What Does an ECG Measure?
An ECG (electrocardiogram) records the electrical activity of the heart via electrodes placed on the chest, arms and legs. Each heartbeat produces a characteristic waveform:
- P wave: Electrical activation of the atria (upper chambers) — represents normal sinus node firing
- PR interval: Time from atrial activation to ventricular — delay through AV node (normal: 120–200ms)
- QRS complex: Electrical activation of the ventricles (lower chambers) — causes the main heartbeat contraction (normal: <120ms)
- ST segment: Period between ventricular activation and recovery — changes here indicate ischaemia or injury
- T wave: Ventricular repolarisation (recovery)
- QT interval: Total ventricular electrical cycle — prolongation increases risk of dangerous arrhythmias
Common ECG Report Terms Explained
| ECG Finding | What it Means | Action |
|---|---|---|
| Normal Sinus Rhythm (NSR) | Regular rhythm, 60–100 bpm, with normal P waves before each QRS | Normal — no action needed |
| Sinus Tachycardia | Regular rhythm >100 bpm from sinus node | Find the cause: fever, anxiety, anaemia, thyroid, dehydration, pain — not the rhythm itself that needs treating |
| Sinus Bradycardia | Regular rhythm <60 bpm from sinus node | Normal in athletes; investigate if symptomatic (dizziness, syncope) or <40 bpm |
| Left Ventricular Hypertrophy (LVH) | Thickened heart muscle from chronically high blood pressure | Investigate and treat underlying hypertension; echo confirms |
| ST Depression | Suggests myocardial ischaemia (reduced blood flow to heart muscle) | Urgent cardiac assessment; stress test or angiography may be needed |
| ST Elevation (STEMI) | Heart attack (blockage of a coronary artery) until proven otherwise | MEDICAL EMERGENCY — call 999/112 immediately; needs immediate reperfusion |
| Left Bundle Branch Block (LBBB) | Delay in left ventricle conduction; can be new (ischaemia) or old (chronic) | New LBBB with chest pain = STEMI equivalent — emergency. Old LBBB: investigate cause (hypertension, cardiomyopathy) |
| Right Bundle Branch Block (RBBB) | Delay in right ventricle conduction | Isolated RBBB is usually benign; full cardiac assessment to exclude structural disease |
| Prolonged QT Interval | Increased risk of Torsades de Pointes (dangerous arrhythmia); normal QTc: <440ms men, <460ms women | Check all medications (many drugs prolong QT); check electrolytes (K+, Mg2+, Ca2+); avoid QT-prolonging drugs |
| Atrial Fibrillation (AF) | Chaotic irregular rhythm from atria firing randomly; no P waves; irregular RR intervals | Anticoagulation to prevent stroke (CHA2DS2-VASc score); rate or rhythm control |
| Ectopic Beats (PVCs / PACs) | Extra beats from ventricles or atria outside normal rhythm | Occasional ectopics are common and usually benign; frequent PVCs (>10,000/day) warrant further workup |
Blood Tests Ordered Alongside ECG
| Blood Test | Why Ordered | Key Finding |
|---|---|---|
| Troponin (I or T) | Heart attack detection | Elevated >99th percentile with clinical context = NSTEMI or STEMI |
| BNP or NT-proBNP | Heart failure | Very elevated suggests heart failure; guides admission and treatment decisions |
| Potassium (K+) | Arrhythmia risk | Low potassium (hypokalaemia) causes T-wave flattening, U waves, dangerous arrhythmias |
| Magnesium (Mg2+) | Arrhythmia, QT prolongation | Low magnesium worsens hypokalaemia and increases arrhythmia risk |
| TSH | Thyroid disease affects heart rate | Hyperthyroidism causes tachycardia, AF; hypothyroidism causes bradycardia, long QT |
| CBC | Anaemia causes compensatory tachycardia | Low haemoglobin → sinus tachycardia and can worsen angina |
When Your ECG is an Emergency: ST elevation in two or more leads = STEMI (heart attack). Call emergency services immediately. New LBBB with chest pain is also treated as a STEMI equivalent. Sustained ventricular tachycardia (wide complex tachycardia) = emergency. Do not drive yourself to hospital — call an ambulance.
Questions to Ask Your Doctor
- Is this ECG finding new, or was it present on a previous ECG?
- Does the ST change on my ECG suggest I need urgent cardiac investigation?
- My QT is prolonged — which of my medications could be causing this?
- Do I have AF — do I need a blood thinning medication?
- Should I have an echocardiogram to look at the heart structure?
Medical Disclaimer: This information is for educational purposes only. ECG interpretation is a clinical skill requiring medical training. Never interpret your own ECG to make treatment decisions — always discuss with your doctor. Chest pain with ECG changes is a potential emergency.