Heart rate typically increases with fever, a phenomenon medically known as tachycardia. Fever is defined as an elevation in the body’s core temperature above the normal range, usually 100.4°F (38°C) or higher. This faster heartbeat is a normal physiological response as the body fights an underlying illness, most often an infection. The increased heart rate helps meet the heightened metabolic demands and circulatory changes that occur when the internal temperature rises.
The Body’s Underlying Mechanism for Increased Heart Rate
The rise in heart rate during a fever is directly linked to the body’s increased metabolic rate. For every one degree Celsius rise in core body temperature, the metabolic rate increases by approximately 10 to 13%. This heightened metabolism requires significantly more oxygen and nutrients to be delivered to the tissues and organs.
The hypothalamus, the brain’s temperature control center, initiates the fever response. When the set-point is raised, the hypothalamus signals the circulatory system to increase its activity. The heart compensates for this increased demand by boosting its output, primarily by increasing the number of beats per minute.
This rapid circulation serves two main purposes: supplying immune cells and dissipating heat. The heart speeds up to deliver immune components, like white blood cells, to the site of infection more quickly. Simultaneously, the body attempts to cool itself through peripheral vasodilation, the widening of blood vessels near the skin’s surface, requiring the heart to pump faster to maintain overall blood pressure.
Typical Measurement of Heart Rate Elevation During Fever
The relationship between temperature and heart rate is consistent, allowing clinicians to rely on a general guideline for measurement. An adult’s heart rate typically increases by approximately 8 to 10 beats per minute (BPM) for every one-degree Celsius (1°C) rise in body temperature. This translates to about 4.4 BPM for every one-degree Fahrenheit (1°F) increase.
For example, if a person’s resting heart rate is 70 BPM and their temperature rises by 2°C (3.6°F), their heart rate is expected to increase to around 86 to 90 BPM. Individual responses can vary based on fitness level, age, and underlying health conditions. This predictable rise helps healthcare providers determine if the heart rate is responding appropriately to the fever or if other complications might be present.
When the Heart Rate Does Not Increase With Fever
In some situations, the heart rate does not increase as expected during a fever, a phenomenon known as relative bradycardia or Faget’s sign. This means the pulse rate is lower than the anticipated increase of 8 to 10 BPM per degree of fever. This unusual dissociation between pulse and temperature can be an important diagnostic clue for specific types of infection.
Certain diseases are known to present with this atypical response. The classic example is typhoid fever, but other infections may include Legionnaires’ disease, dengue fever, Brucellosis, and some viral hemorrhagic fevers. Although the exact mechanism is not fully understood, it is thought to involve increased vagal tone or direct effects of the pathogen on the heart’s conduction system.
Indicators That Require Immediate Medical Consultation
While a moderately increased heart rate is normal with a fever, certain indicators suggest the body is under severe strain and necessitate immediate medical consultation. A sustained rate over 120 or 130 BPM in a resting adult can be a warning sign. This disproportionate increase may indicate conditions like sepsis, severe dehydration, or an underlying heart issue.
Any high heart rate accompanied by other concerning symptoms should prompt emergency care. These red flags signal a potential medical emergency that requires professional assessment:
- Chest pain
- Difficulty breathing or shortness of breath
- Sudden lightheadedness or fainting
- Confusion or extreme fatigue
- A persistent, irregular, or fluttering heartbeat (palpitations)