An EKG (electrocardiogram) shows the electrical activity of your heart, recorded as a pattern of waves on paper or a screen. Each wave corresponds to a specific event in your heartbeat: the upper chambers contracting, the lower chambers contracting, and the heart resetting for the next beat. From this electrical snapshot, doctors can identify irregular rhythms, signs of a heart attack, thickened heart muscle, and even imbalances in blood chemistry like potassium levels.
How the Test Works
Ten small electrode patches are placed on your body: one on each limb and six across your chest. These electrodes detect the tiny electrical signals your heart generates with every beat. Because the signals travel through your body to the skin’s surface, the electrodes can pick them up without any needles or incisions. The machine records 12 different “views” of your heart’s electrical activity, each from a slightly different angle, giving a detailed picture of how electricity moves through the heart from multiple directions.
The entire test takes only a few minutes and is completely painless. You lie still while the machine records, and the results print out almost immediately.
What the Waves Mean
The EKG tracing has three main wave patterns that repeat with each heartbeat:
- P wave: A small bump that represents the upper chambers (atria) firing electrically and contracting to push blood into the lower chambers.
- QRS complex: A sharp, tall spike that represents the lower chambers (ventricles) firing and contracting to pump blood out to the lungs and body. This is the main pumping action of the heart.
- T wave: A broader, gentler wave that represents the ventricles resetting their electrical charge, preparing for the next beat.
The spacing between these waves matters just as much as the waves themselves. The gap between the P wave and the QRS complex (called the PR interval) normally lasts 0.12 to 0.20 seconds. A longer gap means electrical signals are being delayed on their way from the upper to lower chambers. The QRS complex itself normally lasts 0.06 to 0.10 seconds; a wider complex suggests the electrical signal is taking an abnormal path through the ventricles.
Heart Rhythm Problems
One of the most common reasons for an EKG is to check your heart’s rhythm. A normal heart beats in a steady, predictable pattern with consistent spacing between beats. The EKG makes rhythm problems immediately visible.
Atrial fibrillation, the most common serious rhythm disorder, has a distinctive look on an EKG: the spacing between heartbeats is completely irregular (no two gaps are the same length), and the P waves disappear entirely. Instead of the neat little bumps that show the upper chambers firing in an organized way, the baseline becomes chaotic with small, erratic squiggles called fibrillatory waves. This happens because the upper chambers are quivering instead of contracting in a coordinated fashion.
Other rhythm abnormalities the EKG can reveal include a heart rate that’s too fast (tachycardia, over 100 beats per minute), too slow (bradycardia, under 60), or beats that originate from the wrong part of the heart. Extra beats, skipped beats, and dangerous rhythms like ventricular fibrillation all produce recognizable patterns.
Signs of a Heart Attack
During a heart attack, part of the heart muscle loses its blood supply, and the injured tissue produces abnormal electrical signals. This shows up on the EKG primarily in a segment called the ST segment, which is the flat stretch between the QRS spike and the T wave. Normally this segment sits right at the baseline. When heart muscle is actively being damaged, it shifts upward (ST elevation) or downward (ST depression) from the baseline.
ST elevation is the hallmark of a major heart attack where a coronary artery is completely blocked. It’s treated as an emergency. ST depression and inverted T waves typically indicate the heart muscle is under stress from reduced blood flow but the artery isn’t fully blocked.
The EKG is highly reliable for confirming a heart attack when changes are present, with a specificity of 95% to 97%. But here’s the important catch: its sensitivity is only about 30%. That means roughly 7 out of 10 heart attacks may not show obvious changes on a single EKG, especially early on. This is why doctors often repeat the EKG, run blood tests for heart damage markers, and use other imaging when they suspect a heart attack despite a normal-looking tracing.
Thickened Heart Muscle
When the heart’s walls grow thicker than normal, usually from long-standing high blood pressure, the extra muscle generates stronger electrical signals. On the EKG, this appears as taller-than-normal waves, particularly the QRS complex. Doctors use several voltage-based criteria to diagnose left ventricular hypertrophy (thickening of the heart’s main pumping chamber). For instance, one common method adds the height of certain waves together, with different thresholds for men and women.
The EKG can also show signs of strain on the thickened muscle, including changes in the ST segment and T wave shape. These strain patterns suggest the thickened heart is working harder than it should be and may not be getting enough blood flow to meet its increased demands.
Electrolyte Imbalances
Your heart’s electrical system depends heavily on the balance of minerals in your blood, especially potassium. An EKG can reveal dangerous shifts in potassium levels before blood test results come back, which matters because severe imbalances can trigger fatal heart rhythms.
High potassium produces a characteristic progression of changes. The earliest sign is tall, sharply peaked T waves. As levels rise further, the P waves flatten and disappear, the QRS complex widens, and the PR interval stretches out. In severe cases, the widened QRS and tall T wave can merge together into a sinusoidal (wavy) pattern that signals imminent cardiac arrest.
Low potassium causes nearly opposite changes: the T waves flatten, a new wave called a U wave appears after the T wave, and the ST segment dips below baseline. In severe cases, the U wave can become so prominent that it dwarfs the T wave entirely.
What a Normal EKG Doesn’t Rule Out
A normal EKG is reassuring, but it has real blind spots. The test captures a snapshot of electrical activity at one moment in time. If you have coronary artery disease but your arteries aren’t starving the heart of blood at that exact moment, the EKG may look perfectly normal. In one study of patients with chest pain and normal resting EKGs, only about 62% of those with confirmed coronary artery disease showed abnormalities on extended monitoring.
The EKG also doesn’t directly show the structure of your heart. It can’t visualize valve problems, measure how well the heart pumps, or detect fluid around the heart. Those require imaging tests like an echocardiogram (ultrasound of the heart). Similarly, blocked arteries that haven’t yet caused damage won’t show up. A stress test, which records your EKG while you exercise, improves detection by forcing the heart to work harder and potentially revealing problems that a resting EKG misses.
Think of the EKG as an electrical audit, not a complete physical exam of the heart. It’s fast, cheap, painless, and excellent at catching rhythm disorders and active damage. But for a full picture of heart health, it’s often one piece of a larger diagnostic puzzle.