Sphygmo-thermal dissociation or relative bradycardia

Normally, during fever the heart rate is accelerated by 10 beats/minute for every 1 ℃ rise in patient's temperature. But in relative bradycardia, there's fever with normal heart rate, or in other words the heart rate is not proportionate to the high temperature.

It's not below 60 beats/minute, but it's relatively slow for the fever degree.

The expected pulse rate can be calculated using the following formula:
Heart rate = (10 * (temperature in °F)) – 910

Causes of sphygmo-thermal dissociation (also known as relative bradycardia or pulse-temperature deficit):

1)Fevers:

• Influenza.
• Typhoid fever.
• Dengue fever.
• Legionnare's disease.
  
• Leptospirosis.
  
• Mumps.
• Atypical pneumonia.
• Q fever.
• Yellow fever.
• Malaria.
  
• Viral hepatitis.
• Factitious fever.
  

2)Causes related to inhibition of the vasomotor center by a rapid elevation of the intra-cranial tension:

• Brain abscess.
• Infection such as meningitis.
• Brain tumors.
• Pontine haemorrhage.

3)Other causes:

• Beta blockers.
• Malignant lymphoma.
• Drug induced fever.
  

Related articles:

How to cure cold and flue?
Wikipedia: Bradicardia.
Symptoms of Salmonella food poisoning.