An EKG (electrocardiogram) measures the electrical activity of your heart. Every heartbeat is triggered by a tiny electrical signal that spreads across the heart muscle, causing it to contract and pump blood. An EKG picks up these signals through electrodes placed on your skin, then displays them as a series of waves on paper or a screen. Those waves reveal your heart rate, rhythm, and whether the electrical pathways are functioning normally.
How Electrodes Pick Up Your Heartbeat
Each time your heart beats, an electrical impulse travels through the muscle and radiates outward through your body. That signal is strong enough to be detected on the surface of your skin, though it’s tiny, measured in millivolts. The electrodes stuck to your chest, arms, and legs act as sensors that capture these voltage changes and feed them into the EKG machine.
The machine doesn’t generate any electricity or send anything into your body. It simply records what’s already happening. When the electrical wave moves toward a particular electrode, the tracing draws an upward line. When it moves away, the line dips downward. This is how the familiar peaks and valleys of an EKG tracing are created, each one representing a specific phase of your heartbeat.
What Each Wave Represents
An EKG tracing has three main components, and each one maps to a physical event inside your heart.
The P wave is the first small bump. It represents the electrical activation of the upper chambers (atria) as they prepare to push blood down into the lower chambers. The early part of the P wave reflects the right atrium firing, while the middle and tail end reflect the left atrium.
The QRS complex is the tall, sharp spike in the middle. This is the electrical signal sweeping through the lower chambers (ventricles), the powerful muscular walls responsible for pumping blood to your lungs and the rest of your body. Because the ventricles are much larger than the atria, the QRS complex produces the biggest deflection on the tracing.
The T wave is the gentler bump that follows. It represents the ventricles resetting their electrical charge so they’re ready to fire again. This recovery phase is called repolarization, and abnormalities in the T wave can signal problems ranging from electrolyte imbalances to reduced blood flow.
Heart Rate and Rhythm
EKG paper moves at a standardized speed of 25 millimeters per second. Each small square on the grid equals 0.04 seconds, and each large square (made up of five small ones) equals 0.2 seconds. This means 300 large squares pass by every minute. To calculate heart rate from a regular rhythm, you count the number of large squares between two consecutive QRS peaks and divide that number into 300. Five large squares between beats, for instance, means a heart rate of 60 beats per minute.
When the rhythm is irregular, a longer strip of recording is used. Counting the number of beats across a 10-second stretch and multiplying by six gives the average rate per minute. A normal resting heart rate falls between about 54 and 96 beats per minute based on large population data. Rates above 100 are classified as tachycardia, while rates below 60 are called bradycardia.
Beyond simple rate, the EKG reveals whether your heart’s rhythm originates where it should: the sinus node, a cluster of cells in the right atrium that acts as the heart’s natural pacemaker. In normal sinus rhythm, every P wave is upright in certain leads and is followed by a QRS complex. When extra beats, skipped beats, or chaotic patterns appear, the EKG can identify specific types of arrhythmias like atrial fibrillation, heart block, or ventricular tachycardia.
Normal Intervals and What They Mean
The spaces between waves are just as important as the waves themselves. These intervals reflect how quickly electrical signals travel through different parts of the heart, and delays or extensions can point to specific problems.
- PR interval (125 to 196 milliseconds): the time between the start of the P wave and the start of the QRS complex. It shows how long the signal takes to travel from the atria to the ventricles. A prolonged PR interval can indicate a conduction delay known as heart block.
- QRS duration (69 to 103 milliseconds): how long it takes for the ventricles to depolarize. A widened QRS suggests the signal is taking an abnormal path through the ventricle walls, which can happen with bundle branch blocks or certain medications.
- QT interval (corrected for heart rate, roughly 365 to 458 milliseconds): the total time from ventricular activation to full electrical recovery. A prolonged QT interval increases the risk of dangerous rhythm disturbances.
Detecting a Heart Attack
One of the most critical things an EKG can identify is a heart attack in progress. When a coronary artery is blocked and part of the heart muscle loses its blood supply, the electrical signals in that region change in predictable ways.
The earliest sign is often subtle: the T waves over the affected area become taller, more pointed, and symmetrical. These are called hyperacute T waves. Shortly after, the ST segment, the flat line between the QRS complex and the T wave, begins to rise. ST elevation greater than 1 millimeter in limb leads or greater than 2 millimeters in chest leads, seen in at least two neighboring leads, is the primary EKG criterion for diagnosing an acute heart attack. In severe cases, the QRS, ST segment, and T wave can merge into a single massive wave.
As the heart attack evolves over hours and days, the EKG continues to change. The ST segment gradually settles back down, but the T waves invert, and the R waves (the tall upward peaks) may shrink as abnormal Q waves develop, reflecting permanent damage to the muscle. Meanwhile, leads on the opposite side of the heart often show mirror-image changes: ST depression rather than elevation. This “reciprocal change” is a highly sensitive marker of acute infarction, with a positive predictive value above 90%.
Signs of Heart Wall Thickening
An EKG can suggest that the walls of the heart have thickened, a condition called ventricular hypertrophy. When the muscle mass of the left ventricle increases (often from long-standing high blood pressure), the electrical signal it generates grows larger. On the EKG, this shows up as increased voltage in the QRS complexes, along with changes in the ST segment and T wave.
Several scoring systems exist to identify this pattern. The European Society of Cardiology recommends specific voltage-based criteria as part of routine assessment in people with hypertension. These criteria aren’t perfect, with sensitivity in the range of 30 to 35%, meaning they miss more cases than they catch, but their specificity is high (85 to 88%), meaning a positive result is usually real. An EKG can flag the concern, but an echocardiogram (ultrasound of the heart) is typically needed to confirm it.
12-Lead EKG vs. Smartwatch EKG
A standard clinical EKG uses 10 physical electrodes (placed on your chest, arms, and legs) to generate 12 different electrical “views” of the heart. Six of those views come from chest electrodes positioned across specific points on the ribcage, while the remaining six come from the limb electrodes. Together, they create a detailed, three-dimensional picture of the heart’s electrical activity, covering the front, side, and bottom surfaces.
Consumer devices like smartwatches use a single lead, typically recording between two contact points on the wrist or between a wrist sensor and a fingertip. These single-lead recordings are well-proven for detecting rhythm problems like atrial fibrillation. However, they can’t cover the same territory as a 12-lead setup. A single lead has very limited ability to detect reduced blood flow (ischemia) or pinpoint where a heart attack is occurring, because it only sees one narrow slice of the heart’s electrical field. If your watch flags an irregular rhythm, it’s worth following up, but a normal smartwatch reading doesn’t carry the same reassurance as a normal 12-lead EKG.
What an EKG Cannot Tell You
An EKG is a snapshot of electrical activity, not a direct image of the heart. It cannot show how well the heart is pumping, measure blood flow through the coronary arteries, or detect structural problems like valve disease or holes between chambers. It also only captures what’s happening during the seconds it’s recording. Intermittent problems, like arrhythmias that come and go, may not appear on a brief tracing, which is why extended monitoring over 24 hours or longer is sometimes needed.
That said, as a quick, painless, and inexpensive test that takes about 10 seconds to record, the EKG remains one of the most useful tools in medicine. It can confirm a normal heart rhythm, catch a heart attack within minutes, identify conduction delays, flag signs of thickened heart walls, and reveal dangerous rhythm disturbances, all from a few stickers on your skin.