2nd degree heart block


  • Second degree heart block is characterised by a disturbance, delay or interruption of atrial impulses through the AV node resulting in only partial conduction of signals to the ventricles (i.e. in contrast to 1st degree where every P wave has a QRS, this is not the case with 2nd degree heart block)
  • 2nd degree heart block can be classified as either
    • Mobitz Type 1 (Wenckebach)- this is where there is progressive lengthening of the PR-interval until the atrial impulse cannot be conducted and a QRS (and ventricular contraction) does not occur.  The main site of dysfunction is usually the AV node itself and can be vagally mediated.
    • Mobitz Type 2- this is where the PR interval is constant (may be prolonged or not) but there is a ‘dropped beat’.  This may occur in a pattern e.g. 2:1 (can’t actually be differentiated from type 1), or 3:1.  The main site of dysfunction is usually in the bundle of His.
      • Can cause QRS widening and more likely to progress to 3rd degree heart block


  • Although some could be classed as idiopathic, it is important to identify an underlying cause/risks
    • Any cardiac disease (MI, CHD, IHD, heart failure, cardiomyopathy, rheumatic heart disease, infection, tumours etc) can potentially cause heart block
    • Drugs can also cause heart block (local anaesthetics, beta blockers, benzylpenicillin, lithium)
    • other conditions e.g. sarcoidosis, amyloidosis, ank spond, dermatomyositis (and other connective tissue disorders)
    • alcohol may also precipitate/exacerbate heart block


  • Type 1- usually asymptomatic; occasionally presyncopal episodes or syncope, dizziness etc
  • Type 2- more likely to be symptomatic (light-headed, dizzy, presyncope/syncope, palpitations etc)


  • ECG will show features described above.
  • Lying/standing blood pressure
  • Blood count, U&Es


  • Mobitz Type 1 can generally be managed without any active treatment
  • Type 2 (because of risk of development into 3rd degree block) should be given a pacemaker (particularly if symptomatic)

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