Cannon A waves, or cannon atrial waves, are waves seen occasionally in the jugular vein of humans with certain cardiac arrhythmias. When the atria and ventricles contract simultaneously, the blood will be pushed against the AV valve, and a very large pressure wave runs up the vein. It is associated with heart block, in particular third-degree (complete) heart block. It is also seen in pulmonary hypertension. Cannon A waves may also be seen in ventricular tachycardia due to the inherent AV dissociation of the arrhythmia.