Cardiac tamponade, Pulsus Paradoxus

Pulsus Paradoxus

• Abnormally decreased systolic blood pressure during inspiration is termed pulsus paradoxus.
• In fact, this is not a paradoxic response to respiratory changes in intrathoracic pressure.
• Normally, inspiration creates a more negative intrathoracic pressure, which results in enhanced right atrial and right ventricular filling at the minor expense of decreased left ventricular filling.
• A phasic increase in heart rate that is modulated by withdrawal of vagal tone compensates for the decrease in left ventricular preload during inspiration as the aortic baroreceptors signal the brainstem that left ventricular stoke volume has decreased by end inspiration.
• During exhalation, opposite changes occur. Left ventricular filling is increased modestly, resulting in a heart rate that slows slightly by end expiration. Normal sinus arrhythmia results from this phasic respiratory change in ventricular filling. These respiratory changes also are primarily responsible for the normal variation in the splitting of the second heart sound that occurs during the respiratory cycle.
• When there is cardiac tamponade from a large pericardial effusion, the normal decrease in left ventricular preload during inspiration is accentuated, and blood pressure falls. In severe tamponade, this inspiratory fall can be detected easily by the absence of a palpable pulse during inspiration and a return of the pulse during exhalation. 
• More subtle degrees of abnormal pulsus paradoxus may be detected by careful auscultation of the blood pressure as the cuff is deflated slowly. The first Korotkoff sound (K1) is heard, but only intermittently at first. The difference between this systolic blood pressure and the pressure at which a constant audible Korotkoff sound appears is the measured pulsus paradoxus.
• Values greater than 10 torr are abnormal. A measurement greater than 20 torr is considered severe and indicates that cardiac filling is seriously impaired by the effusion around the heart.
• An abnormally large variation in intrathoracic pressure also can produce a pulsus paradoxus, as during a severe asthma exacerbation.

Normal heart rate changes during the respiratory cycle generally are diminished because normal vagal tone is removed during expiration. Sympathetic tone predominates during all respiratory phases in the presence of tamponade. Accordingly, sinus tachycardia is one of the signs of cardiac tamponade.

Jugular venous pulsations may increase inappropriately during inspiration when filling of the right heart is impeded by the large effusion in the pericardial space. This sign is subtle. Increased jugular venous distention is seen in cardiac tamponade without
significant change during respiration.

The systolic blood pressure does fall during both inspiration and expiration once full tamponade results in systemic hypotension. It suggests the imminence of death.

References:
Roberts WC, Spray TL. Pericardial heart disease. Curr Probl Cardiol.
1977;2:1-71
Sagrista-Sauleda J, Merce J, Permanyer-Miralda G, Soler-Soler J.
Clinical clues to the causes of large pericardial effusions. Am J Med.
2000;109:95-101