A classic Step 1 associations is: increased pulsus paradoxus=cardiac tamponade. But like everything in medicine, it’s not that simple. An abnormal pulsus paradoxus is not really sensitive or specific for cardiac tamponade. Pulsus paradoxus may also be present in patients with labored breathing, asthma attacks, pulmonary hypertension, constrictive pericarditis, PE, etc. This summary cites one report that “15% pulsus paradoxus in the face of relative hypotension was found in 97% of patients with moderate or severe tamponade and only 6% of patients with absent or mild tamponade.”
This review in Clinical Cardiology describes pulsus paradoxus as the “end of a spectrum” in cardiac tamponade, which frames it as what it is: an exam finding that if present, means that it’s more likely the patient is ALREADY in tamponade and you should act quickly…but just because it’s absent doesn’t mean the patient does NOT have tamponade. This study reports that NHYA Class III symptoms (comfortable at rest but significant symptoms impairing function) were significantly associated the development of cardiac tamponade, which seems like it would be obvious, but can help in ambiguous cases. In my anecdotal experience, this is true, and relative hypotension, a narrow pulse pressure, and distended neck veins have been more sensitive for predicting tamponade.
NB: when trying to figure out if a pulsus paradoxus is abnormal, 20-30 mm Hg is considered high. But remember to take the pulse pressure into account! If the pulse pressure is narrow (like 110/90), then a pulsus of even 10 mm Hg might be abnormal and warrants immediate ultrasound.