Cannon A waves are related to rhythm disturbances causing changes in the cardiac blood flow. Different dysrhythmias may cause cannon A waves. Heart block may lead to cannon a waves, in particular, third-degree (complete) heart block. It may be seen with ventricular tachycardia as a result of the inherent AV dissociation of the arrhythmia. Another cause is Pacemaker syndrome without proper synchronization of atria and ventricles. 
Cannon A waves need to be distinguished from giant A waves that occur in right heart structural changes such as tricuspid valvulopathies, right ventricular hypertrophy, and pulmonary hypertension. To the observer of the jugular, venous-pressure giant A waves and Cannon A waves may appear similar. From physical exam alone it might be difficult to differentiate between the 2.
Recent literature does not describe the frequency of cannon A waves in rhythm disturbances.
The normal function of the heart is to create forward blood flow. During each cardiac cycle, blood is pushed from the atria to the ventricles passing through the atrioventricular valves. Central venous pressure is measured in the right atrium. The bedside clinician can directly assess the jugular venous pulse by observing the patients neck. When obtaining a central or jugular venous pressure curve, its course can be described as 3 positive and 2 negative deflections. The 3 positive deflections are:
The negative deflections are:
A proper heart function requires a synchronized action of the myocardium. When the cardiac rhythm is disturbed, the heart cannot provide proper blood flow. Cannon A waves are an example of this. Rhythm disturbances cause mechanical problems since atrial contraction can occur when the tricuspid valve is closed. The tricuspid valve is closed because the pressure in the ventricle is greater than in the atrium. The reflection of the pressure wave travels up the venous system and can be examined in the jugular vein as exaggerated a wave pulsation.
A waves are the expression of proper atrial contraction. In opposition to exaggerated A waves, missing A waves might indicate atrial standstill.
Patients may complain of pulsations in the neck and abdomen as the pulse wave travels back the venous system. Other symptoms such as a headache, cough, and jaw pain can occur. Ask the patient for polyuria since increased atrial stress leads to higher BNP levels which in return will cause polyuria. Cannon A waves are associated with higher right atrial pressures. Erlebacher et al. describe that this may result in baroreceptor mediated systemic hypotension.
Inspection of venous jugular pulsation should be done parallel to auscultation of the heart. Remember the relation of the venous pressure curve to the heart sounds. This is important to differentiate the different pulsation phenomena. The A wave is followed closely by the S1 heart sound as the closure of the atrioventricular valves causes it.
The characteristic appearance of the neck vein pulsation is called frog sign according to Contreras-Valdes et al.
According to Ranjith et al. it is helpful to differentiate between regular and irregular Cannon waves since regular Cannon A waves might be caused by junctional or ventricular rhythm. Whereas, atrioventricular dissociation, ectopic atrial beats can result in irregular A waves.
Patients complaining of symptoms that can be related to cannon A waves or direct physical exam positive for cannon A waves should undergo further testing. To identify the cause of Cannon A waves and to distinguish from giant A waves an ECG and echocardiography should be performed. The ECG is helpful to look for rhythm disturbances. For visualization of structural changes, an ultrasound examination of the heart can be performed with attention to the right heart looking for hypertrophy, tricuspid pathology, and pulmonary hypertension.
Treatment of Cannon A waves depends on the underlying pathology.
When inspecting the patient's neck, a variety of pulsation phenomena might be seen. One should first divide between arterial and venous pulsations by location and strength on palpation. There are higher pressures existent in the arterial than in the venous circulation. The venous pressure curve can be altered in different ways and cannon A waves may be mistaken. For example, giant C and V wave can occur in tricuspid regurgitation. This is called Lancisi sign. Jugular vein distension can result from a pulmonary embolism as part of Becks Triad. A number of changes to the jugular venous pressure curve need to be distinguished. This illustrates the importance of venous pressure as a window to the right heart.
Cannon A waves relate to cardiologic pathology. Since then the patient may be referred to cardiologic consultation.
Clinicians (nurse practitioners, physicians, physician assistants) may notice a pulsating sensation in the neck. One cause for this might be a phenomenon called Cannon A wave. It is an exaggerated pulse wave of atrial contraction, usually when the atria contract blood is pushed into the ventricles. When the cardiac action is not well synchronized as might be the case in cardiac rhythm abnormalities, blood pushes against the closed tricuspid valve and create a reverse pulsation into the venous system. The pressure wave might be felt in the neck, jaw, and abdomen. A feeling of pulsation should prompt further investigation. When Cannon A waves are found, further investigations may be needed to identify the reason why the heart rhythm is not well synchronized. A cardiology consultation may be necessary.
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