The Cardiac Conduction Cycle

Before starting to look at ECGs, it is important to understand the mechanics and electrics of what exactly is going on in the heart. Let’s look at the human body’s vessels and organs as a central heating system. Your heart is the boiler, your vessels as the pipes and the radiators as your organs. In your central heating system, the pump, pipes and radiators are of no use unless connected to a power supply. The pump needs electricity to work. The human heart has a similar need for a power source and also uses electricity. Thankfully we don’t need to plug ourselves in to the mains, the heart is able to create it’s own electrical impulses and control the route the impulses take via a specialised conduction pathway.

This pathway is made up of 5 elements:

  • The sino-atrial (SA) node
  • The atrio-ventricular (AV) node
  • The bundle of His
  • The left and right bundle branches
  • The Purkinje fibres

The SA Node

  • The SA node is the natural pacemaker of the heart. You may have heard of permanent pacemakers (PPMs) and temporary pacing wires (TPWs) which are used when the SA node isn’t functioning properly.
  • The SA node releases electrical stimuli at a regular rate
  • The rate is dictated by the needs of the body.
  • Each stimulus passes through the myocardial cells of the atria creating a wave of contraction which spreads rapidly through both atria. (imagine the walls of the atria covered in dominoes, as one falls, they all do, spreading in a wave from the origin)

The AV Node

  • The electrical stimulus from the SA node eventually reaches the AV node and is delayed briefly so that the contracting atria have enough time to pump all the blood into the ventricles.
  • Once the atria are empty of blood the valves between the atria and ventricles close.
  • At this point the atria begin to refill and the electrical stimulus passes through the AV node and Bundle of His into the Bundle branches and Purkinje fibres.

Bundle of His, Bundle Branches and Purkinje fibres

  • The Bundle of His is a band of cardiac muscle fibres connecting the atria with the ventricles
  • Also known as the atrioventricular bundle
  • The bundle branches split from the Bundle of His into the Left Bundle Branch and the Right Bundle Branch 
  • These bundles then split further into very small fibres known as Purkinje fibres.
  • Imagine the bundle branches as motorways, if you like, with the Purkinje fibres as A and B roads that spread widely across the ventricles .
  • In this way all the cells in the ventricles receive an electrical stimulus causing them to contract.

Depolarisation and Repolarisation

  • At this point the ventricles are empty, the atria are full and the valves between them are closed. The SA node is about to release another electrical stimulus and the process is about to repeat itself. However, there is a 3rd section to this process. The SA node and AV node contain only one stimulus. Therefore every time the nodes release a stimulus they must recharge before they can do it again.
  • In the case of the heart, the SA node recharges whilst the atria are refilling, and the AV node recharges when the ventricles are refilling. In this way there is no need for a pause in heart function. Again, this process takes less than one third of a second.
  • The times given for the 3 different stages are based on a heart rate of 60 bpm , or 1 beat per second.
  • The term used for the release (discharge) of an electrical stimulus is “depolarisation”, and the term for recharging is “repolarisation”.

The 3 stages of a single heart beat are:

  • Atrial depolarisation
  • Ventricular depolarisation
  • Atrial and ventricular repolarisation.

This video is very good at visually explaining the process I have tried to explain above. Once you understand how the electrics work, an ECG will make much more sense! It’s only 3 minutes long and well worth a watch! Apologies for the really creepy voice!

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