An EKG (electrocardiogram) shows the electrical activity of your heart, recorded as a series of waves on paper or a screen. It reveals your heart rate, rhythm, and the timing of electrical signals as they travel through each chamber. In doing so, it can detect heart attacks, irregular heartbeats, thickened heart walls, electrolyte imbalances, and problems with the wiring that keeps your heart beating in sync.
How the Test Works
A standard clinical EKG uses 10 small adhesive sensors placed on your arms, legs, and chest. These sensors detect the tiny electrical impulses your heart generates with every beat, and they record them from 12 different angles (which is why you’ll hear it called a “12-lead EKG”). The whole process takes just a few minutes. You don’t need any special preparation, though a technician may shave small patches of chest hair so the sensors stick properly.
The result is a printout of waves, each representing a specific electrical event in the heart. Doctors read the tracing systematically: rate, rhythm, electrical axis, time intervals between waves, and the shape of the waves themselves. Abnormalities in any of these can point to a specific problem.
The Waves and What They Mean
Every heartbeat produces three main wave patterns on an EKG. The P wave is a small bump that represents the upper chambers (atria) firing. The QRS complex is the tallest spike and represents the lower chambers (ventricles) contracting, which is the main pumping action. The T wave follows and represents the ventricles resetting their electrical charge to prepare for the next beat.
The spacing between these waves matters just as much as the waves themselves. A longer-than-normal gap between the P wave and the QRS complex can mean electrical signals are being delayed on their way from the upper to lower chambers. A widened QRS complex can mean signals are taking a detoured path through the ventricles. These timing details turn an EKG into a map of your heart’s internal wiring.
Heart Rate and Rhythm
The most basic thing an EKG shows is how fast your heart is beating and whether the rhythm is regular. A normal resting heart rate falls between 60 and 100 beats per minute, driven by the heart’s natural pacemaker (the SA node). On a normal tracing, you’ll see an identical P wave before every QRS complex, spaced at even intervals.
When the rhythm is abnormal, the pattern changes in specific, recognizable ways. In atrial fibrillation, the most common serious rhythm disorder, the upper chambers fire chaotically with no organized pattern. The P waves disappear entirely, and the spacing between beats becomes “irregularly irregular,” meaning there’s no predictable pattern at all. In atrial flutter, the upper chambers beat at roughly 300 times per minute, creating a distinctive sawtooth pattern on the tracing, though the ventricles typically respond at a slower rate of about 150 beats per minute.
An EKG can also detect rhythms originating from the wrong part of the heart. If the SA node fails, a backup pacemaker in the middle of the heart can take over at a slower rate of 40 to 50 beats per minute. If that fails too, the ventricles themselves can generate a rhythm, but only at 30 to 40 beats per minute, which is often too slow to maintain normal blood flow.
Heart Attacks and Reduced Blood Flow
One of the most critical things an EKG detects is a heart attack in progress. When a coronary artery is completely blocked, the affected heart muscle loses its blood supply, and the electrical signals in that area change. This shows up as an elevation of the ST segment, the flat line between the QRS complex and the T wave. That elevation is the hallmark of what’s called a STEMI, the type of heart attack that requires emergency intervention to reopen the artery.
The location of these changes on the 12-lead tracing tells doctors which artery is blocked. ST elevation in the leads across the front of the chest points to a blockage in the artery supplying the front wall of the heart. Elevation in the bottom leads suggests the artery feeding the underside of the heart is affected. At the same time, the opposite leads often show ST depression, a mirror-image change that helps confirm the diagnosis.
Not all heart attacks produce ST elevation, though. Some cause subtler changes like T-wave inversion or ST depression without elevation. These patterns suggest reduced blood flow without a complete blockage, and they can be harder for both humans and computer algorithms to catch. Studies of automated EKG interpretation have found that computerized readings miss roughly 70% of cases in the broader category of acute coronary syndrome, which includes heart attacks without classic ST elevation. This is one reason doctors always interpret the tracing themselves rather than relying solely on the machine’s printout.
Conduction Blocks
Electrical signals travel from the top of the heart to the bottom through a system of specialized pathways, including a left and right “bundle branch” that carries the signal to each side of the ventricles. When one of these branches is damaged or blocked, the signal has to take a longer route, widening the QRS complex on the EKG. A right bundle branch block and a left bundle branch block each produce a distinctive QRS shape that doctors can tell apart at a glance.
Blocks can also occur higher up, at the junction between the upper and lower chambers. A first-degree block simply slows the signal, stretching the PR interval. A second-degree block means some signals get through and others don’t, causing skipped beats. A third-degree (complete) block means no signals pass through at all, and the ventricles have to beat on their own at their much slower backup rate.
Enlarged or Thickened Heart Chambers
When the heart works harder than normal for months or years, its walls can thicken or its chambers can stretch. An EKG picks up these changes through voltage: thicker muscle generates stronger electrical signals, which show up as taller waves. Left ventricular hypertrophy, often caused by long-standing high blood pressure, produces characteristically tall waves in the leads overlying the left side of the heart. Several scoring systems exist to diagnose it, but they all look for the same thing: voltages that exceed normal thresholds.
Enlargement of the upper chambers shows up differently. An enlarged left atrium produces a wider-than-normal P wave (longer than 120 milliseconds) or a deep downward deflection at the end of the P wave in certain chest leads. These changes can be an early clue that the heart is under strain, even before symptoms appear.
Electrolyte Imbalances
Your heart’s electrical system depends on the right balance of minerals in your blood, and an EKG can reveal when that balance is off. High potassium is one of the most dangerous examples. As levels rise, the T waves become tall and peaked, the P waves flatten and eventually disappear, and the QRS complex widens. In severe cases, the T wave and QRS can merge into a sinusoidal wave that signals the heart is close to stopping.
Low potassium produces a different set of changes: the T waves become shallow, the ST segment dips, and a new wave called a U wave appears just after the T wave. In severe cases, the U wave can grow so large it swallows the T wave entirely, making the overall interval look deceptively prolonged. These EKG clues can prompt a blood test that confirms the diagnosis and guides treatment before the imbalance becomes life-threatening.
What an EKG Can Miss
An EKG is a snapshot of about 10 seconds. If an arrhythmia comes and goes, a standard EKG may catch a perfectly normal moment and miss the problem entirely. This is why doctors sometimes order longer monitoring, such as a 24-hour Holter monitor or a wearable patch worn for days or weeks, to catch intermittent episodes.
Structural problems like valve disease, holes between chambers, or fluid around the heart don’t reliably show up on an EKG either. These typically require an echocardiogram (an ultrasound of the heart) to diagnose. An EKG may hint at structural issues through indirect signs like chamber enlargement or strain patterns, but it can’t image the heart directly.
Wearable EKGs vs. Clinical EKGs
Smartwatches and portable devices that record a single-lead EKG have become widely available, and they can be useful for catching irregular rhythms like atrial fibrillation. But they have real limitations compared to a clinical 12-lead EKG. A single lead gives one viewing angle instead of twelve, which means it can identify the QRS complex reliably but often struggles with the smaller, subtler features like P waves, ST-segment changes, and T-wave shape. In one study comparing a wearable garment device to a standard Holter monitor, the wearable produced diagnostic-quality tracings 70% of the time, while the Holter was readable 99% of the time.
A wearable EKG is a reasonable screening tool for rhythm problems, but it can’t replace a 12-lead EKG when it comes to diagnosing heart attacks, conduction blocks, chamber enlargement, or electrolyte effects. If your wearable flags something abnormal, it’s a prompt to get a full clinical EKG, not a substitute for one.