Electrocardiography is a painless, noninvasive test that records the electrical activity of your heart through small sensors placed on your skin. Often abbreviated as ECG or EKG, it’s one of the most common and widely used heart tests in medicine, taking only a few minutes to perform and providing immediate information about your heart’s rhythm, rate, and overall electrical function.
How Your Heart Creates an Electrical Signal
Every heartbeat begins with a tiny electrical impulse that triggers the heart muscle to contract and pump blood. This impulse starts in a cluster of cells near the top of the heart, spreads through the upper chambers, pauses briefly at a relay point in the middle, then travels rapidly through the lower chambers. The entire sequence takes less than a second and repeats with every beat.
That electrical activity doesn’t stay confined to the heart. It radiates outward through surrounding tissue and can be detected on the surface of your skin. An ECG machine picks up these faint signals through sticky electrode patches, amplifies them, and displays them as a series of waves on paper or a screen. The shape, size, and timing of those waves tell a clinician exactly how electricity is moving through your heart, and whether anything looks abnormal.
What Happens During the Test
A standard ECG uses 10 electrode patches to produce 12 different “views” of the heart’s electrical activity, which is why you’ll sometimes hear it called a 12-lead ECG. Four electrodes go on your limbs (one on each arm and leg), and six are placed across your chest in specific positions along the ribs and sternum. Each combination of electrodes captures the heart’s signal from a different angle, giving a detailed picture of the entire organ.
You’ll lie still on a table for the recording, which typically takes under 10 minutes. The test is completely painless. No electricity passes into your body; the machine only listens. Preparation is minimal: wear a top that’s easy to remove, skip body lotions or oils on your skin beforehand, and some people may need a small area of chest hair shaved so the electrodes stick properly.
Reading the Waves
An ECG tracing looks like a repeating pattern of peaks and dips. Each segment of the pattern corresponds to a specific event in your heartbeat. The first small bump, called the P wave, represents the electrical signal spreading through the upper chambers (atria). The tall, sharp spike that follows, the QRS complex, shows electricity firing through the lower chambers (ventricles), which are the heart’s main pumping force. The gentler wave after that, the T wave, reflects the heart resetting its electrical charge before the next beat.
Clinicians also measure the time intervals between these waves. Textbooks define the normal PR interval (the gap between the start of atrial and ventricular activity) as 120 to 200 milliseconds, and normal QRS duration as 80 to 120 milliseconds. A large study using electronic medical records found that 95% of healthy people fell within slightly tighter ranges: 125 to 196 milliseconds for PR and 69 to 103 milliseconds for QRS. When these intervals stretch or shrink outside normal bounds, it points to specific problems with how electricity travels through the heart.
What an ECG Can Detect
ECGs are used to investigate a wide range of symptoms: chest pain, dizziness, palpitations, fainting, shortness of breath, and unexplained fatigue. The test is also a standard part of preoperative assessments before surgery, sports physicals to screen for hidden heart conditions, and routine monitoring for people taking medications that affect heart rhythm.
Some of the specific conditions an ECG can identify include:
- Arrhythmias. Irregular heartbeats, whether too fast, too slow, or chaotic, show up clearly because the ECG captures every electrical cycle in real time.
- Heart attacks. During an active heart attack, a characteristic pattern called ST elevation appears in specific leads. The location of those changes tells clinicians which artery is blocked. For example, ST elevation in the leads facing the front of the heart points to a blockage in the left anterior descending artery.
- Prior heart damage. Even after a heart attack has passed, the ECG may show lasting changes that reveal old injury to the heart muscle.
- Conduction blocks. If the electrical pathway through the heart is partially or fully interrupted, the ECG shows widened or distorted waveforms that pinpoint where the block is occurring.
- Electrolyte imbalances. Abnormal levels of potassium, calcium, and other minerals alter the shape of the ECG waves in recognizable ways.
- Medication effects. Certain drugs can lengthen the QT interval (the time it takes the ventricles to reset), raising the risk of dangerous rhythms. Regular ECGs help monitor for this.
What an ECG Can Miss
A standard ECG captures only a brief snapshot, typically about 10 seconds of heart activity. If a problem is intermittent, it may not show up during that short window. The test also has real limitations for detecting certain conditions. A study of over 14,000 emergency department patients with chest pain found that ECG criteria for acute coronary syndrome had a sensitivity of roughly 28 to 31%, meaning it correctly flagged fewer than one in three cases. Its specificity was much higher, around 90 to 92%, so a positive finding is meaningful, but a normal ECG doesn’t rule out a heart attack on its own. That’s why clinicians combine ECG results with blood tests, imaging, and your symptoms to make a diagnosis.
Types of ECG Testing
The resting ECG described above is the most common version, but there are several variations designed to catch problems that a brief recording might miss.
Exercise Stress Test
This test records your ECG while you walk on a treadmill or ride a stationary bike, typically for 10 to 15 minutes with gradually increasing intensity. Some heart problems only become visible when the heart is working harder and demanding more blood flow. If you can’t exercise, a medication can be given that mimics the effect of physical exertion on the heart. Blood pressure is monitored throughout the test alongside the ECG.
Holter Monitor
A Holter monitor is a small, portable ECG device you wear continuously for 24 to 48 hours (sometimes longer) while going about your normal routine. It records every heartbeat during that period, making it far more likely to catch arrhythmias or other irregularities that come and go unpredictably. After the monitoring period, a clinician reviews the full recording for abnormalities.
Event Monitor
Similar to a Holter monitor but worn for weeks or even months, an event monitor records only when you activate it (usually when you feel symptoms) or when it automatically detects an abnormal rhythm. This is useful for symptoms that occur infrequently, perhaps only a few times a month.
Digital ECG and Newer Technology
Traditional ECGs were printed on long strips of graph paper, and many clinics still use paper records. Increasingly, though, ECG signals are captured and stored digitally, which opens the door to computer-assisted interpretation. Software can now analyze a digital ECG tracing and flag potential abnormalities, offering a preliminary reading that a clinician then confirms or overrides. Researchers are also developing smartphone-based tools that can photograph a paper ECG, convert it to a digital signal, and run it through diagnostic algorithms trained to identify heart conditions. These tools are especially promising for rural areas where cardiologists and digital ECG equipment are scarce.
Smartwatches and wearable devices have added another layer. Many consumer devices now offer single-lead ECG recordings from your wrist, which can detect atrial fibrillation and other rhythm abnormalities. These aren’t a replacement for a clinical 12-lead ECG, but they can serve as an early warning system that prompts you to seek a full evaluation.