A new study challenges the common assumption that a low resting heart rate is always a sign of good cardiovascular health. Researchers from Imperial College London have found that both very low and very high resting heart rates are associated with an increased risk of stroke.
Study Details
The study, presented at the European Stroke Organisation Conference, followed 460,000 participants from the UK Biobank for approximately 14 years. During this period, 12,290 strokes occurred. After adjusting for age, sex, and cardiovascular risk factors such as atrial fibrillation, the researchers found that stroke risk increased at both extremes: below 50 beats per minute (bpm) and at or above 90 bpm.
Those with very high resting heart rates were 45% more likely to suffer a stroke, while those with very low rates had a 25% increased risk. The lowest stroke risk was observed in individuals with resting heart rates between 60 and 69 bpm.
Biological Signal
Importantly, the relationship persisted even after adjusting for established risk factors like high blood pressure, diabetes, and atrial fibrillation, suggesting a genuine biological signal. However, when participants with atrial fibrillation were analyzed separately, the same risk relationship was not identified.
Dr. Dexter Penn, a neurologist and lead study author, explained: 'This is likely because atrial fibrillation is such a strong risk factor for stroke, increasing risk by fivefold, that it outweighs the contribution of heart rate and limits our ability to detect its effect. Heart rate was therefore most informative in people without atrial fibrillation, where it may provide a valuable additional tool for identifying and stratifying stroke risk.'
Possible Mechanisms
The researchers suggest that very low heart rates, medically known as bradycardia, can reduce blood flow to the brain. Common symptoms include dizziness, lightheadedness, shortness of breath, confusion, chest pain, and fainting. When the heart rate drops dangerously low, the heart cannot pump enough oxygen-rich blood to the brain, potentially increasing the risk of ischemic stroke, which occurs when a blockage cuts off blood supply to the brain. This type accounts for 85% of all strokes.
Conversely, very high heart rates were associated with both ischemic and hemorrhagic strokes, the latter caused by bleeding in or around the brain. The researchers propose that elevated heart rates may increase stress on blood vessel walls, a key marker of high blood pressure, contributing to blockages and a greater predisposition to bleeding. This stress can lead to atherosclerosis, where blood vessels become narrower and stiffer, causing fatty material buildup in the arteries. Clots can then form and travel to the brain, causing a stroke. Elevated resting heart rates can also damage blood vessels in the brain, increasing the risk of a bleed.
Clinical Implications
Professor Alastair Webb, an expert in stroke medicine and study co-author, commented: 'Resting heart rate is a simple, widely available measure that deserves greater attention in cardiovascular risk assessment.' He added that while more research is needed to understand how heart rate may impact treatment, the findings are very promising. 'Very low or very high heart rates should act as a signal for clinicians to look more closely at an individual's overall cardiovascular risk and take action to reinforce lifestyle changes and standard prevention strategies.'
Understanding Resting Heart Rate
Resting heart rate refers to the number of times your heart beats per minute while your body is at rest, such as when sitting or lying down. A normal resting heart rate is between 60 and 100 bpm, but athletes, fit and healthy young adults, and those on beta-blockers tend to have slower rates. This is because the heart learns to pump blood more efficiently with exercise. It is also normal for heart rate to increase to 130-150 bpm or more during exercise.
Staying hydrated, losing weight, and managing stress, alcohol intake, and sleep can help regulate heart rate. Dehydration reduces blood volume, making the heart work harder. Obesity is another known risk factor for stroke, and stress, alcohol, and lack of sleep affect adrenaline and cortisol levels, which can elevate heart rate.
