A 40-year-old woman with tuberculous pericardial effusion has a blood pressure that falls by 18 mmHg during inspiration. Which mechanism best explains this exaggerated inspiratory fall in systolic pressure?
- A Exaggerated negative intrathoracic pressure increasing left ventricular preload
- B Increased pulmonary venous capacitance reducing left ventricular filling while the intact pericardium forces interventricular dependence ✓
- C Inspiratory reduction in systemic vascular resistance out of proportion to cardiac output
- D Compression of the descending aorta by the effusion during inspiration
Correct answer: B. Increased pulmonary venous capacitance reducing left ventricular filling while the intact pericardium forces interventricular dependence
Explanation
In tamponade, all four chambers share a fixed pericardial volume. Inspiration increases right ventricular filling, which, because the septum cannot shift freely within the tense pericardium, displaces the septum leftward and reduces left ventricular filling and stroke volume. Pulmonary venous capacitance also rises on inspiration, further dropping left-sided inflow. Option A describes a normal inspiratory augmentation of venous return, not the fall in LV output.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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