Testing for arrhythmia usually starts with a standard electrocardiogram (EKG), a painless test that takes about 10 minutes and records your heart’s electrical activity through sensors on your skin. But because many arrhythmias come and go, a single EKG often isn’t enough. Depending on how frequently your symptoms appear, your doctor may use portable monitors, stress tests, blood work, or imaging to catch the irregularity and figure out what’s behind it.
What a Standard EKG Looks For
An EKG records the electrical signals that make your heart beat by reading them through small adhesive patches placed on your chest, arms, and legs. A normal reading shows a repeating pattern of three distinct waves: a small bump as the upper chambers contract, a sharp spike as the lower chambers contract, and another small bump as the heart resets before the next beat. These waves should appear at regular intervals, with the same spacing between each cycle.
When the pattern is off, the shape, timing, or spacing of those waves tells your doctor which part of the heart is misfiring. A completely irregular spacing between beats, for instance, is a hallmark of atrial fibrillation. Extra spikes may signal premature contractions. The limitation is that a standard EKG only captures a snapshot of 10 to 15 seconds. If your heart rhythm is normal during that window, the arrhythmia won’t show up, even if it’s been causing symptoms for weeks.
Portable Monitors for Symptoms That Come and Go
When a standard EKG comes back normal but you’re still having palpitations, dizziness, or fainting spells, the next step is usually a portable monitor you wear while going about your daily life. There are two main types, and the choice depends on how often your symptoms occur.
Holter Monitors
A Holter monitor records your heart’s rhythm continuously for 24 to 48 hours. Small electrodes are stuck to your chest (your doctor may shave small patches of chest hair so they adhere properly), and wires connect them to a small recording device you clip to your belt or carry in a pocket. You wear it the entire time, including while sleeping. Because it can’t get wet, you’ll need to shower or bathe before the appointment and skip bathing until it comes off.
While wearing it, you’ll keep a written log of your activities and any symptoms, noting the exact time. This lets your doctor match what you felt with what your heart was doing electrically at that moment. The downside: if your symptoms happen less often than every day or two, the monitor may not capture anything useful. A controlled clinical trial found that 48-hour Holter monitoring produced a diagnostic recording during symptoms only 35% of the time in patients with intermittent palpitations.
Event Monitors
Event monitors are worn for much longer, typically up to three months or until they’ve captured at least two episodes of symptoms. Some record continuously and save data when you press a button; others only start recording when triggered. In the same clinical trial comparing the two approaches, event monitors were twice as likely to capture a diagnostic reading during symptoms, succeeding 67% of the time compared to the Holter’s 35%. For arrhythmias that strike unpredictably, event monitors are both more effective and more cost-efficient.
Implantable Loop Recorders
If standard EKGs, Holter monitors, stress tests, and echocardiograms all come back inconclusive but you’re still experiencing symptoms like fainting or unexplained palpitations, your cardiologist may recommend an implantable loop recorder. This is a tiny device, roughly the size of a small USB stick, inserted just under the skin of your chest in a brief outpatient procedure.
Once in place, it continuously monitors your heart rhythm for up to three years, though it can be removed sooner once it captures the information your doctor needs. It’s particularly useful for arrhythmias that happen so rarely that no external monitor can catch them, and it’s also used to monitor heart rhythm after a stroke or heart attack. The device automatically flags abnormal rhythms and can also be triggered manually when you feel symptoms.
Stress Tests
Some arrhythmias only appear during physical exertion, which is why your doctor may order an exercise stress test. You’ll walk on a treadmill while the speed and incline gradually increase. Throughout the test, you’re connected to an EKG and a blood pressure cuff, and you’re monitored for symptoms like chest pain, dizziness, shortness of breath, or unusual fatigue.
The goal is to push your heart rate to about 85% of your age-predicted maximum (calculated by subtracting your age from 220). At that intensity, rhythm problems that hide at rest may surface. The test is stopped immediately if dangerous patterns appear, such as sustained rapid rhythms from the lower chambers, significant drops in blood pressure, or if your blood pressure spikes above 250 systolic. It also ends if you feel dizzy, develop chest pain, or simply want to stop.
If you can’t exercise due to mobility issues or other conditions, medication can be used to simulate the effect of exercise on the heart while you remain still.
Blood Tests for Non-Heart Causes
Not every arrhythmia originates from a heart problem. Imbalances in blood chemistry can throw off your heart’s electrical system, so blood work is a routine part of the diagnostic process. The key targets are potassium and other electrolytes, which your heart cells need in precise concentrations to fire correctly, and thyroid hormone. An overactive thyroid is a well-known trigger for atrial fibrillation and rapid heartbeats. Correcting these underlying imbalances sometimes resolves the arrhythmia entirely without further cardiac treatment.
Echocardiograms
An echocardiogram is an ultrasound of your heart. It doesn’t detect arrhythmias directly, but it reveals structural problems that may be causing them. The test shows whether your heart chambers are enlarged, whether your walls are thickened (a sign of long-standing high blood pressure or other disease), and whether your heart valves are opening and closing properly. Leaky or stiff valves, congenital heart defects, and other structural abnormalities can all trigger irregular rhythms. Identifying these issues helps your doctor determine not just that you have an arrhythmia but why you have one.
Electrophysiology Studies
An electrophysiology (EP) study is the most detailed test available and is typically reserved for cases where other tests haven’t pinpointed the problem, or when your doctor is planning a procedure to treat a known arrhythmia. It’s an invasive test done in a hospital. Thin, flexible tubes are guided through a blood vessel (usually in the groin) up into the heart, using live X-ray imaging for navigation.
Sensors on the tips of these tubes measure electrical signals at multiple locations inside the heart, creating a detailed map of how electrical impulses travel through each chamber. The doctor can also send small electrical pulses through the tubes to deliberately trigger the arrhythmia in a controlled setting, which helps identify the exact tissue causing it. In many cases, if the source is found, it can be treated during the same procedure by destroying a tiny area of misfiring tissue.
What Smartwatches Can and Can’t Tell You
Consumer smartwatches with built-in EKG features have become a common first alert for arrhythmias, particularly atrial fibrillation. A 2025 meta-analysis of 14 studies found that EKG-based smartwatches have a pooled sensitivity of 83% and specificity of 88% for detecting atrial fibrillation. In practical terms, this means they correctly identify AFib about 83% of the time and correctly confirm a normal rhythm about 88% of the time.
Those numbers are respectable for a screening tool you wear on your wrist, but they also mean roughly 1 in 6 episodes may be missed and about 1 in 8 alerts could be false positives. A smartwatch notification is a reasonable reason to schedule an appointment, but it doesn’t replace a medical-grade 12-lead EKG or any of the tests described above. Similarly, a clean smartwatch reading doesn’t rule out an arrhythmia if you’re having symptoms.