Cannon 'a' waves caused by ventricular pacing

Simultaneous electrocardiographic and right atrial pressure tracings from a patient with a ventricular pacemaker. Arrows indicate underlying atrial activity. Occasional normal a and v waves are interspersed with cannon 'a' waves (ca). Cannon 'a' waves result from atrial contraction against a closed valve, and are a sign of atrioventricular dissociation.