Fagets signs
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Faget's sign yellow fever relative bradycardia clinical significance
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Here is a full breakdown of Faget's Sign:
Faget's Sign (Sphygmothermic Dissociation)
Definition
Faget's sign is the paradoxical combination of fever with relative bradycardia - the heart rate fails to rise proportionally (or even slows down) despite a high body temperature.
Normally, by Liebermeister's rule, for every 1°F rise in temperature, the heart rate increases by ~10 beats/min. When this expected tachycardia is absent, it is called a pulse-temperature deficit.
- The sign is best appreciated when temperature is >38.9°C (102°F), as differences are harder to detect at lower temperatures.
- Named after Jean Charles Faget, a Louisiana physician who first described it in yellow fever patients in 1859.
Causes (per Harrison's 22E, Table 124-1)
Infectious Causes:
| Category | Organisms |
|---|---|
| Gram-negative intracellular bacteria | Salmonella typhi, Francisella tularensis, Brucella spp., Coxiella burnetii (Q fever), Leptospira interrogans, Legionella pneumophila, Mycoplasma pneumoniae |
| Tick-borne organisms | Rickettsia spp., Orientia tsutsugamushi (scrub typhus), Babesia spp. |
| Other bacteria | Corynebacterium diphtheriae, Plasmodium spp. (malaria) |
| Viruses | Yellow fever virus, Dengue virus, viral hemorrhagic fevers (Marburg/Ebola - early), viral myocarditis |
Non-infectious Causes:
- Drug fever
- Beta-blocker use (directly blunts chronotropic response)
- CNS lesions
- Malignant lymphoma
- Factitious fever
Pathophysiology
The exact mechanism is not fully established. The leading hypothesis is that pro-inflammatory cytokines released by intracellular organisms induce increased vagal (parasympathetic) tone, countering the sympathetically-driven tachycardia. The pattern tends to be most prominent with intracellular gram-negative bacteria and viral hemorrhagic fevers.
Clinical Utility
- Faget's sign is not highly sensitive but can be a useful diagnostic clue, particularly in resource-limited settings.
- It helps narrow the differential in febrile illness - for example, distinguishing Legionella pneumonia from Mycoplasma pneumonia in community-acquired pneumonia.
- A 2026 systematic review confirms it is underutilized in practice and its value would be improved by standardizing diagnostic criteria.
- In febrile patients with relative bradycardia, a non-infectious etiology (lymphoma, CNS lesion, drug fever, beta-blockade) must always be considered alongside infectious ones.
Memory Aid
"Yellow fever = Yellow flag on the pulse" - the pulse doesn't race with the fever. Think of it as the temperature and pulse "dissociating" from each other - hence the alternate name sphygmothermic dissociation.
- Harrison's Principles of Internal Medicine 22E (2025), Table 124-1, p. 1003
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