Normal ECG: What to Look For
| Wave/Interval | What It Represents | Normal Value |
|---|---|---|
| P wave | Atrial depolarisation (top chambers contract) | Upright in I & II; <120 ms |
| PR interval | AV node conduction time | 120–200 ms |
| QRS complex | Ventricular depolarisation (main pump contracts) | <120 ms (narrow = normal) |
| ST segment | Plateau between contraction & recovery | Isoelectric (flat) — elevation or depression = ischaemia |
| T wave | Ventricular repolarisation | Upright in most leads; inverted = strain/ischaemia |
| QTc interval | Corrected QT — ventricular recovery time | <450 ms (men); <470 ms (women) |
Common ECG Abnormalities
| Finding | Likely Meaning | Action |
|---|---|---|
| ST elevation | STEMI (heart attack) — emergency | 999/911 immediately — thrombolysis/PCI |
| ST depression | NSTEMI or demand ischaemia | Urgent cardiology |
| AF (irregular rhythm, no P waves) | Atrial fibrillation | Rate control, anticoagulation assessment |
| Wide QRS (>120ms) | Bundle branch block or VT | Further assessment |
| Prolonged QTc | Risk of dangerous arrhythmia (Torsades) | Medication review; cardiology |
| Bradycardia <40bpm | Heart block or sinus node disease | Cardiology — may need pacemaker |
| Right axis deviation + S1Q3T3 | Pulmonary embolism pattern | Urgent assessment |
ST Elevation = EmergencyST elevation in ≥2 contiguous leads = STEMI (heart attack). This is a time-critical emergency — every minute of delay increases myocardial damage. Call 999/911 immediately.
What does 'normal sinus rhythm' mean?
The heart is beating regularly at 60–100 bpm with a normal P wave preceding each QRS — the ideal pattern. It means the heart's natural pacemaker (sinus node) is in control.
What is bundle branch block?
A delay in electrical conduction through one branch of the His-Purkinje system, causing a wide QRS. Right BBB is often benign; Left BBB can indicate underlying heart disease and warrants investigation.
Can a normal ECG rule out heart disease?
No. A resting ECG is normal in ~50% of people who later have a heart attack. Exercise ECG or other investigations may be needed if symptoms suggest ischaemia despite a normal resting ECG.
What causes a prolonged QT interval?
Common causes: medications (many — check the CredibleMeds database), hypokalaemia, hypomagnesaemia, hypothyroidism, congenital Long QT syndrome. Significant as it predisposes to Torsades de Pointes arrhythmia.
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.