Pulsus paradoxus is the term used to describe an exaggerated blood pressure variation with the respiratory cycle. This can be found in cardiac tamponade or during chronic obstructive pulmonary disease or asthma exacerbations.
The image above ― an arterial pressure waveform tracing, superimposed with the respiratory cycle ― shows a significant decrease in arterial pressure with inspiration and return to normal in expiration. This hemodynamic effect of a large pericardial effusion causing cardiac tamponade is referred to as pulsus paradoxus.
To understand the physiologic mechanism of pulsus paradoxus, the normal changes in intrathoracic pressures during the respiratory cycle must first be described. Normal intrapericardial pressures range from -5 to 5 mm Hg. With inspiration, there is a net decrease in intrathoracic pressures as the thoracic cavity volume expands; this allows blood to easily flow into the right heart. Conversely, left heart filling decreases during inspiration, as the intrapericardial volume is fixed (if the right heart is full, the left heart is less full, and vice versa). During expiration, the intrathoracic pressures increase as the thoracic cavity volume decreases; this results in less right heart filling and augments filling of the left heart chambers. These changes within the respiratory cycle result in only small changes in measured systolic blood pressure ― no more than 10 mm Hg.
Any condition that results in increasing pressure variation with the respiratory cycle will exaggerate the measurable BP difference to more than 12 mm Hg, which is considered an abnormal pulsus paradoxus. When fluid accumulates in the pericardial space, and the intrapericardial pressures increase, the right heart is compressed, increasing the right heart pressures. This results in the right heart relying more heavily on the decreased intrathoracic pressures during inspiration to fill, exaggerating the pressure change, which is measurable.
To measure the pulsus paradoxus, place a blood pressure cuff on the patients arm and very very slowly deflate the cuff while listening for brachial pulsations. Note the pressure that you first hear with pulsations during expiration (which will be the highest). Repeat the process, and record the pressure of pulsations heard during inspiration (which will be the lowest). The difference between these two numbers is the pulsus paradoxus.