogen production — which can dangerously mask infection
Heat stroke requires physical cooling, not antipyretics
Aspirin is avoided in children with viral fever — risk of Reye syndrome
Types of fever — continuous (typhoid), intermittent (malaria), remittent, and relapsing
Pyrexia of unknown origin — fever above 38.3 °C for over 3 weeks with no diagnosis after initial investigation
Hyperpyrexia — above 41.5 °C; brain damage occurs above about 42 °C

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