Irregular cannon waves in complete heart block

Irregular cannon waves in complete heart block

Irregular cannon waves in complete heart block are irregular in both timing and amplitude. P occurring later in systole has a taller cannon wave as there is more time for atrial filling. Moreover, during early part of the ventricular systole, atrial pressure drops due to descend of the tricuspid valve, producing a lower amplitude of cannon wave. Cannon waves are irregular in timing in complete heart block because of the varying relation between P and QRS due to atrioventricular dissociation.

Regular cannon waves can be seen in junctional rhthym, ventricular tachycardia with regular retrograde activation and first degree atrioventricular block with sinus tachycardia.

It may be noted that cannon waves in the jugular venous pulse occur when the atria contract over a closed tricuspid valve. As mentioned above, the amplitude of the cannon wave depends on the timing between the phases of ventricular systole and atrial systole. It does not occur when atrial contraction occurs during ventricular diastole when the tricuspid valve is open. When the tricuspid valve is closed, it is the reflux of blood into the great veins rather than the right atrium emptying into the right ventricle as it occurs normally, which causes the cannon waves (sometimes called cannon A waves).

Irregular cannon waves can also occur in atrial, ventricular and junctional ectopic beats, high grade atrioventricular block and ventricular pacing [1].

Reference

1. Mangalachulli Pottammal Ranjith, Kayakkal Shajudeen, Sankaran Prasanth. Regular Cannon Wave. Indian Heart J. 2016 Sep;68 Suppl 2(Suppl 2):S291-S293.