The abnormal ECG: hypertrophy, conduction blocks and main arrhythmias
- Topic
How to recognise the most common abnormal ECG patterns: a thick heart wall, blocked wiring, and fast or irregular rhythms.
What it is
Once the normal waves, intervals and axis are known (see the physiology page on ECG basics), most abnormal tracings fall into a few families: chamber enlargement, bundle branch and fascicular blocks, atrioventricular (AV) blocks and the common tachyarrhythmias. Each has criteria in millimetres or milliseconds that students can check one by one. The ECG suggests; echocardiography, monitoring or an electrophysiology study confirm. Ischemia, infarction and electrolyte patterns have their own physiology pages.
Key facts
- Standard paper: 25 mm per second, so one small square is 40 ms and one large square 200 ms; 10 mm equals 1 mV.
- Normal adult intervals: PR 120–200 ms, QRS up to about 100–110 ms, QTc about 350–450 ms in men and 360–460 ms in women.
- A QRS of 120 ms or more means a complete bundle branch block, pre-excitation, a ventricular rhythm or a paced beat.
- Left bundle branch block hides the usual signs of an old infarction and distorts the ST segment, so it is read with care in chest pain.
- In Colombia and Latin America, right bundle branch block with left anterior fascicular block in an adult from a rural area raises the question of Chagas cardiomyopathy.
- A wide-complex regular tachycardia is treated as ventricular tachycardia until proven otherwise, above all in a patient with known heart disease.
Step by step
- Check name, date, calibration (10 mm = 1 mV) and paper speed (25 mm per second) before reading anything.
- Rate and rhythm: are there P waves, is each P followed by a QRS, are RR intervals regular?
- Measure PR, QRS and QT; correct the QT for heart rate.
- Work out the axis from leads I and aVF (and II if needed).
- Look at P waves for atrial enlargement and at QRS voltage for ventricular hypertrophy.
- If the QRS is wide, decide between right or left bundle branch block, pre-excitation or a ventricular rhythm.
- Look at Q waves, ST segments and T waves for ischemia, infarction or electrolyte change.
- Compare with an old tracing and with the patient's symptoms before concluding.
Main abnormal patterns in adults. Amplitudes in mm at standard calibration (10 mm = 1 mV); durations in ms.
| Finding | ECG criteria | Common causes |
|---|---|---|
| Left ventricular hypertrophy | Sokolow-Lyon: S in V1 + R in V5 or V6 of 35 mm or more, or R in aVL of 11 mm or more; Cornell: R in aVL + S in V3 above 28 mm (men) or 20 mm (women); 'strain': ST depression and T inversion in V5–V6 | Arterial hypertension, aortic stenosis, hypertrophic cardiomyopathy |
| Right ventricular hypertrophy | Right axis deviation (+90° to +180°), dominant R in V1, ST depression and T inversion in V1–V3 | Pulmonary hypertension, chronic lung disease, mitral stenosis, acute pulmonary embolism (right strain) |
| Atrial enlargement | Right: tall peaked P over 2 mm in II, III, aVF (P pulmonale). Left: broad notched P in II and a deep, wide negative final part of P in V1 (at least 1 mm and 40 ms) | Right: lung disease, pulmonary hypertension. Left: mitral valve disease, hypertension |
| Right bundle branch block | QRS 120 ms or more; rSR' ('M') in V1–V2; wide terminal S in I and V6 | Can be normal; atrial septal defect, pulmonary embolism, ischemia, Chagas disease, degeneration of the conduction system |
| Left bundle branch block | QRS 120 ms or more; broad, notched or slurred R in I, aVL, V5–V6 with no Q waves; deep S in V1; ST and T opposite to the QRS | Hypertension, ischemic heart disease, cardiomyopathy, aortic stenosis, conduction-system degeneration |
| Left anterior fascicular block | QRS under 120 ms; axis more negative than −45°; qR in aVL | Ischemia, hypertension, Chagas disease (often with right bundle branch block), degeneration |
| First-degree AV block | Every P is followed by a QRS; PR over 200 ms, constant | High vagal tone, AV-nodal drugs, ischemia, degeneration |
| Second-degree AV block, Mobitz I (Wenckebach) | PR lengthens beat by beat until one P is not conducted; the pause is shorter than two normal cycles | Usually nodal: vagal tone, athletes, inferior infarction, drugs |
| Second-degree AV block, Mobitz II | Constant PR with P waves that suddenly fail to conduct; often a wide QRS | Disease below the AV node (His-Purkinje); anterior infarction, fibrosis; always pathologic |
| Third-degree (complete) AV block | P waves and QRS complexes march independently (AV dissociation); atrial rate faster than ventricular; narrow escape about 35–50 per minute or wide escape about 20–40 per minute | Fibrosis of the conduction system, ischemia, drugs, cardiac surgery, Lyme carditis, Chagas disease |
| Atrial fibrillation | No P waves; irregularly irregular RR intervals; fine or coarse fibrillatory baseline | Hypertension, valve disease, heart failure, hyperthyroidism, alcohol, age |
| Atrial flutter | Sawtooth flutter waves about 300 per minute, best seen in II, III, aVF; often 2:1 conduction with a ventricular rate near 150 | Structural heart disease, chronic lung disease, after cardiac surgery |
| Wolff-Parkinson-White pattern | PR under 120 ms; delta wave (slurred QRS upstroke); QRS 120 ms or more; secondary ST-T changes | Congenital accessory AV pathway |
| Ventricular tachycardia | Three or more wide QRS beats in a row above 100 per minute; AV dissociation, capture or fusion beats support the diagnosis | Old infarction scar, cardiomyopathy, ischemia, Chagas cardiomyopathy, channelopathies |
| Brugada type 1 pattern | Coved ST elevation of 2 mm or more in V1–V2 followed by a negative T wave | Inherited sodium-channel disorder; may appear with fever or some drugs |
Test yourself
Think of the answer, then tap the question to see it.
Which voltage criterion of left ventricular hypertrophy uses V1 and V5 or V6?
Sokolow-Lyon: the S wave in V1 plus the R wave in V5 or V6 adds up to 35 mm or more.
How is right bundle branch block recognised on the ECG?
QRS of 120 ms or more, an rSR' pattern in V1–V2 and a wide terminal S wave in leads I and V6.
What separates Mobitz I from Mobitz II second-degree AV block?
In Mobitz I the PR lengthens beat by beat before the dropped beat; in Mobitz II the PR stays constant and beats drop suddenly, usually from disease below the AV node.
What are the three ECG features of the Wolff-Parkinson-White pattern?
A short PR (under 120 ms), a delta wave at the start of the QRS and a wide QRS (120 ms or more).
Which conduction pattern should make you think of Chagas disease in a Colombian adult?
Right bundle branch block together with left anterior fascicular block, sometimes with AV block or ventricular ectopy.
Elsewhere in DurgiPedia
- ECG basics: waves, intervals and leads
- The cardiac electrical axis
- The ECG in ischemia and infarction
- The ECG in potassium and calcium disorders
Connections
Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.
Sources: Electrocardiography - Cardiology - MSD Manual Professional Edition (MSD Manual Professional Edition, 2026); Bundle Branch Block and Fascicular Block (MSD Manual Professional Edition, 2024); Atrioventricular Block - Cardiology - MSD Manual Professional Edition (MSD Manual Professional Edition, 2026); Wolff-Parkinson-White Syndrome (WPW Syndrome) (MSD Manual Professional Edition, 2024).
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Representative model, for educational purposes only.