A low heart rate has many possible causes, ranging from excellent cardiovascular fitness to thyroid problems to medication side effects. The medical term is bradycardia, generally defined as a resting heart rate below 60 beats per minute, though many cardiologists don’t consider it clinically significant until it drops below 50 bpm. Whether a low heart rate is a sign of health or a sign of trouble depends almost entirely on whether it causes symptoms.
Athletic Fitness Is the Most Common Benign Cause
If you exercise regularly, especially endurance activities like running, cycling, or swimming, your heart likely beats slower at rest than someone who doesn’t. This is one of the most well-understood reasons for a low heart rate, and it’s a sign your cardiovascular system is working efficiently. Your heart has adapted to pump more blood per beat, so it doesn’t need to beat as often to meet your body’s demands.
The numbers can be surprisingly low. In a study of 465 endurance athletes monitored with portable heart trackers, 38% had a minimum heart rate at or below 40 bpm, and about 2% dropped to 30 bpm or lower. Up to 80% of endurance athletes develop a resting heart rate that meets the technical definition of bradycardia. Two things drive this: increased activity of the vagus nerve (which naturally slows the heart) and physical remodeling of the heart’s pacemaker cells in response to years of training. Genetics also play a role. Some people appear to be born with traits that favor a slower heart rate, which may actually predispose them to succeed in endurance sports in the first place.
Medications That Slow the Heart
A long list of prescription drugs can lower your heart rate, sometimes intentionally and sometimes as a side effect. The most common culprits are beta-blockers and certain calcium channel blockers, both widely prescribed for high blood pressure and heart conditions. These drugs work partly by suppressing the heart’s natural pacemaker activity, which is the whole point when treating a fast or irregular heartbeat but can overshoot into a rate that’s too slow.
Other medications that can cause or worsen a low heart rate include:
- Digoxin, used for heart failure, which increases vagal tone and slows the heart
- Certain antidepressants, including citalopram, escitalopram, and fluoxetine
- Clonidine, a blood pressure medication that reduces the release of adrenaline-like hormones
- Amiodarone and other rhythm-control drugs, which directly suppress the heart’s electrical signaling
- Fingolimod, used for multiple sclerosis
- Beta-blocker eye drops for glaucoma, which can be absorbed into the bloodstream and affect heart rate even though they’re applied to the eyes
If your heart rate dropped after starting a new medication, that connection is worth raising with your prescriber. Dose adjustments or switching to a different drug often resolves the issue.
Thyroid Problems
An underactive thyroid (hypothyroidism) is one of the most frequently overlooked causes of a slow heart rate. Thyroid hormones directly influence the cells that generate your heartbeat. They regulate the genes responsible for pacemaker activity in the heart, controlling how quickly those cells fire. When thyroid hormone levels fall, the heart’s electrical cells become sluggish. The heart also contracts less forcefully because thyroid hormones affect how calcium moves in and out of heart muscle cells.
Hypothyroidism is easy to detect with a simple blood test, and treating it with thyroid hormone replacement typically brings the heart rate back to normal.
Problems With the Heart’s Electrical System
Your heart has a built-in electrical system. A cluster of cells called the sinus node generates each heartbeat, and the signal travels through a relay point before reaching the lower chambers. Problems at either of these locations can slow your heart rate.
Sinus Node Dysfunction
Sometimes called sick sinus syndrome, this condition means the heart’s natural pacemaker isn’t firing reliably. It can show up in several ways: the sinus node may simply fire too slowly, it may pause for several seconds between beats, or the signal it generates may get stuck before reaching the rest of the heart. Some people experience a pattern where the heart alternates between unusually slow and unusually fast rhythms, with long pauses in between. Another form, called chronotropic incompetence, means the resting heart rate seems fine but fails to speed up appropriately during physical activity, leaving you winded or exhausted with minimal exertion.
Heart Block
Heart block occurs when electrical signals are delayed or blocked at the relay point between the upper and lower chambers. There are three degrees. First-degree heart block is mild, just a slight delay in signal transmission, and rarely causes symptoms. Second-degree heart block means some signals get through and others don’t, resulting in intermittently dropped beats. Third-degree (complete) heart block is the most serious: the upper and lower chambers lose electrical communication entirely. The lower chambers may generate their own backup rhythm, but it’s often very slow and unreliable.
Electrolyte Imbalances
Potassium levels have a direct effect on the heart’s electrical activity. High potassium (hyperkalemia) is particularly dangerous. At moderately elevated levels, it can impair conduction through the relay point between the upper and lower chambers, slowing or blocking signals. At severely elevated levels, above roughly 7 mmol/L, it can cause the heart’s backup pacemaker cells to fail entirely, leading to heart block or even cardiac arrest. Hyperkalemia is most common in people with kidney disease, since the kidneys are responsible for clearing excess potassium from the blood.
Other Causes Worth Knowing
Sleep apnea, a condition where breathing repeatedly stops during sleep, can trigger episodes of low heart rate overnight. This is common enough that a sleep study is sometimes part of the workup for unexplained bradycardia. Aging also plays a role. The heart’s electrical system can degenerate over time, and sinus node dysfunction becomes more common in older adults. Certain infections and inflammatory conditions can temporarily affect the heart’s conduction system as well.
When a Low Heart Rate Causes Symptoms
A low heart rate that doesn’t cause symptoms generally doesn’t need treatment. Many people walk around with resting rates in the 40s or 50s and feel perfectly fine. The concern arises when the heart rate is too slow to deliver enough blood to the brain and body. Symptoms to watch for include dizziness or lightheadedness, fainting or near-fainting, unusual fatigue, shortness of breath with activities that didn’t previously cause it, and difficulty concentrating or thinking clearly. If your heart rate is low and you’re experiencing any of these, that combination is worth investigating.
How a Low Heart Rate Is Evaluated
The starting point is an electrocardiogram (ECG), which captures the heart’s electrical activity in a few seconds. If the ECG is normal but symptoms come and go, you may be asked to wear a Holter monitor, a portable ECG device that records continuously for a day or more. For symptoms that occur rarely, an event recorder can be worn for up to 30 days and activated with a button press when you notice something unusual.
Blood tests are a standard part of the evaluation. These check thyroid function, potassium and other electrolyte levels, and markers of infection. A stress test, where your heart is monitored during exercise, can reveal whether your heart rate rises appropriately with physical effort. If you’ve had fainting episodes, a tilt table test may be used: you lie flat on a table that’s then tilted upright while your heart rate and blood pressure are tracked to see how your nervous system responds to the position change.
In many cases, the evaluation reveals a straightforward and treatable cause, whether that’s a medication adjustment, thyroid treatment, or monitoring for an electrical conduction issue. For people whose slow heart rate is caused by irreversible damage to the heart’s electrical system and produces symptoms, a pacemaker is the definitive solution, restoring a reliable rhythm by providing electrical signals the heart can no longer generate on its own.