A 70-year-old man with a ventricular rate of 32 bpm has irregular large ('cannon') a waves visible in his jugular venous pulse. The mechanism producing these waves is:
- A Exaggerated right atrial filling during rapid passive ventricular filling
- B Regurgitant flow through an incompetent tricuspid valve during ventricular systole
- C Atrial contraction against a closed tricuspid valve when atria and ventricles beat independently ✓
- D Right atrial contraction against an open tricuspid valve during late diastole
Correct answer: C. Atrial contraction against a closed tricuspid valve when atria and ventricles beat independently
Explanation
In complete heart block the atria and ventricles are depolarised independently. When an atrial contraction happens to fall against a closed tricuspid valve (during ventricular systole), the entire right atrial pressure wave is transmitted to the neck veins as a cannon a wave. The intermittency of this coincidence explains their irregular appearance. C prominent v wave of tricuspid regurgitation, option B, would be systolic and regular, not intermittent.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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