Situation: A 40-year-old man with tuberculous pericarditis, who takes antituberculosis drugs under the National Tuberculosis Program (NTP), is admitted to the medical ward with a large pericardial effusion. Bedside echocardiography shows early cardiac tamponade, and pericardiocentesis is being arranged. Measuring his blood pressure manually, the nurse finds that his systolic pressure falls by 16 mmHg each time he breathes in. He asks why his blood pressure changes with breathing. Which explanation should the nurse give?

This is pulsus paradoxus, an inspiratory fall in systolic pressure of more than 10 mmHg. When he breathes in, more blood returns to the right ventricle; because the fluid-filled sac cannot stretch, the right ventricle expands by pushing the septum into the left ventricle, which then fills and pumps less. This interdependence of the ventricles is why pulsus paradoxus is a sign of tamponade.

Why the systolic pressure drops during inspiration

The finding is pulsus paradoxus: an inspiratory fall in systolic blood pressure of more than 10 mmHg. In this patient, the drop is 16 mmHg, which is clearly abnormal and consistent with early cardiac tamponade from a large pericardial effusion ^[1].

The mechanism is not a vagal slowing of the heart, and it is not simply fluid being pulled away from the heart. The key is ventricular interdependence inside a stiff, fluid-filled pericardial sac ^[2][3].

During inspiration, negative intrathoracic pressure increases venous return to the right side of the heart. The right ventricle fills more. However, because the pericardial sac is already tense with fluid, the sac cannot stretch to accommodate the extra right ventricular volume. The expanding right ventricle therefore pushes the interventricular septum toward the left ventricle, reducing left ventricular filling and stroke volume. That lower left ventricular stroke volume is transmitted to the arterial tree as a fall in systolic pressure during inspiration ^[1][3].

This sequence explains why option 3 is correct: inspiration fills the right ventricle, which bulges into the left ventricle through septal shift. The left ventricle then fills and ejects less, producing the inspiratory pressure drop.

Option	Proposed mechanism	Why it is incorrect or correct
1	Vagal slowing of the heart lowers pressure	Inspiration normally increases heart rate slightly; vagal slowing is not the mechanism of pulsus paradoxus.
2	Inspiration pulls fluid out of the sac away from the heart	Pericardial fluid is not removed by breathing; the effusion remains and restricts filling.
3	Inspiration fills the right ventricle, which bulges into the left one	Correct. This describes ventricular interdependence and septal shift in tamponade.
4	Inspiration makes the lungs squeeze both ventricles against the sac	Lung compression is not the primary mechanism; the septal shift from right ventricular filling is central.

Key point! Pulsus paradoxus is defined as an inspiratory drop in systolic pressure greater than 10 mmHg. It reflects exaggerated ventricular interdependence, not a change in heart rate or direct lung compression ^[1][3].

Watch out! Pulsus paradoxus also occurs in severe acute asthma and COPD exacerbations, where wide intrathoracic pressure swings or right ventricular distension produce a similar inspiratory fall in left ventricular stroke volume ^[1][2]. The presence of a large pericardial effusion with early tamponade makes tamponade the most likely cause in this patient.

The hemodynamic consequence is a low cardiac output state. In tamponade, the clinical diagnosis requires evidence of reduced cardiac output together with the echocardiographic finding of effusion; echocardiography alone is insufficient to confirm hemodynamically significant tamponade . The bedside finding of pulsus paradoxus is therefore a critical piece of the hemodynamic assessment while pericardiocentesis is being arranged.