Relative Bradycardia

Synonyms: Faget sign, Sphygmo-thermic dissociation, Sphygmo-thermal dissociation

Definition of Relative Bradycardia

Physiologically, for each 1 °F rise in body temperature, there is a commensurate increase in the heart rate of 10 beats/min . When temperature elevations are not accompanied by a physiologic increase in the pulse, the patient is said to have a pulse-temperature deficit.

However, the term ‘relative bradycardia’ should only be applied to patients with temperatures in excess of 102 ° F since the difference between pulse and temperature readings of ≤ 102 ° F is insufficient to discern pulse temperature abnormalities.

Diagnosis of Relative Bradycardia

Criteria for using relative bradycardia in clinical diagnosis:

1. Age of patient >= 13 years
2. Temperature >= 102°F and <= 106°F
3. The pulse is taken simultaneously with the temperature.
4. The patient has normal sinus rhythm with no arrhythmias, second or third degree heart block, or pacemaker.
5. The patient must not be taking beta-blockers.

Calculation of Expected pulse rate:

a. Method 1:

• Subtract 1 from the last digit of the farenheit temperature, multiply by 10, and then add the number to 100.
• Example: 103°F = (3-1) X 10 + 100 = 120/min
• Any pulse <120/min with a temperature ≥103°F  is considered relative bradycardia.

b. Method 2:

• Multiply farenheit temperature by 10 and subtract 910 from it.
• Example: 103°F  = (103 X 10) – 910 = 120/min

c. Method 3:

• Just use the unitary method.
• Add 10 for each 1°F  above 102°F to 110
• Example: 103°F  = 110 + 10 = 120

Appropriate temperature-pulse relationship:

Temperature	Pulse in Beats per Minute
102°F (38.9 °C)	110
103°F (39.5 °C)	120
104°F (40.0 °C)	130
105°F (40.6 °C)	140
106°F (41.1 °C)	150

Causes of relative bradycardia

a. Infectious:

1. Flavivirus:
   • Dengue fever
   • Yellow fever
2. Bacteria:
   • Salmonella typhi
   • Salmonella paratyphi
   • Leptospira
   • Brucella
   • Chlamydia psitacci
   • Chlamydia pneumoniae
   • Ricketssia prowazeki (epidemic typhus)
   • Coxiella burnetti (Q fever)
   • Ehlrichia chafeensis
   • Legionella
3. Parasites: Malaria

b. Non-infectious:

1. Rise in ICP (Cushing’s reflex):
   • Brain abscess
   • Meningitis
   • Brain tumors
   • Pontine hemorrhage
2. Other: Lymphoma, Drug fevers

Aids to Diagnosis

1. Pneumonia + Relative bradycardia = Think of Atypical pneumonia (Legionella, Q fever, Chlamydia pneumoniae)
2. Hospitalization + Multiple drugs for treatment + Other causes of fever excluded + Relative bradycardia = Think of Drug fever
3. Rashes + Relative bradycardia = Think of Typhus or RMSF
4. Hemorrhagic rash + Systemic toxemia + Relative bradycardia = Think of viral hemorrhagic fever
5. Headache + Constipation/Diarrhea + Cough + Relative bradycardia = Think of 1st week of enteric fever

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References:

1. Hospital Medicine edited by Robert M. Wachter, Lee Goldman (MD.), Harry Hollander
2. Infectious Disease Secrets By Robert H. Gates
3. Infectious Diseases in Critical Care Medicine By Burke A. Cunha
4. Cunha, B. A. (2000), The diagnostic significance of relative bradycardia in infectious disease. Clinical Microbiology and Infection, 6: 633–634. doi:10.1046/j.1469-0691.2000.0194f.x