In a report published in the Archives of Internal Medicine half a century ago, Katz and Gauchat1 called attention to the nonparadoxical nature of pulsus paradoxus. They pointed out that the rhythmical diminution of the pulse and blood pressure on inspiration, which Kussmaul described 50 years previously as a characteristic of chronic cardiac compression, is not a paradoxical phenomenon but an exaggeration of the normal response. The blood pressure in the normal individual also drops on inspiration, but the fall of the systolic peak never exceeds 5 mm Hg. By contrast, in the pathologic "paradoxical" pulse, the fall is greater than 10 mm Hg.
The mechanism of the normal phenomenon probably reflects an imbalance between the outputs of the right and left ventricles. Reduced intrapleural and intrathoracic pressures on inspiration result in augmented venous return to the right atrium and thus an increased right ventricular volume. The simultaneous pooling