The definition
In healthy people, systolic blood pressure dips slightly during inspiration, usually by a few mmHg. Pulsus paradoxus is when that normal dip becomes exaggerated, conventionally a fall of more than 10 mmHg with each breath. The name, coined by Adolf Kussmaul in 1873, is a little misleading: the pulse is not truly paradoxical, it is an amplified version of a normal phenomenon. The heart sounds remain regular while the palpable pulse weakens or disappears on inspiration.
Why it happens (briefly)
The short version: when something couples the filling of the two sides of the heart abnormally tightly, or when large swings in chest pressure are transmitted to the heart, inspiration produces a larger-than-normal drop in left-heart output, and therefore in the pulse. In cardiac tamponade, fluid under pressure around the heart is the coupling mechanism. In severe asthma and COPD, large swings in intrathoracic pressure are the driver. The clinically important point for detection is not the mechanism but the result: the pulse develops a pronounced, breath-linked variation.
How pulsus paradoxus is measured
There are three practical ways to capture it, with increasing convenience:
1. Manual sphygmomanometer (the classic method)
Inflate the cuff above systolic pressure, then deflate slowly. Note the pressure at which Korotkoff sounds are first heard (these appear only during expiration). Continue deflating and note the pressure at which the sounds are heard throughout the respiratory cycle. The difference between those two pressures is the magnitude of the pulsus paradoxus. A difference greater than 10 mmHg is abnormal.
2. Arterial line
An indwelling arterial catheter shows the respiratory variation directly on the pressure tracing, which is why it is often first noticed in the ICU. It is accurate but invasive and confined to monitored settings.
3. The pulse oximeter waveform
The photoplethysmography (PPG) waveform that every pulse oximeter already produces also carries the respiratory variation, as a rising and falling envelope on the pulse waveform. This is the signal PulSentry analyzes, because it is continuous, non-invasive, and available from hardware that is already in use.
The conditions it accompanies
- Cardiac tamponade, where it is one of the most specific bedside signs.
- Severe asthma and COPD exacerbations, where its magnitude tracks with severity.
- Tension pneumothorax and large pulmonary embolism, less commonly.
- Constrictive pericarditis, variably.
Because more than one condition can produce it, pulsus paradoxus is interpreted in clinical context rather than in isolation, and persistence over time, rather than a single transient reading, is what distinguishes a developing hemodynamic problem from a momentary artifact.
When pulsus paradoxus is absent or misleading
Pulsus paradoxus can be absent even when tamponade is present. Classic situations include a large atrial septal defect (which equalizes the respiratory effect between the two atria), significant aortic regurgitation, severe left ventricular dysfunction, and regional or loculated tamponade, which is common after cardiac surgery, where a localized clot compresses only part of the heart. This is one reason post-surgical tamponade is easy to miss and why continuous, objective monitoring of the signal is valuable.
From a bedside sign to a continuous signal
The limitation of pulsus paradoxus has never been its usefulness, it is that measuring it has traditionally required a clinician at the bedside with a cuff, or an arterial line. The respiratory variation in the pulse oximeter waveform makes it possible to track the same physiology continuously. PulSentry computes a pulsus index, the ratio of the respiratory component to the cardiac component in the frequency domain of the PPG signal, and follows how it changes over time.