The resting heart rate (RHR) is the number of times your heart beats per minute while your body is at rest, typically ranging from 60 to 100 beats per minute for most adults. Many people who monitor their RHR notice a distinct increase when they begin to feel unwell. This temporary acceleration is a common biological phenomenon indicating the body is actively responding to a challenge. This article details the mechanisms behind this rise in heart rate during illness.
Understanding the Physiological Connection
When the body encounters an infection, the immune system initiates a defense response that requires significant energy, increasing the overall metabolic demand. This heightened internal activity triggers a rise in body temperature, known as a fever, which is a primary driver of the elevated heart rate. The heart must pump faster to deliver the necessary oxygen and nutrients to support this accelerated metabolism.
The relationship between body temperature and heart rate is consistent: for approximately every one degree Celsius (1.8 degrees Fahrenheit) rise in core body temperature, the heart rate increases by about 10 beats per minute. This response is a form of sinus tachycardia, where the heart’s natural pacemaker signals it to beat faster than its normal resting rate. The resulting increase in cardiac output ensures that white blood cells and other immune components are circulated quickly to the site of infection.
This process is heavily influenced by the sympathetic nervous system, which controls the body’s “fight-or-flight” response. Illness is perceived as physiological stress, leading to the activation of this system. Sympathetic activation stimulates the heart, increasing its rate and contractility to meet the elevated demands of the stressed body.
Secondary Contributors to Elevated Heart Rate
Several external and internal factors related to illness contribute to an elevated heart rate beyond the direct metabolic demands of a fever.
One common secondary cause is dehydration, which frequently occurs due to fluid loss from vomiting, diarrhea, or reduced liquid intake. When fluid volume decreases, the heart compensates by pumping more frequently to maintain adequate blood pressure and ensure tissues receive sufficient blood flow. This faster rate helps move the reduced blood volume rapidly to prevent a drop in circulation to vital organs.
Systemic inflammation, pain, or anxiety also prompt the release of stress hormones, such as catecholamines like adrenaline. These hormones act as stimulants, directly accelerating the heart rate as part of the body’s generalized stress response. Additionally, some over-the-counter cold and flu preparations contain decongestant ingredients that have stimulant effects on the cardiovascular system, independently contributing to a faster heart rhythm.
Warning Signs and Medical Intervention
While a temporary elevation in heart rate is a normal response to illness, specific thresholds and accompanying symptoms signal the need for professional medical attention. Tachycardia is medically defined as a resting heart rate exceeding 100 beats per minute. A sustained rate significantly above this level, especially one disproportionate to the degree of fever, may indicate a more severe underlying issue like sepsis or cardiovascular compromise.
Warning signs that prompt immediate care include an elevated heart rate combined with severe symptoms, such as chest pain, severe shortness of breath, or episodes of dizziness or fainting. Confusion or severe weakness alongside a rapid pulse are also serious indicators requiring urgent medical evaluation.
If the heart rate remains high even after the fever has fully subsided and other symptoms have resolved, consult a healthcare provider. This persistence may suggest a lingering infection, an issue with the heart’s electrical system, or a lasting effect of the illness that warrants further investigation.