A 34-year-old man with tuberculous pericardial effusion has pulsus paradoxus with an inspiratory fall in systolic blood pressure of 16 mmHg. The mechanism of this exaggerated inspiratory drop is:

• A Inspiration increases pulmonary venous return to the left atrium, overdistending the compressed left ventricle
• B Inspiration augments right ventricular filling, and the shared pericardial space forces the septum leftward, reducing left ventricular filling ✓
• C Inspiration reduces systemic venous return, lowering right-sided output and hence left-sided preload
• D Inspiration increases negative intrathoracic pressure, raising left ventricular afterload disproportionately

Explanation

In tamponade all four chambers compete for a fixed pericardial volume. Inspiration lowers intrathoracic pressure, increasing systemic venous return and expanding the right ventricle; because the pericardium cannot accommodate this, the interventricular septum bulges leftward, curtailing left ventricular filling and dropping systolic pressure by more than 10 mmHg. Option A reverses the physiology since pulmonary venous gradient actually falls with inspiration, and option C contradicts the known inspiratory rise in venous return.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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