The Cannon Wave

1. What is a cannon wave?

ANS: It is the high A wave caused by atrial contraction against a closed tri-cuspid valve.

2. When will the atrium contract against a closed tricuspid valve?

ANS: When there is atrioventricular dissociation or when a very early P wave occurs on a T wave, as with a premature atrial contraction or a junctional pacemaker.

Cannon Cannon Cannon n | 11

Cannon Cannon Cannon

Note that in this patient with complete atrioventricular block, every other P wave happens to fall on a T wave, i.e., it occurs during ventricular systole when the tricuspid valve is closed. Thus, there is a cannon A wave with every other P wave.

3. Are cannon waves easy or difficult to see?

ANS: Cannon waves are usually very difficult to see unless the atrial contraction is very strong, as when there is increased stiffness of the RV, such as in pulmonary hypertension. Because they are easily recorded, they were probably given the name cannon waves from the appearance on pulse tracings rather than from the appearance of the slight variations in jugular pulsation seen in the neck.

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