ECG Rhythms......... without the Blues! - 1/5

14th August, 2006


Welcome to the first in this series of ECG rhythm recognition. Over the coming weeks we will examine a number of common ECG rhythm disturbances, how to recognise them and their significance.We begin this series with a review of normal ECG rhythms.


Heart muscle contracts in response to electrical activity spreading across it (depolarisation). This electrical activity is carried across the heart by the electrical conducting system. This is a specialised collection of cells and structures that are involved in initiating and conducting the electrical impulses. An ECG is a recording of these impulses.

Image showing a typical PQRS ComplexLet’s now look at the names of the various blips and squiggles that make up a normal ECG complex.

The first wave of the normal ECG is called the P wave, normally a small upwardly humped wave - it represents depolarisation of both atria. Having travelled across the atria impulses enter the atrioventricular node (AV node) where they are slowed to allow ventricular filling.

The straight line that follows the P wave comprises part of the PR interval, the time from the initiation of the impulse in the SA node to the moment of ventricular depolarisation. A normal PR interval, measured from the start of the P wave to the start of the QRS complex, is 0.12 – 0.20 secs. (3 to 5 wee squares on ECG paper!)

Impulses then travel through the Bundle of His to the right and left bundle branches and are released into the ventricles via the purkinje fibres. Subsequent depolarisation of the ventricles gives rise to the larger QRS complex. The QRS complex is followed by the ST segment and the T wave, the latter representing ventricular repolarisation.

If all that happens as it should you will see something looking like the image above.


Systematic guide to ECG rhythm interpretation.

It is best to approach your ECG rhythm recognition in a very systematic way. We’d recommend that you apply the following every time – you’ll soon be doing it without thinking!

Step 1: If the rhythm doesn’t look right check your patient!

Step 2: Is the rhythm regular or irregular (i.e. are the QRS complexes coming in regularly)

Step 3: What is the heart rate (to calculate this quickly count the number of big squares between any two consecutive QRS complexes into 300)

Step 4: Can you identify P waves

Step 5: Can you identify QRS complexes & T waves

Step 6: What is the ratio of P waves to QRS complexes

Step 7: What is the PR interval

Step 8: Anything else you notice that shouldn’t be there


Sinus Rhythm

The normal heart rhythm is called sinus rhythm:

What to look for on the ECG:

  • The rhythm will be regular
  • The rate will be between 60 and 100
  • There will be P waves and QRS complexes
  • The P to QRS ratio is 1 : 1 The PR interval is normal

Look for these features in this example of sinus rhythm:

Example ECG rhythm strip showing Sinus Rhythm


Sinus Bradycardia

When sinus rhythm is present at a rate of less than 60 we call it sinus bradycardia.

Sinus bradycardia may be seen in:

  • Normal finding in fit, athletic individuals
  • Increased vagal tone e.g. during vomiting
  • Drug effect e.g. beta blockers
  • Hypothermia
  • Under active thryroid
  • Myocardial Infarction
  • Raised intracranial pressure
  • Electrolyte disorders

Example of sinus bradycardia:

Example ECG rhythm strip - Sinus Bradycardia


Sinus Tachycardia

Conversely when sinus rhythm is present at a rate of over 100 it is called sinus tachycardia.

Sinus tachycardia may be seen in:

  • Exercise
  • Anxiety, pain, fear
  • Hypotension
  • Over active thyroid
  • Drug effect e.g. atropine Stimulants e.g. caffeine

Example of sinus tachycardia:

Sample ECG rhythm strip - Sinus Tachycardia

You may come across a rhythm that looks very much like sinus rhythm except for a slight variability in the regularity of the QRS complexes. This is called sinus arrhythmia. This is often due to transient vagal stimulation that occurs due to a breathing effect and in such cases is not generally significant.

Charles Bloe BSc RGN NDN ITU cert
Lead Lecturer and CEO - Charles Bloe Training Ltd

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