Mechanism of Pulsus Paradoxus in Cardiac Tamponade

Pathophysiological Mechanism

Pulsus paradoxus is defined as an inspiratory decrease in systolic arterial pressure of >10 mmHg during normal breathing, while diastolic blood pressure remains unchanged 2
During cardiac tamponade, the pericardial space fills with fluid, creating a fixed total heart volume constrained by the stiff pericardium 1
This creates exaggerated ventricular interdependence, where chambers must compete for space within the confined pericardial sac 3
During inspiration, there is increased venous return to the right heart, causing the right ventricle to expand 1
This rightward septal shift reduces left ventricular filling, decreasing left ventricular stroke volume and systolic blood pressure during inspiration 1
The opposite occurs during expiration, with decreased right heart filling allowing increased left heart filling 1

The stiffness of the pericardium determines how fluid accumulation affects cardiac function, creating a characteristic pressure-volume curve with an initial slow ascent followed by an almost vertical rise 1
This makes tamponade a "last-drop" phenomenon, where the final increment of fluid produces critical cardiac compression 1
The magnitude of pulsus paradoxus depends on several factors, including rate of fluid accumulation in the pericardial space, total amount of pericardial contents, distensibility of the pericardium, and filling pressures and compliance of cardiac chambers 3

Pulsus paradoxus can be detected by feeling the pulse, which weakens or disappears during inspiration 2
For accurate measurement using sphygmomanometry, inflate blood pressure cuff above systolic pressure, and during deflation, note when the first Korotkoff sound appears only during expiration, and continue deflation until the first Korotkoff sound is audible throughout the respiratory cycle 2
The difference between these two pressure points is the measure of pulsus paradoxus 2

Echocardiographic signs that correlate with pulsus paradoxus include exaggerated respiratory variability (>25%) in mitral inflow velocity, inspiratory decrease and expiratory increase in pulmonary vein diastolic forward flow, respiratory variation in ventricular chamber size, and abnormal ventricular septal motion 1

The presence of pulsus paradoxus should prompt immediate echocardiographic evaluation and consideration of pericardiocentesis in unstable patients 1