An ECG (electrocardiogram) works by detecting the tiny electrical signals your heart produces every time it beats, then translating those signals into a line tracing that reveals your heart’s rhythm, timing, and health. Ten small sensor patches placed on your skin pick up voltage changes as electrical impulses sweep through the heart muscle, and a machine amplifies and records those changes as the familiar peaks and valleys you see on the printout.
The Electrical System Inside Your Heart
Your heart doesn’t wait for instructions from the brain to beat. It has its own built-in pacemaker: a cluster of specialized cells in the upper right chamber called the SA node. Roughly 60 to 100 times per minute, the SA node fires an electrical impulse that spreads across both upper chambers (the atria), causing them to contract and push blood into the lower chambers.
The signal then funnels through a gateway called the AV node, which briefly delays it so the upper chambers finish emptying before the lower chambers fire. From there, the impulse travels down a highway of fibers (the bundle of His and its branches) and fans out through a dense network of Purkinje fibers that trigger the thick-walled lower chambers (the ventricles) to contract in a coordinated squeeze. That squeeze is what pumps blood to your lungs and the rest of your body. Every peak and dip on an ECG tracing maps directly to one stage of this electrical journey.
How Electrodes Pick Up the Signal
Each electrode is a small adhesive patch containing a silver/silver-chloride disc surrounded by conductive gel. Your heart’s electrical impulses generate tiny voltage differences on the surface of your skin, and the gel bridges the gap between your skin and the metal sensor so those voltages can flow into the machine’s wiring. The machine amplifies signals that are only about one millivolt strong, far too faint for you to feel, and converts them into the moving line on the screen or paper strip.
Because the signal is so small, anything that disrupts skin contact introduces noise. That’s why technicians clean your skin with alcohol, shave chest hair if needed, and ask you to lie still and breathe normally without talking during the recording. Even slight muscle movement can create electrical interference that muddies the tracing.
Electrodes vs. Leads: Why There Are 12 Views
A standard clinical ECG uses 10 electrode patches (four on the limbs, six across the chest) but produces 12 “leads.” A lead isn’t a physical wire. It’s a particular perspective on the heart’s electrical activity, calculated from the voltage difference between two or more electrodes. Think of it like photographing a building from 12 angles: each photo shows the same structure, but some angles reveal details the others miss.
The three basic limb leads form what’s called Einthoven’s triangle, with the heart roughly at the center. Lead I measures the voltage difference between the right arm and left arm. Lead II measures right arm to left leg. Lead III measures left arm to left leg. A fourth electrode on the right leg acts as a reference point. When you collapse these three sides of the triangle and overlay them on the heart, each lead “looks” at electrical activity from a different angle: 0°, +60°, and +120°. A wave of electricity heading straight toward a lead’s positive electrode produces a tall upward spike on that lead. A wave heading perpendicular to a lead produces a flat line. By comparing which leads show the biggest deflections, clinicians can figure out the direction and strength of each electrical impulse.
The six chest electrodes add views from the front and side of the heart, plus three additional calculated limb leads round out the picture. Together, the 12 leads can pinpoint problems in specific regions of the heart muscle that a single viewpoint would miss entirely.
Reading the Waves: P, QRS, and T
The repeating pattern on an ECG strip has three main landmarks, each tied to a specific electrical event in the heart.
- P wave: A small, rounded bump representing the electrical activation of the right and left atria. When this wave looks normal, the upper chambers are firing on schedule.
- QRS complex: The tall, sharp spike (sometimes with a small dip before or after it) that represents the simultaneous activation of both ventricles. Because the ventricles are much larger than the atria, this signal is the biggest feature on the tracing.
- T wave: A broader, gentler bump that shows the ventricles resetting their electrical charge (repolarizing) to prepare for the next beat.
The flat segments between these waves matter too. The gap from the start of the P wave to the start of the QRS complex (the PR interval) normally lasts 0.12 to 0.20 seconds. A longer gap can mean the electrical signal is being delayed at the AV node. The QRS complex itself normally takes 0.06 to 0.10 seconds; a wider complex suggests the impulse is taking a detour through damaged or abnormal tissue. The QT interval, from the start of the QRS to the end of the T wave, has an upper normal limit of about 0.44 seconds after correcting for heart rate. A prolonged QT interval increases the risk of dangerous rhythm disturbances.
What an ECG Can Detect
The 12-lead ECG is one of the first tests ordered when someone has chest pain, shortness of breath, palpitations, or unexplained dizziness, because it can flag a range of problems quickly and painlessly.
Heart attacks leave a distinctive signature. When a coronary artery is blocked and muscle tissue is being starved of oxygen, the segment between the QRS complex and the T wave (the ST segment) rises above the baseline in the leads facing the damaged area. This pattern, called ST-segment elevation, tells emergency teams exactly which part of the heart is in trouble. An inferior wall heart attack, for example, shows ST elevation in leads II, III, and aVF, which look at the bottom of the heart. A blockage in the left main coronary artery can produce ST elevation in lead aVR along with widespread ST depression in seven or more other leads. Some heart attacks don’t produce dramatic ST changes but instead show evolving shifts in the ST segment and T wave over hours, which is why serial ECGs (repeated tracings) are sometimes needed.
Rhythm disorders also show up clearly. Atrial fibrillation replaces the organized P waves with a chaotic, irregular baseline and an unpredictable spacing between QRS complexes. Heart blocks appear as abnormally long PR intervals or dropped QRS complexes. Conditions like an enlarged heart chamber, electrolyte imbalances, or the effects of certain medications can all alter wave shapes or timing in recognizable ways.
Smartwatch ECGs vs. Clinical ECGs
Consumer smartwatches can now record a single-lead ECG by measuring voltage between sensors on the back of the watch and a finger placed on the crown. That single lead is useful for spotting irregular rhythms like atrial fibrillation, but it captures only one of the 12 perspectives a clinical ECG provides. Diagnosing a heart attack requires seeing ST-segment changes in specific lead groupings, which a single lead simply cannot do.
Researchers at Harvard have tested a workaround: holding the back of a smartwatch at eight specific locations on the chest and abdomen to simulate a multi-lead ECG. In that scenario, the watch-generated tracings were 93% to 95% accurate at identifying different types of heart attacks. But the process is awkward, the baseline can be wavy if the watch isn’t pressed firmly, and the results still need a physician to interpret them. For now, a smartwatch is best thought of as a screening tool for rhythm problems, not a replacement for the full 12-lead test you’d get in a clinic or emergency room.
What the Test Feels Like
A standard ECG takes about five to ten minutes and is completely painless. You’ll be asked to lie on an exam table while a technician cleans small areas of your skin with alcohol and attaches the adhesive electrode patches. If you have thick chest hair, they may shave small spots so the patches stick properly. Once everything is connected, you breathe normally and stay still for roughly 10 seconds while the machine records. The electrodes peel off like small bandages, and you can go right back to your normal activities.
No electricity is sent into your body. The machine is only listening to what your heart already produces. There’s no preparation needed on your part: no fasting, no medication changes, no recovery time. The tracing is available almost instantly, which is one reason the ECG remains the most widely used heart test in medicine more than a century after it was invented.