A patient with complete atrioventricular dissociation is examined. The jugular venous pulse shows intermittent large, abrupt 'a' waves occurring at random intervals relative to ventricular systole. What is the mechanism of these findings?
- A Regurgitation of blood through an incompetent tricuspid valve during ventricular systole
- B Right atrial contraction against a closed tricuspid valve when atrial and ventricular systoles coincide ✓
- C Elevated right ventricular end-diastolic pressure transmitted backward during inspiration
- D Loss of atrial relaxation causing continuous elevation of the venous column
Correct answer: B. Right atrial contraction against a closed tricuspid valve when atrial and ventricular systoles coincide
Explanation
Cannon 'a' waves occur when the right atrium contracts against a closed tricuspid valve, which happens intermittently in complete heart block whenever a P wave falls just before or during ventricular systole. Their random timing reflects atrioventricular dissociation. Tricuspid regurgitation instead produces a large systolic 'v' or 'cv' wave with each beat, and option C describes the Kussmaul-related finding seen in constrictive pericarditis rather than cannon waves.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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