Mechanism of Pulsus Paradoxus

Fluid and Air Dynamics

Not only is air affected by pressure, but so is fluid. That’s why the turkey baster has a bulb and why it’s called a turkey baster. You can use it to suck up turkey droppings and squirt them on the bird when cooking your Thanksgiving dinner. This is one example of using pressure—in this case, to baste.

Blood Return During Inhalation

The thorax works the same way when it comes to fluid (blood) as it does for air. When the chest expands, not only does air rush into the lungs, but the drop in pressure inside the chest helps to suck blood into the vena cavae, the large veins that bring blood back to the heart. The difference is that blood is coming from the rest of the body rather than from outside like air. The thorax is a cavity inside the body that is isolated from the other cavities. So when it sucks, it pulls in air and fluid.

Even the heart itself works this way. When the ventricles contract (systole) they squeeze blood out of the heart and into the arteries—especially the aorta. When the ventricles relax and expand back to their original size (diastole), they help to pull blood into the chambers. Blood is moved through the process as the heart expands and contracts. Blood is also moved through the process as the muscles of the body move and as pressures change within the chest cavity.

All of these changes in pressure would only move blood back and forth if it wasn’t for the one-way valves throughout the venous system to keep it moving forward. It’s the one-way valves in the heart and in the veins that make blood flow in the right direction.

CPR and Thoracic Pressure

Researchers are beginning to understand that this is an important part of how cardiopulmonary resuscitation (CPR) works. Not only is it important to push on the chest-deep enough and at the right rate, but it’s also equally important to release the chest and allow it to fully recoil.

The expansion of the chest in between compressions encourages critical blood return from the brain and abdomen.

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