"Cannon" - a wave is seen in-

cannon a wave occurs with atrioventricular (AV) dissociation and right atrial contraction against a closed tricuspid valve. In a patient with a wide complex tachycardia, the appreciation of cannon a waves in the jugular venous waveform identifies the rhythm as ventricular in origin. The a wave is not present with atrial fibrillation. The x descent defines the fall in right atrial pressure after inscription of the a wave. ref : ( Harrison&;s principle of internal medicine,20th edition,pg no.1823 )