Cannon waves occur when Atria contracts against a closing tricuspid valve of right ventricle .( There would be a equivalent left atrial cannon which goes into pulmonary vein as well , it is discussed elsewhere !)
Cannon waves happen only when P waves fall within QT interval in ECG as QT represents the electro-mechanical systole of ventricles. (Since P wave represents atrial systole , it is simple to understand when it falls within QT both atria and ventricular contractions collide to produce a cannon wave into the neck or pulmonary veins.)
The following two images of cannon waves taken from the legend Dr Paul woods own tracing .
Regular cannon waves
Occur during SVT with 1:1 VA conduction.*
1 : 1 VA conduction can be considered as absence of AV dissociation (Rather disciplined VA association with every beat ) This is essential to create a hemodynamic milieu for regular cannon waves.
* In AVNRT , VA conduction in strict sense is a misnomer .It is simply a retrograde conduction thorough the AV node .
Irregular cannon waves
- Complete heart block .
- Multiple random VPDs
- Some patients with VT.*(Who are those patients ? Those with AV dissociation when retrograde “P” wave falls within QT interval cannon occurs. As expected this occurs in random fashion which makes the cannon fire irregular.
Can we get regular cannon in VT ?
Yes , but rare . As explained earlier this can happen only if AV association occur on a retrograde fashion.
Further reading in this site
Dr.S.Venkatesan MD 