An electrocardiogram (ECG, or EKG in American English) records the electrical activity of your heart as a series of waves across a paper or digital trace. The report your doctor receives is written in cardiology shorthand — and most patients who receive a copy have little way of interpreting it.
This guide explains the key measurements and findings that appear on a standard ECG report, in plain language.
What an ECG measures
The heart generates electrical impulses that travel in a predictable sequence, causing the chambers to contract in the right order. An ECG captures this electrical journey from multiple angles — using electrodes placed on the chest, arms and legs — and records it as a series of waves: P, Q, R, S and T.
Heart rate
Normal resting heart rate: 60–100 beats per minute. Below 60 is bradycardia; above 100 is tachycardia. Both can be normal in context (athletes often have resting heart rates in the 40s; anxiety or exercise raises it) or a sign of pathology. The ECG distinguishes between different types of fast and slow rhythms.
Heart rhythm: normal sinus rhythm
Normal sinus rhythm means the heart's natural pacemaker (the sinoatrial node) is in charge and producing a regular, well-organised beat. An ECG report that states 'normal sinus rhythm' is generally reassuring. Any departure from this — atrial fibrillation, ventricular ectopics, heart block — will be named specifically.
Atrial fibrillation (AF) is the most common abnormal rhythm found on ECG. It appears as an irregularly irregular beat with no identifiable P waves. AF significantly raises stroke risk and usually requires treatment.
Key intervals
- **PR interval** (normal 120–200 ms) — time from the start of atrial activation to the start of ventricular activation. Prolonged PR indicates a conduction delay (first-degree heart block).
- **QRS duration** (normal < 120 ms) — time for the ventricles to depolarise. A wide QRS suggests a bundle branch block or ventricular pacing.
- **QT interval** — time from ventricular depolarisation to repolarisation. A prolonged QT raises risk of dangerous arrhythmias and can be caused by some medications, electrolyte disturbances and inherited conditions.
ST segment: the critical finding
The ST segment sits between the end of the QRS complex and the start of the T wave. ST elevation in two or more adjacent leads is the hallmark of a STEMI (ST-elevation myocardial infarction) — a full heart attack requiring emergency intervention. ST depression and T wave inversion can signal ischaemia (reduced blood flow to the heart) and are also taken seriously.
Common benign findings
Not every abnormal-sounding ECG finding requires action. Early repolarisation (a slightly elevated ST segment in young, healthy people) is typically benign. Left ventricular hypertrophy (larger-amplitude waves) often reflects high blood pressure and indicates the need for better control rather than an acute problem. Isolated occasional ectopic beats are common and usually harmless.
The correct interpretation of an ECG always requires clinical context — symptoms, age, risk factors and previous tracings. An ECG stored in your health record gives your doctor a baseline to compare against at every future test.