QRS Interval

QRS Complex Morphology

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 Complex Naming Convention
Naming-of-the-QRS-complex-ECGWAVES 2
Courtesy of ECGwaves.com

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 due to aberrant conduction of supraventricular complexes (e.g. due to bundle branch block, hyperkalaemia or sodium-channel blockade).
ECG Strip QRS narrow and wide
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 QRS Complex Morphology

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:

ECG Normal sinus rhythm strip
Sinus rhythm: Each narrow complex is preceded by a normal P wave.

ECG Strip Atrial flutter
Atrial flutter: Narrow QRS complexes are associated with regular flutter waves.

Junctional tachycardia: Narrow QRS complexes with no visible P waves.

Broad QRS Complex Morphology

Broad/Wide QRS 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:

Example of a Broad Complex Rhythm:

ECG Strip Ventricular tachycardia VT
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:

High Voltage QRS Morphology

  • 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.

Low Voltage QRS Morphology

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

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.

Spot Diagnoses

These cardiac diseases produce distinctive QRS morphologies that are important not to miss:

ECG Library Basics

Advanced Reading



LITFL Further Reading


Emergency Medicine Specialist MBChB FRCEM FACEM. Medical Education, Cardiology and Web Based Resources | @jjlarkin78 | LinkedIn |

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.