Conduction disorders

• Atrioventricular block is also known as heart block. The most common cause is idiopathic fibrosis and sclerosis. It is caused by partial or complete interruption of impulse from the atria to ventricles.
• First degree AV block is common and usually an incidental finding. ECG shows consistent prolongation of the PR interval with no dropped QRS complexes.
• Management of first degree AV block is to treat any underlying cause such as electrolyte derangement and thyroid dysfunction. Any AV blocking drugs such as beta should be stopped. Pacemaker could be considered if patient symptomatic.
• Second degree AV block type 1 is also called Mobitz type 1
• Mobitz type 1 is usually asymptomatic and doesn't cause haemodynamic compromise. Can sometimes cause syncope due to symptomatic bradycardia.

  Management is usually treating the underlying cause and stopping AV blocking drugs.
• Mobitz type 1 causes progressive prolongation of PR interval until atrial impulse is not conducted and the QRS complex is dropped. This causes an irregular pulse.
• Investigations for conduction disorders:

  • ECG to determine subtype
  • FBC, U&Es, TSH and troponin for any underlying cause
• Second degree AV block type 2 is also called Mobitz type 2
• Mobitz type 2 is always pathological. 80% of the time it is caused by a block at the bundle branches. The block can also be at the bundle of His.
• The most common causes of Mobitz type 2 are myocardial infarction and disorders affecting the heart wall muscles such as cardiomyopathies
• ECG for Mobitz type 2 shows consistent PR interval duration with intermittently dropped QRS complexes due to failure of conduction. Typically follows a pattern. Every 3rd QRS dropped would be a 3:1 block.
• Patients with Mobitz type 2 typically present with:

  • Chest pain
  • Dyspnoea
  • Palpitations
  • Pre-syncope/ syncope
• When examining a patient with Mobitz type 2 you may detect an irregularly irregular pulse where there is a 3:1 or 4:1 pattern
• Patients with Mobitz type 2 can become haemodynamically unstable if too many P waves aren't conducted. They are also likely to progress to complete heart block. Reversible causes to be treated. Patients will need temporary cardiac pacing until a permanent pacemaker can be fitted.
• Complete heart block occurs when there is no communication between the atria and ventricles due to complete failure in conduction.
• In complete heart block the P waves and QRS complexes have no association with each other as the atria and ventricles are functioning independently.
• Symptoms of complete heart block:

  • Palpitations
  • Pre-syncope/ syncope
  • Confusion
  • SOB due to acute heart failure
  • Chest pain
• Patient with complete heart block are at risk of sudden cardiac death
• On examination, patients with complete heart block will have an irregular pulse, profound bradycardia and signs of haemodynamic compromise
• Patients with complete heart block will need temporary cardiac pacing and an isoprenaline infusion (increases cardiac output). They will eventually need a permanent pacemaker.
• A complete heart block is suggestive of AV node damage. In 80-90% of the population the AV node is supplied by the right coronary artery
• First degree heart block is defined as a PR interval >200 ms (5 small squares)
• The AV node is supplied by the right coronary artery in most people, so an inferior MI can result in a bundle branch black
• AV node blocking drugs - amiodarone, beta blockers, calcium channel blockers and digoxin