In the presence of low arterial blood pressure and a rapid and feeble pulse, it is necessary to rule out the presence of cardiac tamponade. A valuable clinical sign suggestive of cardiac tamponade is the presence of a marked paradoxical pulse (also known as a pulsus paradoxus), which is characterized by an exaggeration of the normal inspiratory fall in systolic pressure. There is much confusion about the definition of a normal paradoxical pulse. A normal paradoxical pulse should be defined as the normal fall (about 5 mm Hg) in systolic arterial pressure during inspiration. It is the magnitude of the phenomenon that should determine whether the pulsus paradoxus is normal or abnormal.
The technique for assessing the magnitude of a paradoxical pulse is as follows: Have the patient breathe as normally as possible. Inflate the blood pressure cuff until no sounds are heard. Gradually deflate the cuff until sounds are heard in expiration only. Note this pressure. Continue to deflate the cuff slowly until sounds are heard during inspiration. Note this pressure. If the difference in these two pressures exceeds 10 mm Hg, a marked (abnormal) pulsus paradoxus is present; cardiac tamponade may be the cause. Cardiac tamponade results when there is an increase in intrapericardial pressure that interferes with normal diastolic filling. A marked paradoxical pulse is not a specific phenomenon for tamponade because it is also seen in large pericardial effusions, in constrictive pericarditis, and in conditions associated with increased ventilatory effort, such as asthma and emphysema.
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