A paradoxical pulse is defined as a marked decrease in the pulse amplitude during normal quiet inspiration or a decrease in the systolic arterial pressure by more than 10 mmHg. The normal small decline in systolic blood pressure probably is produced predominantly by relative pooling of blood in the pulmonary vessels during inspiration and also may reflect the delayed transmission through the lungs of the preceding expiratory fall in venous pressure and RV cardiac output.176
In patients with cardiac tamponade, fluid accumulation in the pericardium increases intrapericardial pressure, and the heart's filling capacity is reduced. During inspiration, the expected augmentation of venous return to the right side of the heart occurs despite the elevated intrapericardial pressure.191 The diminished thoracic pressure also causes a pooling of blood in the pulmonary veins and capillaries and diminishes pulmonary venous return to the left atrium. Since the high intrapericardial pressure limits flow to the heart and the total cardiac filling capacity is limited, the increase in right-sided heart volume with inspiration causes an obligatory decrease in left-sided heart filling. This, along with the pooling of blood in the pulmonary bed, produces a decline in LV stroke volume and systolic blood pressure during inspiration.192
Pulsus paradoxus is common with cardiac tamponade but infrequent with constrictive pericarditis (see Chap. 72). Different hemodynamic mechanisms contribute to the production of a paradoxical pulse in certain patients with superior vena cava obstruction, asthma, or obstructive airways disease; in some patients with pulmonary embolism or shock; and in some patients after thoracotomy.176
The extent of pulsus paradoxicus can be quantitated by cuff sphygmomanometry as the pressure difference between the first discernible Korotkoff sound on expiration and the pressure level at which Korotkoff sounds are audible during all phases of respiration.
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