A pounding or racing heart is one of the most common things people notice when they come down with a fever, flu or another infection. In most cases it isn’t a sign that something is wrong with the heart. It means the body is working harder to fight the illness, and the effort is measurable enough that smartwatches can sometimes pick up an infection from a rising resting heart rate before a person feels sick.1The Conversation. Wearable fitness devices deliver early warning of possible COVID-19 infection
How Much Fever Speeds Up Your Heart
A normal resting heart rate, your heartbeat while sitting quietly, is about 60 to 100 beats per minute for adults. In adults seen in emergency departments, each 1 °C rise in temperature went with about 7 to 10 more beats per minute.2PubMed. Relationship between body temperature and heart rate in adults and children: A local and national study In adults admitted to hospital for medical illness, the rise was about 6 to 7 beats per minute for each 1 °C.3PubMed. Fever increases heart rate and respiratory rate: a prospective observational study of acutely admitted medical patients So an adult whose resting rate is usually 70 might sit around 85 to 90 with a 39 °C (102 °F) fever.
To use the rule, you need the temperature in the right units. One °C equals about 1.8 °F, and normal is about 37 °C or 98.6 °F. A 38.5 °C fever is about 101.3 °F, roughly 1.5 °C above normal, so you would expect around 10 to 15 extra beats per minute.
To check your own rate, sit or lie still for five minutes first. Then count your pulse at the wrist or neck for 30 seconds and double it. Note your temperature at the same time so you can compare the two. The useful comparison is with your usual healthy resting rate, not a textbook number.
Children’s hearts respond more strongly. In children under 16 seen in urgent and emergency care, heart rate rose by about 12 beats per minute for each 1 °C.4PubMed Central. The association between temperature, heart rate, and respiratory rate in children aged under 16 years attending urgent and emergency care settings The rise was largest in the youngest children, about 14 beats per minute per degree, and smallest in teenagers, about 9.5PubMed Central. The association between temperature, heart rate, and respiratory rate in children aged under 16 years attending urgent and emergency care settings
The rule is only a rough guide. Age, fitness, medicines and hydration all shift the numbers. A rate far above what the fever predicts is worth noticing.
Why Fever Makes the Heart Work Harder
A fever is not just a body that has overheated. The brain resets the body’s thermostat higher to fight infection. Shivering and narrowed skin vessels then generate and hold heat until the body reaches the new setting. Sweating and flushing come when the fever breaks.
As body temperature rises, the speed at which the body burns energy rises along with it.6Bolt Pharmacy. Metabolic Rate and Body Temperature: Understanding Their Vital Relationship The body uses more oxygen, and the heart must pump more blood each minute to deliver it.7Bolt Pharmacy. Metabolic Rate and Body Temperature: Understanding Their Vital Relationship People with weak hearts or lungs feel this extra demand most.8Bolt Pharmacy. Metabolic Rate and Body Temperature: Understanding Their Vital Relationship
Chills pull the heart two ways. During the chill phase as a fever climbs, rising metabolism pushes the heart to pump more.9Journal of Clinical Investigation. Circulation and Respiration During an Episode of Chill and Fever in Man At the same time, blood vessels in the skin clamp down tightly, which can reduce blood returning to the heart.10Journal of Clinical Investigation. Circulation and Respiration During an Episode of Chill and Fever in Man
From the inside, this often feels like pounding in the chest, neck or ears, especially when lying down. Breathing also speeds up slightly with fever, and feeling worse when you stand up quickly is common.
Dehydration, Stress and the Infection Itself
Fever, sweating, vomiting, diarrhea and not drinking all drain body fluid. Blood is mostly fluid, so when you lose fluid there is less blood in circulation, and each beat carries a bit less. The heart compensates by beating faster to keep oxygen moving, even at rest.11Stat Specialty Hospital. Dehydration and Heart Rate: Key Warning Signs Losing even about 1.6% of body weight in fluid can raise heart rate noticeably.12Stat Specialty Hospital. Dehydration and Heart Rate: Key Warning Signs
Signs you are dehydrated include dark yellow or scanty urine, a dry mouth, and dizziness on standing. Your heart may race more when you stand up. Thirst is a late signal, so sip regularly rather than waiting for it.
Illness also activates the body’s stress system, releasing adrenaline-like signals that speed the heart. Immune chemicals released during infection can directly affect the heart’s electrical rhythm.13PubMed Central. Fir(e)ing the Rhythm
Worry about being sick adds to this, and watching a smartwatch number can create a loop of alarm. Slow breathing can help calm a stress-driven rise.
Rarely, the virus reaches the heart. Some viruses can infect and inflame the heart muscle, a condition called myocarditis.14PubMed Central. Viruses in the Heart: Direct and Indirect Routes to Myocarditis and Heart Failure For most colds and flu this does not happen, and a fast heartbeat comes from fever and fluids.
Heart inflammation feels different from a fast but ordinary pulse. Look for chest pain or pressure, not just pounding, and breathlessness out of proportion to the illness. Fainting or an irregular, skipping rhythm also counts. These need a doctor promptly.
When a Racing Heart Is a Warning Sign
Doctors use set thresholds for children. In children’s emergency triage, a heart rate of 160 or more in babies under 1, or 140 or more in children aged 1 and older, is treated as abnormal.15PubMed Central. Improving the prioritization of children at the emergency department: Updating the Manchester Triage System using vital signs A rate of 170 or more in a baby under 1 is treated as very urgent.16PubMed Central. Improving the prioritization of children at the emergency department: Updating the Manchester Triage System using vital signs
In serious infection, a persistently fast heart signals higher risk. Among hospital patients with septic shock, a life-threatening reaction to infection, those whose heart rate stayed above 85 had a higher risk of dying.17PubMed Central. Association between heart rate and mortality in patients with septic shock: an analysis revealed by time series data This is a hospital finding, not a rule for someone with flu at home, but it shows why a fast heart that won’t settle matters in severe illness.
Get urgent help for a racing heart with chest pain, severe breathlessness, fainting, confusion or blue lips. Call a doctor if an adult’s resting heart rate stays above about 120, or the heart rhythm feels irregular.
Sepsis has its own pattern: a fast heart plus fast breathing and feeling very unwell. Skin may turn cold, mottled or clammy. Confusion or extreme drowsiness and passing very little urine are further warnings.
A rate that falls with rest and fluids is reassuring. If your resting rate stays high after the fever has gone and you have drunk and rested, something else may be driving it, and that is a reason to call a doctor, especially if you also feel breathless or unwell. Some people should be more cautious: those with existing heart or lung disease, older adults, young babies, and pregnant people.
What Helps Bring Your Heart Rate Down
Fever reducers help a little. In critically ill adults with fever, acetaminophen lowered temperature by about half a degree Celsius and heart rate by about 6 beats per minute.18PubMed. Effects of IV Acetaminophen on Core Body Temperature and Hemodynamic Responses in Febrile Critically Ill Adults: A Randomized Controlled Trial It also lowered blood pressure in those patients, and the effect faded by about 4 hours.19PubMed. Effects of IV Acetaminophen on Core Body Temperature and Hemodynamic Responses in Febrile Critically Ill Adults: A Randomized Controlled Trial Fever medicine can ease the pounding but will not bring the heart rate fully back to normal on its own, and it does nothing for dehydration, a separate cause that fluids address. So drink water, broth or oral rehydration drinks regularly, especially with sweating, vomiting or diarrhea. Rest lying down and stand up slowly to avoid dizziness. Avoid caffeine and decongestants containing pseudoephedrine, which can speed the heart further.
When using acetaminophen or ibuprofen, follow the label dose and timing. Don’t double up on products that contain the same drug, and use child-specific dosing by weight for children.
Your resting rate may stay slightly high for a few days after the fever breaks, so return to exercise gradually.
Why Your Heart May Still Race After You Recover
Nerve control of the heart can stay off-balance. Weeks after COVID-19, young adults showed higher stress-nerve activity and a bigger heart rate jump when tilted upright.20PubMed Central. COVID-19 is getting on our nerves: sympathetic neural activity and haemodynamics in young adults recovering from SARS-CoV-2 In post-COVID patients, about 15% had problems with this automatic nerve control, mostly blood pressure dropping on standing.21PubMed Central. Heart rate variability as a marker of cardiovascular dysautonomia in post-COVID-19 syndrome using artificial intelligence
Standing is the trigger because gravity pulls blood toward the legs, and the nervous system normally tightens vessels and nudges the heart to compensate. If that automatic control is off after an infection, the heart over-speeds to make up for blood pooling below. That is why symptoms often appear on standing and ease when lying down.
POTS (postural orthostatic tachycardia syndrome) is one recognized result. It means heart rate rises by at least 30 beats per minute on standing and stays up.22PubMed Central. Cardiovascular autonomic dysfunction in “Long COVID”: pathophysiology, heart rate variability, and inflammatory markers It can bring palpitations, chest pain and trouble tolerating exercise or standing.23PubMed Central. Cardiovascular autonomic dysfunction in “Long COVID”: pathophysiology, heart rate variability, and inflammatory markers About 80% of people with POTS are women, and viral illnesses can trigger it.24PubMed Central. Cardiovascular autonomic dysfunction in “Long COVID”: pathophysiology, heart rate variability, and inflammatory markers
Day to day, people feel lightheaded or near fainting on standing, especially after a hot shower or while waiting in a queue. A pounding or fluttering heart, fatigue after small efforts and shakiness are common. Symptoms often worsen when dehydrated, so fluids and salt are commonly discussed with a doctor, but only under medical advice.
Big jumps on standing that last weeks are concerning, and brain fog and exhaustion often travel with them. See a doctor if a racing heart on standing lasts weeks after an illness.
A simple home test gives the doctor something concrete. Lie down for 5 to 10 minutes and note your heart rate. Then stand still and record it at 2, 5 and 10 minutes, stopping to sit if you feel faint. Write down your symptoms alongside the numbers.
What Your Smartwatch Can Tell You
Wearables pick up early changes. Fitness tracker resting heart rate and activity data have been used to spot people likely developing flu-like illness.25The Conversation. Wearable fitness devices deliver early warning of possible COVID-19 infection In one reported case, a wearable showed raised heart rate and skin temperature at the start of Lyme disease, before a rash appeared.26PubMed Central. Digital Health: Tracking Physiomes and Activity Using Wearable Biosensors Reveals Useful Health-Related Information More serious, inflamed illnesses are more likely to show up than mild ones.27PubMed Central. Digital Health: Tracking Physiomes and Activity Using Wearable Biosensors Reveals Useful Health-Related Information
Most trackers log resting rate overnight or at quiet moments, so a sustained rise of several beats over a few days stands out against your own baseline. Sleeping heart rate is often the cleanest signal because movement and caffeine do not interfere. A single high day matters little; a trend does. Alcohol, poor sleep, stress and hard workouts also raise resting rate, so a rise plus symptoms is worth noting but not a basis to self-diagnose.
Readings can also be wrong. A loose fit or movement gives false spikes, so confirm odd readings with a manual pulse count. Trackers can’t reliably detect irregular rhythms on all models.