The weakening of the arterial pulse during inspiration was recorded in 1854 by Vierordt, in a case of purulent pericarditis, and published the next year in his book Die Lehre vom Arterien Puls. By the time Kussmaul1 reported 4 cases of adhesive pericarditis, in 1873, it was known that rapid, deep inspiration caused a weak pulse in normal people, and normal breathing might cause it in cases of bronchial obstruction. In the cases without pericarditis it was apparent that venous pressure fell sharply during inspiration. But Kussmaul found that in 2 of his cases the jugular bulb distended even more during inspiration, and in the others little or no fall in venous pressure occurred. Pulsus paradoxus may be defined as an inspiratory drop in arterial pressure and pulsation with a rise or no significant fall in venous pressure. This occurs only in adhesive pericarditis, pericardial tamponade, or tense paramediastinal