Which fevers are associated with relative bradycardia?

Fevers Associated with Relative Bradycardia

Typhoid fever, Legionnaires' disease, psittacosis (Chlamydia pneumonia), scrub typhus, and dengue fever are the key infectious diseases associated with relative bradycardia. 1

Definition and Clinical Significance

Relative bradycardia refers to a pulse rate that is lower than expected for a given body temperature. It is typically defined as an increase in heart rate of less than 10 beats per minute for each 1°C rise in temperature 2. This phenomenon occurs in approximately 53% of patients with certain infections 2.

Specific Infections Associated with Relative Bradycardia

Confirmed by Evidence:

1. Typhoid Fever

   • Caused by Salmonella typhi
   • Statistically significant association with relative bradycardia (P = 0.003) 1
   • Listed in CDC case definitions as a characteristic clinical feature 3
   • Often presents with sustained fever, headache, malaise, anorexia, and constipation or diarrhea 3

2. Legionnaires' Disease

   • Caused by Legionella pneumophila
   • Statistically significant association with relative bradycardia (P = 0.005) 1
   • Listed as a cause of sinus bradycardia in guidelines 3

3. Psittacosis/Chlamydia Pneumonia

   • Caused by Chlamydia species
   • Strongest statistical association with relative bradycardia (P = 0.0005) 1

4. Scrub Typhus

   • Caused by Orientia tsutsugamushi
   • 53% prevalence of relative bradycardia in infected patients 2

5. Dengue Fever

   • Viral hemorrhagic fever mentioned in guidelines as associated with bradycardia 3

Other Notable Causes:

• Leptospirosis 3
• Malaria 3
• Viral hemorrhagic fevers 3
• Listeria infection 3

Pathophysiological Pattern

Interestingly, relative bradycardia appears to be predominantly associated with infections caused by organisms that are both Gram-negative and intracellular 1. This pattern suggests a common pathophysiological mechanism potentially related to:

• Release of inflammatory cytokines
• Increased vagal tone
• Direct pathogenic effects on the myocardium
• Electrolyte abnormalities 4

Clinical Pearls

1. Diagnostic Value: While relative bradycardia is a sensitive clinical sign, it lacks specificity for any single disease 4. However, its presence should prompt consideration of the infections listed above.

2. Changing Presentation: The classical "stepladder fever" and relative bradycardia of typhoid fever are becoming less commonly observed in clinical practice 5.

3. Diagnostic Challenge: Relative bradycardia can be a key diagnostic clue in fever of unknown origin (FUO) cases, particularly when differentiating between infectious and non-infectious causes 6.

4. Baseline Factors: Patients who exhibit relative bradycardia during infection often have higher resting pulse rates after recovery, suggesting that baseline cardiovascular parameters may influence this phenomenon 2.

Clinical Application

When evaluating a febrile patient with unexpectedly low heart rate:

1. Consider the infections listed above in your differential diagnosis
2. Look for other characteristic features of these infections
3. Order appropriate diagnostic tests (blood cultures, serologies)
4. Remember that relative bradycardia may be the earliest or only clinical sign pointing to these specific infections

This clinical sign can be particularly valuable in resource-limited settings where advanced diagnostic testing may not be immediately available.