Pulsus paradoxsus is a condition in which during inspiration systolic blood pressure drop is more than 10mmHg.During inspiration in normal condition intrathoracic pressure becomes more negative which pulls the venous blood from superior and inferior venacava i.e venous return increases and right ventricle slightly overfilled and expanding in all direction which causes filling in left side of heart to be less so end diastolic volume and pressure is less and contractility is less and less cardiac output which determines systolic bp so there is drop in systolic bp but whereas during expiration it's reverse and systolic bp increase.so fall in systolic bp should be less than 10mmHg and this is considered to be normal. Hope this is helpfulππ