An electrocardiogram (ECG or EKG) measures the electrical activity of your heart. Every heartbeat is triggered by a tiny electrical impulse, and an ECG detects those signals from the surface of your skin, producing a tracing that reveals your heart’s rhythm, rate, and the timing of each phase of its pumping cycle. These electrical signals are remarkably small, ranging from 50 microvolts to about 4 millivolts, yet they carry enough information to diagnose dozens of heart conditions.
How the Heart Creates Electrical Signals
Your heart has its own built-in pacemaker: a cluster of specialized cells in the upper right chamber that fires an electrical impulse with each beat. That impulse travels through a network of specialized fibers, spreading first across the upper chambers (the atria) and then down into the lower chambers (the ventricles). As the signal passes through each region, the muscle contracts and pushes blood forward. An ECG captures this wave of electricity as it moves through the heart, translating it into a line graph that doctors can read in real time.
What Each Wave on the Tracing Means
The ECG tracing has three main components, and each one corresponds to a specific event inside the heart.
The P wave is the first small bump on the tracing. It represents the electrical signal spreading from the heart’s natural pacemaker across both upper chambers, causing them to contract and push blood into the ventricles below.
The QRS complex is the tallest, sharpest spike. It marks the moment the ventricles receive the electrical signal and contract forcefully, sending blood out to the lungs and the rest of the body. This signal is so much larger than the one in the atria that it actually hides another event happening at the same time: the upper chambers resetting their electrical charge. That atrial “reset” wave is too small to see beneath the powerful ventricular spike.
The T wave is the gentler bump that follows the QRS complex. It represents the ventricles recovering their electrical charge, preparing for the next beat.
The Gaps Between Waves Matter Too
Doctors pay close attention to the intervals between these waves, because they reveal how quickly electrical signals travel through different parts of the heart. Two intervals are especially important.
The PR interval, measured from the start of the P wave to the start of the QRS complex, reflects the time it takes for the signal to travel from the upper chambers to the lower chambers. In adults, a PR interval shorter than 120 milliseconds can suggest the signal is taking a shortcut through an abnormal pathway, a condition called preexcitation.
The QRS duration tells you how long it takes for the electrical signal to spread across the ventricles. In adults over 16, this should be 110 milliseconds or less. The average in healthy adult men is about 95 milliseconds. A QRS that’s wider than normal can indicate a blockage in one of the heart’s electrical pathways, meaning parts of the ventricle are contracting out of sync.
How a Standard 12-Lead ECG Works
A standard ECG uses 10 adhesive electrodes placed on your chest, arms, and legs. Despite having only 10 physical sensors, the machine combines their readings in different ways to produce 12 distinct “views” of the heart’s electrical activity. Each view highlights a different region of the heart muscle.
- Leads II, III, and aVF look at the bottom (inferior) wall of the heart.
- Leads V1 through V4 look at the front (anterior) wall.
- Leads I, aVL, V5, and V6 look at the left side (lateral) wall.
This matters because problems like heart attacks affect specific regions. If the tracing looks abnormal in leads II, III, and aVF but normal everywhere else, the issue is likely in the inferior wall, which is supplied by a particular coronary artery. That kind of localization helps doctors act quickly.
What Conditions an ECG Can Detect
Because the ECG captures electrical timing and rhythm, it’s useful for a wide range of diagnoses. Irregular heart rhythms (arrhythmias) are the most straightforward: the tracing will show beats that come too fast, too slow, or in an erratic pattern. Atrial fibrillation, for example, replaces the normal P wave with a chaotic, irregular baseline.
Heart attacks leave distinct electrical fingerprints. During an acute heart attack, the segment of the tracing between the QRS complex and the T wave (called the ST segment) rises above the baseline. After a heart attack has occurred, new abnormal Q waves can appear at the very beginning of the QRS complex, serving as a permanent scar on the electrical record. These changes, combined with symptoms and blood tests, are central to confirming the diagnosis.
An ECG can also reveal thickening of the heart muscle, electrolyte imbalances (particularly potassium and calcium), effects of certain medications on the heart’s electrical system, and congenital abnormalities in the heart’s wiring.
What an ECG Cannot Tell You
An ECG only measures electrical activity. It doesn’t show the physical structure of the heart. It cannot tell you the size of each chamber, whether a valve is leaking or narrowed, or how strongly the heart is pumping blood (a measurement called ejection fraction). For those questions, doctors turn to an echocardiogram, which uses ultrasound to create a detailed picture of the heart’s anatomy and movement. It’s common for an abnormal ECG to be followed up with an echocardiogram to determine the structural cause behind the electrical abnormality.
A normal resting ECG also doesn’t rule out all heart problems. Some conditions only produce abnormal readings during physical exertion or at random moments. That’s why doctors sometimes order stress tests (ECGs recorded while you exercise) or Holter monitors (portable ECGs worn for 24 to 48 hours) to catch intermittent issues.
What the Test Feels Like
A resting ECG is painless and quick. A technician cleans each electrode site with alcohol and gauze to remove oils and ensure a good connection, then sticks 10 adhesive patches to your skin: six across the chest and one on each arm and leg. The actual recording takes only a few seconds with modern automated machines, though the entire visit, including prep, typically lasts under 10 minutes. You lie still, breathe normally, and the machine does the rest. There’s no radiation, no injection, and no recovery time.