[1] The paradox in pulsus paradoxus is that, on physical examination, one can detect beats on cardiac auscultation during inspiration that cannot be palpated at the radial pulse.
As is usual with inspiration, the heart rate is slightly increased,[2] due to decreased left ventricular output.
[3] During inspiration, the negative intra-thoracic pressure results in an increased right venous return, filling the right atrium more than during an exhalation.
[4] Normally during inspiration, a person's systolic blood pressure decreases by ≤10 mmHg[1] and heart rate slightly increases.
Pulsus paradoxus occurs not only with severe cardiac tamponade but also with asthma, obstructive sleep apnea and croup.
The mechanism, at least with severe tamponade, is likely very similar to those of hypertrophic and restrictive cardiomyopathies (diastolic dysfunction), where a decrease in Left Ventricular (LV) filling corresponds to an increasingly reduced stroke volume.
So during inspiration, since LV filling is lesser relative to that during expiration, the diastolic dysfunction is also proportionally greater, so the systolic pressure drops >10 mmHg.
[12] Pulsus paradoxus may not be seen with cardiac tamponade if an atrial septal defect or significant aortic regurgitation is also present.