ECG Interpretation

A systematic approach to the 12-lead ECG in general practice and urgent care: the patterns that change immediate management, and when a trace needs an urgent second opinion. Management of individual arrhythmias, ischaemia and heart failure lives on their own pages.

Indications

Symptoms are the most common trigger: palpitations, chest pain, dizziness, syncope, dyspnoea. It is also indicated for suspected electrolyte disturbance, before starting a QT-prolonging or cardiotoxic drug, in the preoperative assessment of at-risk patients, and as part of some sports and occupational screening. There are no absolute contraindications; the practical limits are patient refusal and adhesive allergy.

Reading the trace

Read every ECG in the same order, every time, rather than jumping straight to the abnormality that prompted the request — a fast, consistent method catches incidental findings a targeted look misses.

Rate. Divide 300 by the number of large squares between two R waves (or 1500 by the number of small squares) for a regular rhythm; for an irregular rhythm, count beats on the 10-second strip and multiply by 6. Normal is 60–99 bpm; below 60 is bradycardia, above 100 is tachycardia.

Rhythm. Look at leads I, II, aVF and V1 for five things: regular P waves or not, QRS width, whether each P is followed by a QRS, regular or irregular overall rhythm, and P-wave morphology. Normal sinus rhythm shows upright P waves in I, II and aVF, and negative in aVR. Absent discrete P waves with an irregularly irregular narrow-QRS rhythm is the hallmark of atrial fibrillation.

Axis. A quick estimate: QRS positive in both I and aVF is a normal axis; positive in I but negative in aVF is left axis deviation; negative in I but positive in aVF is right axis deviation; negative in both is extreme/ indeterminate axis. Left axis deviation occurs with old inferior MI, LVH and left bundle branch block; right axis deviation with RVH, pulmonary hypertension, hyperkalaemia and WPW.

Intervals.

• PR interval (normal 120–200 ms): prolonged in first-degree AV block; short with an accessory pathway (WPW, Lown-Ganong-Levine).
• QRS duration (normal <120 ms): widened by bundle branch block, a ventricular rhythm, or hyperkalaemia.
• QT interval (normal <440 ms, corrected for rate): prolongation raises the risk of torsades de pointes — see Prolonged QT & Torsades for causes and management. A short QT (<360 ms) suggests hypercalcaemia, hyperkalaemia or short QT syndrome.

Waveform morphology.

• P wave: abnormal size/shape suggests atrial enlargement (right atrial — tall peaked P in II; left atrial — broad notched P, terminal negativity in V1).
• QRS/Q wave: a pathological Q wave (≥40 ms wide, ≥1 mm deep, or ≥25% of QRS amplitude) suggests prior infarction. Reduced R-wave progression across the precordial leads suggests prior anteroseptal MI or LVH.
• ST segment: elevation ≥1 mm in a limb lead or ≥2 mm in a precordial lead, in two or more contiguous leads, is highly specific for acute MI — see Acute Coronary Syndromes for the coronary-territory lead groupings and reperfusion pathway. Diffuse ST elevation with PR depression suggests pericarditis rather than infarction.
• T wave: tall, tented T waves suggest hyperkalaemia; widespread flattening or inversion has a broad differential including ischaemia, strain and electrolyte disturbance.
• U wave: a prominent U wave suggests hypokalaemia.

Red flags

• ST elevation meeting the criteria above, or new left bundle branch block with a compatible history — treat as STEMI until proven otherwise.
• Complete (third-degree) heart block, or Mobitz type II second-degree block — both carry a real risk of progression to complete block and usually need same-day cardiology assessment for pacing; Mobitz type I (Wenckebach) is usually benign and asymptomatic.
• Broad-complex tachycardia, ventricular tachycardia or fibrillation — see VT/VF — treat as ventricular in origin until proven otherwise, especially in a patient with known structural heart disease.
• Pre-excitation (delta wave, short PR) in a symptomatic patient — see SVT & WPW.
• ECG changes of significant hyperkalaemia (peaked T waves progressing to widened QRS, loss of P waves, a sine-wave pattern) — treat as a medical emergency regardless of the potassium result pending.
• A markedly prolonged QT, especially with a compatible drug history or electrolyte disturbance.

Technical pitfalls

A poor-quality or unusual trace is more often a technical problem than a new diagnosis.

Limb-lead misplacement is the commonest cause of an unexpected axis or morphology change between two ECGs on the same patient. Check lead placement and repeat before acting on an isolated, otherwise unexplained abnormality.

Artefact from tremor, poor skin contact or patient movement can mimic atrial or ventricular ectopy.

Safety-netting

Treat an equivocal trace in a patient with a concerning history — chest pain, syncope, palpitations with haemodynamic symptoms — as high-risk pending clarification, not as normal. Repeat the ECG, compare with any prior trace, and discuss with cardiology or refer to the emergency department rather than waiting on an outpatient opinion.

A technically difficult trace does not defer a clinical decision that can’t safely wait.