Module 3: The QRS

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When it comes to the QRS complex there are 4 questions to ask:

  • Is it TALL or SMALL?
  • Is it WIDE/NARROW?
  • Does it have abnormal MORPHOLOGY i.e. DELTA wave
  • Are the complexes CLUMPED or irregularly irregular?
  • Are there Q waves?

Password is ‘ecg’


If the complexes are TALL– think of left ventricular hypertrophy.

There are several voltage criteria that can be used. I use:
S in V1 + R in V5/6 > 35mm

The lateral leads are the best place to view this. There is a large R wave, with a down-sloping ST segment and an asymmetrical inverted T wave.

In situations where the complexes are tall, think about cardiomyopathy, especially Hypertrophic Obstructive Cardiomyopathy.

Some distinct features to look out for include:

  1. High voltages
  2. Very thin QRS complexes, especially in the anterior leads
  3. Q waves in the lateral leads, that are <1mm wide and so do not fit the normal pattern of post infarction Q waves.

If the complexes are SMALL, think of:

  1. Pericardial Effusion
  2. Pleural Effusion
  3. Obesity
  4. COPD

The story may give it away. In a previous fellowship examination, the ecg below was given, with the story of a 63 year old female patient with recent CABG, who now presents short of breath with a tachycardia.

The story may give it away. In a previous fellowship examination, the ecg below was given, with the story of a 63 year old female patient with recent CABG, who now presents short of breath with a tachycardia.

Notice the small complexes. Also notice the electrical alternates, with every alternate QRS complex being of difference amplitude. This is due to a pericardial effusion. The heart swings in an abnormal fashion due to the effusion and the complexes become abnormal.


A NARROW complex is defined as < 3 small squares i.e., <0.12ms. This indicates normal conduction through the Atrioventricular node.

A WIDE complex is defined as > 3 small squares.

A wide complex can be:

  1. a bundle branch block- LBBB or RBBB
  2. Hyperkalaemia
  3. Na channel blockade

If the rate is faster than 120 beats per minute think of:

  1. Ventricular Tachycardia
  2. SVT with aberrancy
  3. Antidromic SVT

a side note on axis

One of the few times I calculate axis is with a Right Bundle Branch Block. If this is present then it is important to see if there is a leftward axis. A left axis is caused by a Left Anterior Fascicular Block. If there is a right bundle branch block and a left anterior fascicular block, it means that only the right posterior fascicle is still intact. It may explain the patients syncopal episode. If there is also a first degree block, this is names a triphasicular block and the patient, if symptomatic requires a pacemaker.

The other times that I look at an axis are:

  1. If I’m looking for an extreme axis in ventricular tachycardia; although this will not usually make the diagnosis./li>
  2. If I’m looking for a right axis in a potential pulmonary embolism

Back to WIDE Complex

To diagnose VENTRICULAR TACHYCARDIA, I look for the following:

  1. Is the patient presenting with the right history for this? i.e., are they older, with risk factors, or know heart disease?
  2. The rate must be > 120 beats per minute
    1. Beware: If there is a wide complex tachycardia, but the rate is not > 120, it may not be ventricular tachycardia. Think of:
      1. Hyperkalaemia
      2. Na channel blockade
  3. The complex must be wide- wider the better but > 3 small squares
  4. Is there concordance?- Are the complexes in anterior leads pointing in the same direction?
  5. Is there AV dissociation? Can you see P waves?
  6. Are there Fusion Beats?
  7. Are there Capture Beats?
  8. Is there an Rr complex in V1? Is there a rabbit ear in V1?
  9. Is there a Josephson’s Sign?
  10. Is there a Brugada Sign?
  11. Is the Axis Extreme?

Are the QRS’s clumped?

This is a reminder to look for the MOBITZ block. If the QRS complexes are clumped, usually regularly, unless you have a variable block or atrial fibrillation, then think of a MOBITZ Block.

Are there Q waves?

If there are Q waves, look for two things:

  1. Are the Q waves long and thin i.e. < 1mm, think Hypetrophic Cardiomyopathy
  2. If they are > 1mm, the patient may have had a previous infarction.
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Dr Peter Kas

Emergency Physician, Educator. Key Interests: Resuscitation, Airway, Emergency Cardiology, Clinical Examination. Creator

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