A systematic order for reading an ECG aloud, normal values, axis, chamber enlargement and bundle branch blocks, heart block, infarct territories, and the patterns examiners show — from hyperkalaemia to pericarditis.
Internal Medicine
Cardiovascular system
All colleges
Read it in this order
Patient details, date and time; calibration of 10 mm/mV and 25 mm/s
Rate — 300 divided by the number of large squares between beats; if irregular, count the QRS complexes in the 10-second strip and multiply by 6
Rhythm — regular or irregular; a P wave before every QRS?
Axis
P waves, PR interval, QRS width and shape
ST segments, T waves, QT interval
Anything extra — U waves, delta waves, pacing spikes
Normal values
Measure
Normal
PR interval
120–200 ms (3–5 small squares)
QRS duration
Below 120 ms (under 3 small squares)
QTc
Below 440 ms in men, below 460 ms in women
P wave
Under 120 ms wide and 2.5 mm tall in lead II
Corrected QT (Bazett): QTc = QT ÷ √RR, with RR in seconds.
Axis
Axis
Leads I and aVF
Causes
Normal (−30° to +90°)
Both positive (or I and II positive)
—
Left axis deviation
I positive, II negative
Left anterior fascicular block, inferior infarction, primum ASD, Wolff–Parkinson–White
Right axis deviation
I negative, aVF positive
Right ventricular hypertrophy, pulmonary embolism, lateral infarction, left posterior fascicular block, dextrocardia, tall thin young adults
Chamber enlargement
Finding
Criteria
Left ventricular hypertrophy
Sokolow–Lyon: S in V1 plus R in V5 or V6 of 35 mm or more; Cornell: R in aVL plus S in V3 above 28 mm (men) or 20 mm (women); lateral strain pattern
Right ventricular hypertrophy
Dominant R wave in V1 (R larger than S, or R of 7 mm or more), right axis deviation, strain in V1–V3
Left atrial enlargement
Bifid P wave over 120 ms in II (P mitrale); deep negative terminal part of the P wave in V1
Right atrial enlargement
Peaked P wave over 2.5 mm in II (P pulmonale)
Bundle branch block
Block
Features
Common causes
Right bundle branch block
QRS 120 ms or more; RSR′ in V1; wide slurred S in I and V6
Normal variant, atrial septal defect, pulmonary embolism, right ventricular strain, ischaemic heart disease
Left bundle branch block
QRS 120 ms or more; broad notched R in I, aVL, V5–V6; deep S in V1; no septal q waves
Usually benign; often inferior infarction or high vagal tone
Second degree, Mobitz II
Constant PR with sudden dropped beats
Pacing — risk of complete block
Third degree (complete)
P waves and QRS complexes independent; slow escape rhythm
Pacing; atropine or isoprenaline as a bridge
Infarction and ischaemia
STEMI: new ST elevation at the J point in two contiguous leads — 1 mm or more in all leads except V2–V3. In V2–V3 the threshold is 2 mm in men aged 40 or over, 2.5 mm in men under 40, and 1.5 mm in women.
Territory
Leads
Usual artery
Inferior
II, III, aVF
Right coronary (sometimes circumflex)
Anterior
V1–V4
Left anterior descending
Lateral
I, aVL, V5–V6
Circumflex or diagonal branch
Posterior
ST depression with tall R in V1–V3; ST elevation in V7–V9
Right coronary or circumflex
Right ventricle
ST elevation in V4R
Right coronary
Evolution: hyperacute T waves, then ST elevation, then Q waves, then T-wave inversion.
Patterns to recognise
Condition
ECG
Hyperkalaemia
Tall peaked T waves, flat P waves, long PR, wide QRS, sine wave, then VF or asystole
Hypokalaemia
Flat T waves, U waves, ST depression, apparent long QT
Hypercalcaemia
Short QT
Hypocalcaemia
Long QT
Acute pericarditis
Widespread saddle-shaped ST elevation, PR depression, PR elevation in aVR, no reciprocal changes
Pericardial effusion
Low voltage, electrical alternans
Pulmonary embolism
Sinus tachycardia (commonest), right bundle branch block, right axis deviation, S1Q3T3, T inversion in V1–V4
Digoxin effect
Sagging "reverse tick" ST depression, short QT; toxicity causes almost any arrhythmia
Wolff–Parkinson–White
Short PR, delta wave, wide QRS
Hypothermia
J (Osborn) waves, bradycardia, shivering artefact
Atrial fibrillation
No P waves, irregularly irregular QRS
Atrial flutter
Sawtooth flutter waves at 300/min; 2:1 block gives a rate of 150
Ventricular tachycardia rather than SVT with aberrancy: QRS wider than 140 ms, AV dissociation, capture or fusion beats, concordance across the chest leads, extreme axis, and a history of ischaemic heart disease.
Causes of a long QT: congenital syndromes; drugs such as macrolides, fluoroquinolones, antipsychotics, methadone, ondansetron, quinine and chloroquine; hypokalaemia, hypomagnesaemia, hypocalcaemia; hypothermia.
Examiner questions
References
Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson JL, Loscalzo J, et al., eds. Harrison's Principles of Internal Medicine. 22nd ed. New York: McGraw Hill; 2025.