The Electrocardiograph (ECG) is clinically very useful, as it shows the electrical activity within the heart, simply by placing electrodes at various points on the body surface. This enables clinicians to determine the state of the conducting system and of the myocardium itself, as damage to the myocardium alters the way the impulses travel through it.

When looking at an ECG, it is often helpful to remember that an upward deflection on the ECG represents depolarisation moving towards the viewing electrode, and a downward deflection represents depolarisation moving away from the viewing electrode. Below is a normal lead II ECG.

  • The P wave represents atrial depolarisation- there is little muscle in the atrium so the deflection is small.
  • The Q wave represents depolarisation at the bundle of His; again, this is small as there is little muscle there.
  • The R wave represents the main spread of depolarisation, from the inside out, through the base of the ventricles. This involves large ammounts of muscle so the deflection is large.
  • The S wave shows the subsequent depolarisation of the rest of the ventricles upwards from the base of the ventricles.
  • The T wave represents repolarisation of the myocardium after systole is complete. This is a relatively slow process- hence the smooth curved deflection.
ECG animation

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