An EKG (also written as ECG) records the electrical signals that travel through your heart with each beat. It shows your heart’s rhythm, how fast it’s beating, and whether the electrical impulses are following their normal path. Beyond rhythm, it can reveal signs of a heart attack, enlarged heart chambers, electrolyte imbalances, and other conditions that alter the heart’s electrical behavior. It’s one of the most common and fastest cardiac tests, taking less than 10 minutes from start to finish.
How an EKG Reads Your Heartbeat
Every heartbeat starts as an electrical impulse that spreads through the heart muscle in a specific sequence. First, the upper chambers (atria) receive the signal and contract to push blood into the lower chambers. Then the lower chambers (ventricles) fire and pump blood out to the lungs and body. Finally, the ventricles reset their electrical charge to prepare for the next beat.
An EKG captures each phase of this cycle as a distinct wave on paper or a screen. The first small bump, called the P wave, represents the upper chambers contracting. The tall, sharp spike that follows (the QRS complex) represents the lower chambers contracting, which is the powerful squeeze that sends blood through your arteries. The rounded wave after that (the T wave) represents the lower chambers resetting electrically. Together, these three waveforms repeat with every single heartbeat, and changes in their shape, height, timing, or spacing tell a clinician what’s happening inside your heart.
In a healthy heart, the interval between the start of the P wave and the start of the QRS complex is 120 to 200 milliseconds. The QRS complex itself lasts 80 to 100 milliseconds. When these intervals fall outside normal ranges, it points to specific problems with how electrical signals are conducting through the heart.
Heart Rhythm Problems
Detecting abnormal heart rhythms (arrhythmias) is one of the most common reasons for ordering an EKG. The test can identify whether your heart is beating too fast, too slow, or irregularly. Atrial fibrillation, for example, replaces the normal P wave with a chaotic, irregular pattern because the upper chambers are quivering instead of contracting in a coordinated way. A heart that’s beating dangerously slowly might show long pauses between QRS complexes, suggesting the electrical signal is getting blocked somewhere along its path.
Because the EKG records the precise timing between each wave, it can also pinpoint where a rhythm problem originates. An irregular signal starting in the upper chambers looks different from one starting in the lower chambers, and the distinction matters because the causes, risks, and treatments are different.
Signs of a Heart Attack
An EKG is one of the first tests performed when someone arrives at a hospital with chest pain. During a heart attack, part of the heart muscle loses its blood supply, and the injured tissue generates abnormal electrical signals. These show up as characteristic changes in the waveform.
The most critical pattern is called ST-segment elevation, which appears in the flat section between the QRS spike and the T wave. When this segment rises above its normal baseline, it typically means one of the coronary arteries is completely blocked. This type of heart attack, called a STEMI, carries a higher risk of serious complications and death than a partial blockage, so identifying it quickly on an EKG can trigger immediate treatment to reopen the artery.
That said, an EKG isn’t perfect at catching every heart attack. A study in the Journal of the American Heart Association found that physician sensitivity for identifying true ST-elevation heart attacks on an EKG was about 65%, with a specificity of 79%. This means some heart attacks, particularly those caused by partial blockages, may not produce obvious EKG changes. Doctors typically combine EKG results with blood tests and imaging to get the full picture.
Enlarged Heart Chambers
When the heart has to work harder than normal, over months or years, the muscle walls of the chambers can thicken. High blood pressure is a common cause. An EKG can pick up signs of this thickening because enlarged muscle generates stronger electrical signals, which show up as taller-than-normal waves.
For left ventricular hypertrophy (thickening of the heart’s main pumping chamber), clinicians measure the height of specific waves and compare them against established thresholds. The test is quite good at ruling it out when results look normal, with specificity above 90%. Its sensitivity is more modest, catching roughly 35% to 62% of confirmed cases depending on which measurement criteria are used. So a normal EKG doesn’t guarantee the heart chambers are normal size, but abnormal voltage patterns are a reliable red flag.
Electrolyte Imbalances
Your heart’s electrical system depends on minerals like potassium, calcium, and magnesium dissolved in your blood. When levels of these electrolytes shift too high or too low, the EKG waveform changes in predictable ways.
High potassium is one of the clearest examples. As potassium levels rise, the T wave becomes unusually tall and peaked because the electrical reset of the lower chambers happens faster and more uniformly. As levels climb further, the QRS complex widens and the P wave can disappear entirely. These changes can signal a life-threatening situation, so an EKG provides a fast visual clue even before lab results come back. Low potassium, on the other hand, tends to flatten the T wave and can produce an extra small wave (called a U wave) that doesn’t normally appear.
What an EKG Cannot Show
A standard resting EKG captures a snapshot of your heart’s electrical activity at the exact moment you’re lying still on a table. This means it can miss problems that only appear during physical exertion or stress. A partially blocked artery might supply enough blood at rest to produce a perfectly normal EKG, while causing symptoms during exercise. That’s why stress tests, which record an EKG while you walk on a treadmill, exist as a separate test.
The U.S. Preventive Services Task Force has found no clear evidence that routine EKG screening in people without symptoms improves health outcomes. Some EKG abnormalities do correlate with future cardiovascular events, but the test wasn’t designed to detect the buildup of plaque inside arteries. It reads electrical signals, not blood flow or anatomy. Conditions like valve disease, small structural defects, or the degree of artery narrowing require imaging tests such as echocardiograms, CT scans, or cardiac catheterization.
What the Test Feels Like
A standard 12-lead EKG uses 10 small electrode stickers placed on your body: four on your limbs (near each shoulder and near each ankle) and six across the left side of your chest. The chest electrodes follow a precise path starting beside the breastbone and curving around toward your left side. Despite the name “12-lead,” the 12 refers to 12 different electrical views of the heart calculated from those 10 electrodes.
The test is painless. You lie still, the electrodes are attached, and the machine records for about 10 seconds. There’s no electricity sent into your body. The electrodes only detect the tiny electrical signals your heart already produces. Results are available almost immediately, printed as a strip of waveforms that a clinician interprets on the spot.
In some cases, a single snapshot isn’t enough. If your symptoms come and go, your doctor may recommend a Holter monitor, which is essentially a portable EKG worn for 24 to 48 hours to catch intermittent rhythm problems that a brief recording might miss.