Mechanims of arrhythmias:

• Enhanced automaticity
• Triggered activity
• Re-entry

What is automaticity?

• property of spontaneous depolarisation - usually only occurs in SAN, AVN & conducting tissues
• in certain conditions other myocardial cells can exhibit this property, leading to spontaneous depolarisatoin at other sites giving rise to ectopic beats and tachycardias

Enhanced Automaticity:

Enhanced (= accelerated) automaticity may result from factors that either reduce threshold (yellow line) or increase slope of Phase 4 depolarisation (red line).

Factors that may enhance automaticity include:

• Catecholamine excess
• Ischaemia
• Abnormal pH
• Electrolyte abnormalities

Triggered Activity

This results from the premature activation of the cardiomyocytes by afterdepolarisatoins

• Afterdepolarizations are abnormal depolarizations of cardiac myocytes that disrupt phase 2, phase 3, or phase 4 of the cardiac action potential in the electrical conduction system of the heart

They can be be early or delayed

DAD = Delayed afterdepolarisation

EAD = Early afterdepolarisation

Afterdepolarisations can for be caused by:

• electrolyte abnormalities or
• certain medications - e.g. digoxin toxicity.