What is the significance of cannon waves in a wide QRS tachycardia?

Anyone working in the emergency department or intensive care unit would have experienced difficulty in deciding whether a given wide QRS tachycardia is of supraventricular or ventricular origin. Differentiation is quite important from the point of view of therapeutic options and prognostic significance. Supraventricular tachycardia with either a pre-existent bundle branch block or aberrancy during tachycardia, can be mistaken for a ventricular tachycardia. Close observation of the jugular venous pulse for cannon waves can give a clue to the ventricular origin of the tachycardia. Cannon waves occur when the atria contract against closed atrioventricular valves. This occurs in ventricular tachycardia which does not usually have atrioventricular synchrony. Irregular cannon waves in the jugular venous pulse indicates ventricular tachycardia with interference AV dissociation. In the rare instance of ventricular tachycardia with regular retrograde conduction, there could be regular cannon waves. Irregular cannon waves are not seen in supraventricular tachycardia because of AV synchrony. Regular cannon waves may be noted in junctional tachycardia with regular retrograde conduction.