Heart Blocks
2nd Degree
Heart Blocks
3rd Degree
Pathophysiology
3rd degree heart block is also known as complete heart block and is a very serious arrhythmia. Any patient with complete heart block will be bradycardic with a high risk of developing ventricular asystole.
In complete heart block none of the impulses from the atria are able to reach the ventricles. Despite this, all cardiac cells have the ability generate an electrical rhythm, and so usually one cell within the ventricles will begin to depolarise spontaneously. This is called a ventricular escape rhythm. The QRS complex will likely be broad, as the pathway the electrical activity will take will not follow the His-purkinje network.
Remember, the sino-atrial node is still firing, it's just that the signals will be blocked at the level of the AV node. Because of this p-waves will still be present on the ECG trace, but they will not match up in any meaningful way to the QRS complexes. There will be a complete dissociation between atrial activity, and ventricular activity because two independent pacemakers are present
ECG findings in complete heart block
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No relationship between p-waves and QRS complexes
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Bradycardia
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Potentially broad QRS complexes if in ventricular escape rhythm
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p-waves may be superimposed on top of other waves
Complete heart block can occur for several reasons, such as acute myocardial infarction, progression of Mobitz type II heart block, AV node blocking drugs, idiopathic degeneration of the conducting system (Lenegre’s or Lev’s disease) and iatrogenic causes such as ablation of the AV node.
Watch the animation below. Notice how some of the p-waves are superimposed onto other waves. The p-wave rate and QRS rate are both regular but are completely independent of each other. This patient then goes on to develop p-wave asystole (no ventricular activity).
