An EKG (electrocardiogram) checks for problems with your heart’s electrical activity, including irregular heartbeats, signs of a current or previous heart attack, and structural changes like thickened or enlarged heart chambers. The test records the timing and strength of electrical signals as they move through your heart, producing a pattern of waves that reveals how well each part of your heart is functioning. It’s one of the most common cardiac tests, takes only a few minutes, and is completely painless.
Heart Rhythm and Rate
The most basic thing an EKG checks is how fast your heart beats and whether the rhythm is steady or erratic. A normal resting heart rate falls between 60 and 100 beats per minute, and the EKG can spot rates that are too fast, too slow, or jumping between the two. Beyond simple speed, it reveals the pattern of each beat, showing whether the electrical signal follows its normal path from the upper chambers down to the lower chambers.
Irregular heartbeats, called arrhythmias, are among the most common findings. Atrial fibrillation (AFib) and atrial flutter both show distinct patterns on an EKG tracing. Some arrhythmias are harmless. Others increase the risk of stroke or sudden cardiac events. The EKG gives your doctor the specific type and pattern of irregularity, which determines what happens next.
Heart Attacks, Current and Past
An EKG is one of the first tests performed when someone arrives at a hospital with chest pain because it can diagnose a heart attack in progress. When part of the heart muscle loses blood supply, the electrical signals in that area change in recognizable ways. Specific wave patterns on the tracing point to which part of the heart is affected and how severe the damage is.
What surprises many people is that an EKG can also detect heart attacks that happened in the past, even ones you didn’t realize you had. Previous damage leaves a permanent electrical “scar” that shows up as abnormal wave shapes, particularly deeper or wider waves in certain leads. One study in JACC: Advances found that a specific wave pattern in one of the standard leads could identify prior heart attacks with 100% sensitivity and roughly 89% specificity, meaning very few cases are missed. That said, the EKG isn’t perfect for this purpose. Subtle or older damage can sometimes look similar to normal variations, so doctors often pair EKG findings with other tests when the picture isn’t clear.
Blocked or Narrowed Arteries
When the arteries supplying blood to your heart are partially blocked (coronary artery disease), the EKG may show changes in the electrical pattern, particularly during episodes of chest pain when the heart isn’t getting enough oxygen. A resting EKG done while you’re symptom-free is less reliable for catching this. Exercise EKGs, where you walk on a treadmill while being monitored, improve detection because the heart’s demand for oxygen increases and blockages become more apparent.
Even with exercise, EKG detection of coronary artery disease has sensitivities and specificities in the range of 60% to 70%. That means roughly 3 out of 10 cases can be missed, and some false alarms occur as well. For this reason, an EKG is considered a diagnostic aid rather than a standalone test for artery blockages. If your doctor suspects coronary artery disease, you’ll likely need imaging studies for a definitive answer.
Enlarged or Thickened Heart Chambers
The strength and timing of the electrical signals can reveal whether parts of your heart have grown larger or thicker than normal, a condition called hypertrophy. The left ventricle (the heart’s main pumping chamber) thickens in response to chronic high blood pressure or valve disease. The right ventricle thickens from conditions that increase pressure in the lungs.
EKG detection of thickened heart walls is highly specific but not very sensitive. The specificity is above 90%, which means if the EKG says you have hypertrophy, you almost certainly do. But the sensitivity is only around 50%, so roughly half of people with actual hypertrophy will have a normal-looking EKG. An echocardiogram (ultrasound of the heart) is far more reliable for measuring chamber size and wall thickness, so doctors use the EKG as an initial flag rather than the final word.
Other Conditions on the Checklist
Beyond the major categories, an EKG can provide clues about several other heart-related conditions:
- Heart failure: Changes in wave patterns and chamber size indicators can suggest the heart isn’t pumping effectively.
- Heart valve disease: Malfunctioning valves often cause chamber enlargement or rhythm disturbances that appear on the tracing.
- Congenital heart defects: Structural problems present from birth produce distinctive electrical patterns.
- Cardiomyopathy: Disease of the heart muscle itself alters the way electrical signals travel and shows up as abnormal wave shapes.
Monitoring Medications and Devices
Certain medications affect your heart’s electrical system, and an EKG is the primary tool for making sure they aren’t causing dangerous changes. The key measurement here is called the QT interval, which represents the time it takes for your heart to electrically recharge between beats. Some drugs stretch this interval longer than normal, a condition called QT prolongation. When the heart takes too long to reset, it becomes vulnerable to a potentially fatal rhythm disturbance.
If you’re taking a medication known to affect the QT interval, your doctor may order periodic EKGs to check that the interval stays within a safe range. If the drug is causing prolongation, stopping or switching the medication is often the only treatment needed. EKGs also verify that pacemakers and other implanted cardiac devices are working correctly, confirming that the device is pacing the heart at the right rate and the signals are being captured properly.
What the Test Feels Like
An EKG involves placing small adhesive electrode patches on your chest, arms, and legs. A standard 12-lead EKG uses 10 electrodes positioned at specific points to capture the heart’s electrical activity from multiple angles. The skin may be lightly cleaned or exfoliated at each site to ensure a good signal. You lie still for about 5 to 10 minutes while the machine records. There’s no pain, no electrical shock, and no recovery time. Results are usually available within minutes.
When Screening Isn’t Recommended
Despite how useful an EKG is for people with symptoms, routine screening of healthy adults without heart-related complaints is a different story. The U.S. Preventive Services Task Force recommends against EKG screening for adults at low risk of cardiovascular disease (those with a 10-year event risk below 10%). The reasoning is straightforward: in low-risk people, the chance of a false positive, which leads to unnecessary follow-up testing, anxiety, and cost, outweighs the chance of catching a real problem early. For adults at intermediate or higher risk, the evidence is less clear, and the decision comes down to a conversation between you and your doctor about your individual risk factors.