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Basic Interpretation

There are 3 main parts to the ECG waveform.

- P-wave

- QRS complex

- T-wave

 

When all of these occur in the correct order with the correct time interval in between them, this is called sinus rhythm. All the examples we are looking at on this page are using the readings from limb lead II

 

 

Click the arrows underneath each section to progress to the next point

 

Atrial Depolarisation

  • The SA node discharges an action potential

  • This flows through special fibres and causes the atria to depolarise/contract

  • The depolarisation moves through to the AV node

  • The atrial depolarisation forms the P wave of the ECG

PR interval

  • The AV node delays the action potential reaching the ventricles allowing the atria to contract before the ventricles

  • This delay can be seen on an ECG as a brief flat section

  • It forms part of the PR interval (start of P wave to start of QRS complex)

  • The only way the depolarisation can spread from the atria to the ventricles is through the AV node. 

  • This is because there is an insulating layer at the bottom of the atria called the annulus fibrosus which prevents electrical activity from spreading

 

Note that even though it is called the PR interval it is actually from the start of the P wave to the start of the Q wave

Ventricular Depolarisation

  • The action potential then spreads through a network of fibres to the ventricles

  • This happens very quickly

  • The pattern of ventricular depolarisation is called the QRS complex

  • At the same time, the atria are repolarising which can't be seen on the ECG (obscured by the QRS complex)

ST Segment

  • There is then a brief period of net zero electrical activity

  • This is a time between depolarisation and repolarisation

  • This is called the ST segment

T-Wave

  • Having depolarised and contracted, the ventricles move onto repolarising

  • Moving ions against a concentration gradient requires energy: repolarisation takes a little bit longer than depolarisation

  • This should appear as an upward deflection 

  • The deflection is called a T wave

QT interval

  • So far, we have considered ventricular depolarisation and ventricular repolarisation as separate entities

  • There is scope to combine the two and then consider ventricular activity as a whole

  • We use the QT interval to do this

  • This is often corrected for heart rate (giving us a QTc measurement) by using a mathematical formula

U-Wave

  • Occasionally, the repolarisation process may not be completed in one smooth wave

  • There may be a second phase to repolarisation (the papillary muscles and Purkinje fibres repolarise late)

  • If this is the case, there may be a distinct upward deflection following the T wave (like the T wave but smaller)

  • This is called the U wave 

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Use this outline if you have no prior knowlege of ECGs, how they work or how to set them up. This map will take you through the basic physics and anatomy, set up of the ECG and will touch on interpretation. There is a test at the end to examine your understanding.

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Use this outline if you have some prior knowlege of how an ECG works, its set up and a good understanding of the anatomy of the heart's conducting system. This will focus on interpreting the readings on different leads of the ECG.

 

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Use this map if you already have a thourough understanding of heart anatomy, different arrythmias and their appearance on ECGs. This map focuses on testing your existing knowledge of interpreting ECGs.

 

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