IntroductionBradycardias (also known as bradyarrhythmias) are rhythm disorders characterised by a heart rate lower than 60 beats per minute. Causes include atrioventricular block, bundle branch block and sick sinus syndrome.
First degree heart blockDelay in the conduction of impulses from the sinoatrial node to the ventricles. May be idiopathic or associated with heart disease. No treatment is usually necessary. ECG findings: Prolonged PR interval (>200 ms)
Second degree heart blockIntermittent failure of conduction of impulses through the atrioventricular node or bundle of His (dropped beats). Dropped beats have a P wave but no QRS complex. Can be:
- Wenckebach (Mobitz I): progressive PR prolongation with beat until a P wave fails to conduct.
- Mobitz II: constant PR interval but intermittent failure of P wave conduction. Usually due to cardiac pathology
- 2:1 block: consistent failure of P wave conduction such that there are 2 P waves for every 1 QRS complex. It is impossible to classify this as either Wenckebach or Mobitz II as the PR interval cannot be evaluated as progressively prolonging nor constant.