A fever is a temporary elevation of the body’s core temperature, typically occurring as a response to infection or inflammation. Blood pressure is the measurement of the force exerted by circulating blood against the walls of the body’s arteries. While a fever itself does not cause chronic hypertension, the physiological stress of an illness that causes a fever can lead to noticeable, temporary changes in blood pressure readings. This temporary elevation is a frequent concern for individuals tracking their health while ill.
The Acute Relationship Between Fever and Blood Pressure
Illness, particularly when accompanied by a fever, often results in a temporary, mild increase in blood pressure readings. This elevation is a physical response to the body’s heightened metabolic demand as it works to fight off an infection. For many people, this change is not dramatic and resolves completely once the fever breaks and the underlying illness subsides.
The variability of blood pressure during illness is significant, meaning a rise is not guaranteed. While the body’s stress response often pushes blood pressure higher, other factors related to severe illness can cause the opposite effect. For instance, significant dehydration from vomiting or diarrhea can lead to a decrease in blood volume, which may cause a dip in blood pressure. Furthermore, a severe, systemic infection like sepsis can cause profound vasodilation, or widening of the blood vessels, resulting in a dangerously low blood pressure reading.
Physiological Mechanisms Driving Temporary BP Changes
The temporary increase in blood pressure during a fever is primarily mediated by the activation of the sympathetic nervous system (SNS). As the immune system releases signaling molecules to raise the body’s temperature, this process acts as a physical stressor that activates the “fight or flight” response. This prompts the adrenal glands to release catecholamines, such as adrenaline and noradrenaline, into the bloodstream. These hormones cause the constriction of small arteries and arterioles (vasoconstriction), which directly raises the resistance to blood flow and increases blood pressure.
Another significant factor is the increase in cardiac output, the volume of blood the heart pumps per minute. To meet the higher metabolic needs of the body during a fever, the heart rate increases substantially, a condition known as tachycardia. The heart beats faster to deliver more oxygen and nutrients to tissues fighting the infection. This elevated rate of pumping blood into the vascular system contributes directly to a higher pressure reading.
The inflammatory process itself also plays a role in regulating vascular tone. When the body detects a pathogen, immune cells release inflammatory signaling molecules called cytokines. These substances can affect the lining of the blood vessels, promoting the release of peptides like endothelin-1, a powerful vasoconstrictor. Cytokines can also interact with the Renin-Angiotensin-Aldosterone System (RAAS), a hormonal pathway that regulates fluid balance and blood pressure.
When High Readings Require Medical Attention
A high blood pressure reading taken while a person is acutely ill with a fever should not automatically be interpreted as a diagnosis of chronic hypertension. Blood pressure should always be rechecked when the individual is healthy, well-rested, and free of symptoms, which provides a more accurate baseline for diagnosis. The temporary nature of the elevation means it is an acute response, not necessarily a long-term condition.
However, certain readings require prompt medical evaluation regardless of the fever. A blood pressure reading of 180/120 mm Hg or higher is considered a hypertensive crisis and warrants immediate medical attention. If this extremely high reading is accompanied by symptoms such as severe headache, chest pain, sudden shortness of breath, numbness, or changes in vision, it constitutes a hypertensive emergency. These symptoms indicate potential organ damage and should be reported to emergency services immediately.
It is also important to consider the effects of over-the-counter fever-reducing and pain medications. Nonsteroidal Anti-Inflammatory Drugs (NSAIDs), such as ibuprofen and naproxen, can interfere with blood pressure regulation. In patients who already have controlled hypertension, NSAIDs can raise systolic blood pressure by an average of 3 to 6 mm Hg by causing sodium retention and reducing the effectiveness of certain antihypertensive medications. Patients with pre-existing hypertension should discuss analgesic options with a healthcare provider, as alternatives like acetaminophen may be preferred to manage fever and pain without negatively impacting blood pressure control.