Variation of carotid venous wave shape: cannon a-wave
The mechanism for most of the cause of cannonballs is due to the difference in systolic time between the atria and ventricles, resulting in the atrium contracting while the tricuspid valve is closed.
A large, sudden flashing wave occurs after the T1 hour and immediately after the carotid artery bounces. The cannon a wave usually does not have a distinct ebb wave that occurs after the v wave.
Atrioventricular dissociation and complete atrioventricular block.
External atrial systole.
External systolic connector.
Severe tricuspid stenosis.
Grade 1 atrioventricular block with a prolonged PR interval.
Figure. Mechanism of cannon a-wave
The mechanism for most of the causes of cannonballs is due to the difference in systolic time between the atria and ventricles, resulting in the atrium contracting while the tricuspid valve is closed.
The a-wave reflecting the atrium begins to contract, while the pressure in the atrium increases slightly and the atrial size decreases momentarily. Normally, the tricuspid valve opens and atrial pressure drops as blood flows into the ventricles, while the ventricles contract and the tricuspid valve closes again.
When there is a difference between atrial contraction and ventricular dilation (regardless of cause), the atrium contracts strongly against the closing of the tricuspid valve, producing a wave due to increased pressure in the atria. into the carotid vein - cannon a wave.
Of all the causes of the a-wave listed, there is some degree of atrioventricular dissociation, when the atria are contracting at some point when the tricuspid valve is closed.
For example, in atrial flutter, the atria beat 2 to 4 times faster than the ventricle, depending on the degree of AV block. This means that there are atrial equilibria that contract while the valve is closed after the ventricles contract.
In a complete atrioventricular block, the atria and ventricles act at different pacing points that stimulate contractions at different times.