An ECG (electrocardiogram, also called EKG) records the electrical activity of the heart as a series of waves on graph paper or a digital display. It's quick, painless and non-invasive — 12 electrodes attached to the chest and limbs produce a 12-lead ECG in around 10 seconds. The resulting trace can reveal an enormous amount about the heart's rate, rhythm and condition.
Reading the trace: the key components
- P wave — atrial depolarisation; represents the electrical signal that causes the upper chambers to contract
- PR interval — time from atrial to ventricular activation; prolonged PR suggests a conduction delay (first-degree heart block)
- QRS complex — ventricular depolarisation; represents the main heart contraction; wide QRS suggests a bundle branch block
- ST segment — the period between ventricular depolarisation and repolarisation; elevation suggests STEMI (heart attack); depression may suggest ischaemia
- T wave — ventricular repolarisation; inverted T waves can indicate ischaemia or other conditions
- QT interval — the total time for ventricular depolarisation and repolarisation; prolonged QT raises the risk of dangerous arrhythmias
Common findings explained
- Normal sinus rhythm — regular rhythm, rate 60–100 bpm, all waves normal; this is what 'normal ECG' means
- Sinus tachycardia — fast heart rate with a normal pattern; often caused by fever, anxiety, exercise or medication
- Atrial fibrillation — irregular rhythm with no clear P waves; requires anticoagulation assessment
- Left ventricular hypertrophy — tall QRS complexes indicating a thick left ventricle; associated with longstanding hypertension
- Right bundle branch block — a specific QRS morphology; common as an incidental finding, not always significant
- ST elevation — urgent finding; requires immediate evaluation for a heart attack