ECG – The Basics

To understand how to read an ECG and the terminology involved, it is essential you have a good idea of the cardiac conduction system. If you haven’t already, give the last post ‘The Cardiac Conduction System’ a quick read. There is a useful short video that explains it really well! 

To recap on that electrical process, this diagram shows the electrical conduction in relation to the waves on an ECG. Before we look at what each wave means, we need to first look at the rate and the rhythm on an ECG.

The Rate

  • The standard paper speed of an ECG is 25mm/sec
  • 1mm (small square) = 0.04 sec (40ms)
  • 5mm (large square) = 0.2 sec (200ms)
  • 5 large squares = 1 second
  • An ECG rhythm strip = 250 small squares / 50 large squares / 10 seconds
  • Normal – 60-100 beats / min
  • Tachycardia – > 100 beats / min
  • Bradycardia – < 60 beats / min

Calculating the Rate

  • There are multiple methods to estimate the rate 
  • We can calculate the beats per minute (BPM) by dividing 300 by the number of large squares between the two R waves (R-R interval = one beat)
  • This works for regular rhythms only
  • For irregular rhythms try Rate = Number of R waves x 6 on a 10 second rhythm strip


The Rhythm

  • The heart rhythm can be regular or irregular
  • An irregular rhythm can either be Regularly Irregular (a recurrent pattern of irregularity) or Irregularly Irregular (completely disorganised)

Waves and Intervals on the ECG

Atrial and ventricular depolarisation and repolarisation are represented on the ECG as a series of waves: the P wave followed by the QRS complex and the T wave.

The P Wave

  • The P wave is the first positive deflection of the ECG
  • It represents atrial depolorisation
  • Duration: < 0.12 s (< 120 ms or 3 small squares


PR Interval 

  • The PR interval is the time from the onset of the P wave to the start of the QRS complex. It reflects conduction through the AV node. 
  • The normal PR interval is between 120-200 ms (0.12-0.20s or 3-5 squares)
  • If the PR interval is longer than 200ms (5 squares) first degree heart block is said to be present. It is caused by delayed conduction through the AV node.
  • If the PR interval is < 120ms (3 squares) it suggests pre-excitiation (the presence of an accessory pathway between the atria and ventricles)

The Q Wave 

  • A Q wave is any negative deflection that precedes an R wave
  • The Q wave represents the normal left-to-right depolarisation of the inter ventricular septum 
  • Small Q waves are normal in most leads
  • Deeper Q waves (> 2mm) may be seen in leads III and aVR as a normal varient
  • Under normal circumstances, Q waves are not seen in the right-sided leads (V1-3)

QRS Complex

A QRS complex with QRS deflections
  • The second wave on the ECG is a QRS complex.
  • Typically the complex has a series of 3 deflections that represent right and left ventricular depolarisation.
  • If the first deflection is negative it is called a Q wave.  
  • The first positive deflection in the complex is called an R wave.
  • Any negative deflection after an R wave is an S wave.
  • A normal QRS width is 70-100 ms.
  • The QRS width is useful in determining the origin of each QRS complex:
  • Narrow complexes (QRS < 100 ms) are supra ventricular in orginin. 
  • Broad complexes (QRS > 100 ms) may be either ventricular in origin, or due to conditions such as  bundle branch block 

S-T Segment 

  • The ST segment is the flat, isoelectric section of the ECG between the end of the S wave (the J point) and the beginning of the T wave. 
  • The ST segment represents the interval between ventricular depolarisation and repolarisation. 
  • The most important cause of ST segment abnormality (elevation or depression) is myocardial ischaemia or in infarction

S-T Segment Elevation Morphology

  • Acute STEMI may produce ST elevation with either concave, convex or obliquely straight morphology

The T Wave

  • The T wave is the positive deflection after each QRS complex
  • It represents ventricular replarisation
  • It is upright in all leads except aVR and V1
  • Amplitude (height) < 5mm in limb leads, < 15mm in precordial leads 

Interpreting an ECG

When looking at ECG’s, it is important to be systematic. Firstly, ensure the print out is at 25mm/s. Then confirm the patient’s name and date of birth (patient label) is on the ECG and that the date and time are correct (important for tracking ECG changes over time).  I then answer the following question:

  • What is the rate? (<60 Bradycardia >100 Tachycardia)
  • Is it regular or irregular? If irregular is it regularly irregular or not?
  • Is there a P Wave? If yes is it less than 3 small squares? I no, likely AF.
  • Is the PR interval between 120-200 ms? (If it’s longer, likely a heart block)
  • Is the QRS width between 70-100 ms?
  • Is there a QRS complex after every P wave?
  • Is there ST segment elevation or depression?
  • Are the T waves upright? (except aVR and V1)

You will see lists of ECG rules everywhere, some of which go into much more depth. I think for what we are doing

 Now you have an understanding of what each wave means, why it looks like it does on the ECG and what rules to follow to confirm a normal ECG, have a look at this ECG below and apply the rules and what you’ve learnt. It is only when you are happy at looking at normal ECGs that you can start to understand and spot abnormal ones.

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