A “cannon A wave” occurs when the right atrium contracts against a closed tricuspid valve causing a large pulsation to occur in the jugular venous pulsation. This occurs at times of electrical “AV dissociation”, meaning with the P wave on the ECG overlaps with the QRS complex and thus atrial systole occurs simultaneously with ventricular systole. This can result in significant stretch of the atrium causing ANP (Atrial Natriuretic Peptide) to be released causing polyuria.