An EKG (electrocardiogram) works by detecting the tiny electrical signals your heart produces each time it beats. Ten small electrodes placed on your skin pick up these signals and translate them into a line tracing on paper or a screen, giving a detailed picture of your heart’s electrical activity in real time. The entire test takes about 10 minutes and is completely painless.
Your Heart’s Built-In Electrical System
To understand what an EKG is measuring, it helps to know that your heart runs on electricity. Every heartbeat starts with a natural pacemaker called the SA node, a small cluster of cells in the upper right chamber. This node fires an electrical signal that spreads across both upper chambers (the atria), causing them to contract and push blood downward into the lower chambers.
The signal then hits a relay station called the AV node, which sits near the center of the heart. The AV node deliberately pauses the signal for a fraction of a second. That tiny delay is important: it ensures the upper chambers finish emptying before the lower chambers start squeezing. From there, the signal travels down a bundle of specialized fibers through the center of the heart and fans out through a network called the Purkinje fibers. When those fibers deliver the signal to the lower chambers (the ventricles), the ventricles contract powerfully, sending blood to the lungs and out to the rest of the body.
This entire sequence, from the SA node’s initial spark to the ventricles contracting, happens in less than a second. An EKG captures every stage of that sequence as a specific wave on the readout.
What the Waves on an EKG Mean
The line tracing on an EKG isn’t random. Each bump and dip corresponds to a specific electrical event in the heart:
- P wave: The first small bump. It records electrical activity moving through the upper chambers as they contract.
- QRS complex: The tall, sharp spike in the middle. This captures the electrical impulse firing through the lower chambers, triggering the heart’s main pumping action.
- T wave: The smaller bump after the QRS. It shows the lower chambers resetting electrically and preparing for the next beat.
The spaces between these waves matter just as much as the waves themselves. The gap between the P wave and the QRS complex (the PR interval) shows how long the AV node delays the signal. In a healthy heart, this interval lasts 120 to 200 milliseconds. The QRS complex itself should be narrow, around 80 to 100 milliseconds, meaning the electrical signal moves through the ventricles quickly and efficiently. A wider QRS can suggest the signal is taking an abnormal path. The QT interval, which spans from the start of the QRS to the end of the T wave, reflects the total time the ventricles take to fire and reset. At a resting heart rate of 60 beats per minute, this should be 420 milliseconds or less.
When any of these intervals fall outside their normal range, or when the waves themselves look distorted, stretched, or missing, it points a cardiologist toward specific problems.
How the Electrodes Are Placed
A standard clinical EKG uses 10 physical electrodes to generate 12 different “views” of the heart, which is why you’ll hear it called a 12-lead EKG. Four electrodes go on the limbs: one on each arm and one on each leg. The remaining six are placed across the chest in precise locations. Two sit on either side of the breastbone at the fourth rib space. The others continue in a line around the left side of the chest, following specific landmarks down to the midpoint under the left arm.
Each pair of electrodes measures the heart’s electrical activity from a different angle. Think of it like photographing an object from 12 directions instead of one. A problem that’s invisible from one angle often shows up clearly from another. This is why the chest electrodes are positioned so specifically: shifting one even a couple of inches can change the reading.
What an EKG Can Detect
An EKG is one of the first tests ordered when heart trouble is suspected because it reveals a wide range of problems quickly. Irregular heart rhythms (arrhythmias) show up as disruptions in the normal wave pattern. A heart that beats too fast, too slow, or out of sequence produces a distinctly abnormal tracing. Damage from a heart attack leaves characteristic changes in the wave shapes, particularly in the segment between the QRS and T wave. Thickened heart walls, enlarged chambers, and electrolyte imbalances like abnormal potassium levels all leave their own signatures on the readout.
The limitation is that a standard EKG captures only about 10 seconds of activity. If your heart rhythm problem comes and goes, a brief snapshot might miss it entirely. That’s where longer monitoring comes in. A Holter monitor is essentially a portable EKG recorder you wear for 24 hours or more, providing a continuous “movie” of your heart’s electrical activity instead of a quick snapshot.
How the Paper Grid Works
EKG tracings are printed on special graph paper with a precise grid. Time runs along the horizontal axis: each small square represents 0.04 seconds, and five small squares (one large square) equal 0.20 seconds. Voltage runs along the vertical axis: 10 millimeters of height equals 1 millivolt of electrical activity. This standardization is what allows doctors to measure intervals down to the millisecond and compare your results against established norms. A wave that’s too tall might indicate a thickened heart muscle. One that’s too flat could suggest damage or fluid around the heart.
Smartwatch EKGs vs. Clinical EKGs
Consumer devices like the Apple Watch and Samsung Galaxy Watch can now record a single-lead EKG from your wrist. These work on the same basic principle, detecting electrical signals through skin contact, but they capture only one view of the heart instead of twelve. That’s a significant difference.
Single-lead smartwatch EKGs are primarily designed to screen for atrial fibrillation, the most common serious heart rhythm disorder. Their detection algorithms score well in studies, with sensitivity and specificity in the 91 to 99 percent range. But there’s a practical catch: roughly 20 to 24 percent of smartwatch recordings come back uninterpretable, meaning the device can’t produce a clean enough signal to analyze. Motion, poor skin contact, and sweat all contribute to this. A six-lead portable device, by comparison, achieved 99 percent sensitivity and 97 percent specificity in head-to-head testing, significantly outperforming both single-lead smartwatches.
For detecting conditions beyond atrial fibrillation, such as heart attacks, conduction blocks, or electrolyte problems, a single-lead consumer device simply doesn’t capture enough information. The 12 views from a clinical EKG remain necessary for anything beyond basic rhythm screening.