A 40-year-old woman with tuberculous pericardial effusion has muffled heart sounds, distended neck veins, and systolic blood pressure that falls by 18 mmHg during inspiration. The primary mechanism of this inspiratory fall in systolic pressure is:
- A Direct compression of the thoracic aorta by the pericardial effusion during inspiration
- B Exaggerated negative intrathoracic pressure increasing venous return to the right ventricle at the expense of left ventricular filling ✓
- C Inspiratory increase in pulmonary venous capacitance diverting blood away from the left atrium
- D Reflex vagal withdrawal caused by distension of the pericardium
Correct answer: B. Exaggerated negative intrathoracic pressure increasing venous return to the right ventricle at the expense of left ventricular filling
Explanation
In tamponade, the rigid fluid-filled pericardium fixes total cardiac volume, so the ventricles share a fixed space (interdependence). Inspiration lowers intrathoracic pressure, augmenting systemic venous return and right ventricular filling; the septum bulges leftward, reducing left ventricular filling and stroke volume, producing an exaggerated inspiratory drop in systolic pressure. Option C describes a minor contributor seen normally, but septal shift from right-sided filling is the dominant mechanism in tamponade.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.