Main Features to Consider
- Width of the complexes: Narrow versus broad.
- Voltage (height) of the complexes.
- Spot diagnoses: Specific morphology patterns that are important to recognise.
QRS Width
- Normal QRS width is 70-100 ms (a duration of 110 ms is sometimes observed in healthy subjects).
- The QRS width is useful in determining the origin of each QRS complex (e.g. sinus, atrial, junctional or ventricular).
- Narrow complexes (QRS < 100 ms) are supraventricular in origin.
- Broad complexes (QRS > 100 ms) may be either ventricular in origin, or may be due to aberrant conduction of supraventricular complexes (e.g. due to bundle branch block, hyperkalaemia or sodium-channel blockade).
Example ECG showing both narrow and broad complexes:
- Sinus rhythm with frequent ventricular ectopic beats (VEBs) in a pattern of ventricular bigeminy. The narrow beats are sinus in origin, the broad complexes are ventricular.
Narrow Complexes
Narrow (supraventricular) complexes arise from three main places:
- Sino-atrial node (= normal P wave)
- Atria (= abnormal P wave / flutter wave / fibrillatory wave)
- AV node / junction (= either no P wave or an abnormal P wave with a PR interval < 120 ms)
Examples of Narrow Complex Rhythms:
- Sinus rhythm: Each narrow complex is preceded by a normal P wave.
- Atrial flutter: Narrow QRS complexes are associated with regular flutter waves.
- Junctional tachycardia: Narrow QRS complexes with no visible P waves.
Broad Complexes
- A QRS duration > 100 ms is abnormal
- A QRS duration > 120 ms is required for the diagnosis of bundle branch block or ventricular rhythm
Broad complexes may be ventricular in origin or due to aberrant conduction secondary to:
- Bundle branch block
- Hyperkalaemia
- Poisoning with sodium-channel blocking agents (e.g. tricyclic antidepressants)
- Pre-excitation (i.e. Wolff-Parkinson-White syndrome)
- Ventricular pacing
- Hypothermia
- Intermittent aberrancy (e.g. rate-related aberrancy)
Example of a Broad Complex Rhythm:
- Ventricular tachycardia: Broad QRS complexes with no visible P waves.
Ventricular vs supraventricular rhythms
Differentiation between ventricular complexes and aberrantly conducted supraventricular complexes may be difficult.
- In general, aberrant conduction of sinus rhythm and atrial rhythms (tachycardia, flutter, fibrillation) can usually be identified by the presence of preceding atrial activity (P waves, flutter waves, fibrillatory waves).
- However, aberrantly conducted junctional (AV nodal) complexes may appear identical to ventricular complexes as both produce broad QRS without any preceding atrial activity.
- In the case of ectopic beats, this distinction is not really important (as occasional ectopic beats do not usually require treatment).
- However, in the case of sustained tachyarrhythmias, the distinction between ventricular tachycardia and SVT with aberrancy becomes more important. This topic is covered in more detail here.
Fortunately, many causes of broad QRS can be identified by pattern recognition:
- Right bundle branch block produces an RSR’ pattern in V1 and deep slurred S waves in the lateral leads.
- Left bundle branch block produces a dominant S wave in V1 with broad, notched R waves and absent Q waves in the lateral leads.
- Hyperkalaemia is associated with a range of abnormalities including peaked T waves
- Tricyclic poisoning is associated with sinus tachycardia and tall R’ wave in aVR
- Wolff-Parkinson White syndrome is characterised by a short PR interval and delta waves
- Ventricular pacing will usually have visible pacing spikes
- Hypothermia is associated with bradycardia, long QT, Osborn waves and shivering artefact
Follow the links above for more detailed descriptions of the different causes of broad QRS
Low Voltage
The QRS is said to be low voltage when:
- The amplitudes of all the QRS complexes in the limb leads are < 5 mm; or
- The amplitudes of all the QRS complexes in the precordial leads are < 10 mm
Low voltage is discussed in more detail here.
Electrical Alternans
- This is when the QRS complexes alternate in height.
- The most important cause is massive pericardial effusion, in which the alternating QRS voltage is due to the heart swinging back and forth within a large fluid-filled pericardium.
Click here for an example of electrical alternans.
High Voltage
- Increased QRS voltage is often taken to infer the presence of left ventricular hypertrophy.
- However, high left ventricular voltage (HLVV) may be a normal finding in patients less than 40-45 years of age, particularly slim or athletic individuals.
- There are multiple “voltage criteria” for left ventricular hypertrophy.
- Probably the most commonly used are the Sokolov-Lyon criteria (S wave depth in V1 + tallest R wave height in V5-V6 > 35 mm).
- Voltage criteria must be accompanied by non-voltage criteria to be considered diagnostic of left ventricular hypertrophy.
Left ventricular hypertrophy is discussed in more detail here.
Spot Diagnoses
These cardiac diseases produce distinctive QRS morphologies that are important not to miss:
- Brugada syndrome (partial RBBB with ST elevation in V1-2)
- Wolff-Parkinson White Syndrome (delta wave)
- Tricyclic poisoning (wide QRS with dominant R wave in aVR)
Further Reading
- ECG BASICS – Waves, Intervals, Segments and Clinical Interpretation
- ECG CLINICAL CASES – Your favourite ECG’s placed in clinical context with a challenging Q&A approach
- ECG and Cardiology Eponymous Syndromes – Cheats guide to eponymous emancipation
- ECG Reference Sites on the WEB – the best of the rest
Author Credits
About Edward Burns
