An electrocardiogram (ECG or EKG) reveals a surprising amount about your heart and overall health from just a few seconds of recording. It captures the electrical signals that trigger every heartbeat, and the shape, timing, and size of those signals tell clinicians whether your heart is beating at the right speed, in the right rhythm, and whether the muscle itself is healthy. A standard resting ECG takes under 10 minutes, involves no needles or radiation, and produces a tracing that can flag everything from a heart attack in progress to dangerously abnormal potassium levels.
What the Waves Actually Represent
An ECG tracing looks like a repeating series of bumps and dips. Each one corresponds to a specific electrical event happening inside the heart.
The P wave is the first small bump. It represents the electrical signal spreading across the upper chambers (atria), telling them to squeeze and push blood into the lower chambers. The first half of the P wave comes mainly from the right atrium, the second half from the left.
The QRS complex is the tall, sharp spike that follows. This is the electrical signal firing through the lower chambers (ventricles), the heart’s main pumping force. The entire QRS normally lasts only 80 to 100 milliseconds, and different slices of that brief window represent different parts of the ventricle walls activating in sequence.
The T wave is the broader, gentler bump after the QRS. It shows the ventricles resetting their electrical charge, preparing for the next beat. Changes in the T wave’s shape or direction are some of the most clinically important clues an ECG provides.
Heart Rate and Rhythm
The most basic information an ECG gives is how fast and how regularly your heart beats. Heart rate is calculated from the distance between consecutive QRS spikes: divide 300 by the number of large grid squares between two beats. Three squares means a rate of 100 beats per minute, five squares means 60. Normal resting heart rate falls between roughly 60 and 100.
Rhythm is just as important as rate. A normal heartbeat originates from the heart’s natural pacemaker and follows a predictable sequence: P wave, then QRS, then T wave, repeating at steady intervals. Some variation with breathing is normal and harmless. But when the ECG shows beats arriving at irregular intervals, missing P waves, or extra spikes in unexpected places, it points to an arrhythmia.
Arrhythmias and Abnormal Rhythms
ECGs are the primary tool for identifying heart rhythm disorders. Atrial fibrillation, the most common sustained arrhythmia, produces a distinctive pattern: an “irregularly irregular” rhythm with no identifiable P waves and a chaotic, shifting baseline between beats. The ventricular rate can be normal, slow, or fast. When it exceeds 100 beats per minute, it’s described as atrial fibrillation with rapid ventricular response, which often causes palpitations, shortness of breath, or dizziness.
Other rhythm problems the ECG can catch include extra beats originating from the ventricles, abnormally fast heart rhythms from either the upper or lower chambers, and conduction blocks where electrical signals are delayed or stopped on their way through the heart. Each leaves a specific fingerprint on the tracing.
Signs of a Heart Attack
One of the most urgent reasons to run an ECG is to check for a heart attack. When a coronary artery is completely blocked and heart muscle is being starved of oxygen, the segment of the tracing between the QRS complex and the T wave (called the ST segment) rises above its normal baseline. This pattern, called ST-segment elevation, typically means a major artery is fully obstructed and the heart muscle needs blood flow restored immediately.
Not all heart attacks produce this classic change. Some cause subtler shifts, like ST depression or T-wave inversion, which still signal damage but suggest the blockage may be partial. After a heart attack has healed, the ECG can also show permanent changes, such as abnormal Q waves, that serve as a lasting record of where the damage occurred.
Heart Chamber Size and Wall Thickness
When the heart’s walls thicken or its chambers enlarge, the electrical signals they generate become larger or take longer to travel. The ECG picks this up as taller or wider waves than normal. Left ventricular hypertrophy, a thickening of the heart’s main pumping chamber often caused by long-standing high blood pressure, is one of the most commonly detected structural changes. Clinicians look at the height of certain waves across multiple leads and apply voltage criteria that differ for men and women.
Similarly, an unusually wide or tall P wave can suggest enlargement of one or both atria, a finding that sometimes accompanies valve disease, heart failure, or chronic lung conditions. These aren’t definitive diagnoses on their own, but they flag the need for further imaging like an echocardiogram.
Electrical Conduction Problems
The heart has a built-in wiring system that routes electrical signals in a specific order. When part of that system is damaged or slow, the ECG reveals it through timing abnormalities. The PR interval, which measures the delay between the atria firing and the ventricles firing, normally lasts 120 to 200 milliseconds. A PR interval that’s too long suggests a conduction block between the upper and lower chambers. A QRS complex wider than 100 milliseconds suggests a block in one of the branches that carry signals through the ventricles.
The QT interval, which captures the full cycle of ventricular electrical activity, is also closely watched. A QT interval that’s too long (above about 420 milliseconds at a normal heart rate) increases the risk of dangerous rhythm disturbances. Some medications, genetic conditions, and electrolyte imbalances can all stretch the QT interval, and the ECG is the simplest way to monitor it.
Electrolyte Imbalances
This is one of the less obvious things an ECG reveals: the chemical balance of your blood. Potassium levels, in particular, leave dramatic marks on the tracing. When potassium is too high, the T waves become tall, narrow, and “peaked.” As levels climb further, the P wave flattens and eventually disappears, the QRS complex widens, and the heart is at risk of stopping entirely. Severely elevated potassium (above roughly 7.0 mmol/L) can cause heart block, cardiac arrest, or fatal rhythm disturbances.
Low potassium creates different but equally dangerous changes. It extends the electrical recovery phase of each heartbeat, which can trigger a type of chaotic ventricular rhythm that may degenerate into cardiac arrest. Calcium abnormalities also alter the ECG, primarily by shortening or lengthening the interval between the QRS and T wave.
What an ECG Cannot Tell You
For all its usefulness, an ECG has real blind spots. It captures a snapshot of electrical activity at a single moment in time. If an arrhythmia comes and goes, a standard ECG may look completely normal if the rhythm happens to be stable during the recording. That’s why doctors sometimes use portable monitors worn for 24 hours or longer to catch intermittent problems.
An ECG also cannot reliably detect partially blocked arteries that aren’t currently causing damage. Exercise stress testing with ECG improves detection somewhat, but even then, sensitivity and specificity for coronary artery disease hover around only 60% to 70%. Significant blockages can exist without producing any ECG abnormality at rest. Imaging tests like CT angiography or invasive angiography remain the gold standard for mapping the coronary arteries.
Structural details like valve function, blood flow patterns, and the precise thickness of heart walls require ultrasound or other imaging. The ECG hints at these problems through indirect electrical clues, but it can’t measure them directly.
How the Test Works
A standard 12-lead ECG uses 10 small adhesive electrode patches placed on your chest, arms, and legs. Four go on the limbs (one near each wrist or shoulder and one near each ankle), and six are positioned across the left side of the chest in a specific arc from the breastbone to the side of the ribcage. Despite being called “12-lead,” this refers to 12 different electrical perspectives calculated from those 10 electrodes, giving a three-dimensional picture of the heart’s activity.
The recording itself takes about 10 seconds of lying still. The entire process, including placing and removing the electrodes, is painless and typically finished in under 10 minutes. No special preparation is needed, though you may be asked to avoid lotions on your chest so the electrodes stick properly.
AI-Enhanced ECG Interpretation
Newer artificial intelligence models are expanding what a standard ECG can detect. One AI system designed to identify complete coronary artery blockages correctly flagged 93.8% of patients with an obstructed artery in a recent study published in JACC: Advances. The same system also identified most patients who did not have a blockage, which could reduce unnecessary emergency procedures. Still, 6% of cases were missed, and these diagnostic algorithms remain aids to diagnosis rather than standalone tools. The ECG’s power has always come from combining a simple, fast recording with skilled interpretation, and AI is sharpening that interpretation rather than replacing it.