Mechanism of Pulsus Paradoxus

Significance of Pulsus Paradoxus

No one is quite sure why certain conditions lead to a bigger drop in blood pressure during inhalation than the drop seen in healthy individuals. There is probably a clue in the types of conditions that cause pulsus paradoxus. They all have to do with extra pressure being exerted on the heart.

The following is a list of medical conditions that can cause pulsus paradoxus.

Cardiac Tamponade

Bleeding from the heart can sometimes lead to blood getting trapped between the heart and the membrane that surrounds the heart (pericardium). The pericardium is very tough and unforgiving. When blood seeps into it, the pericardium doesn’t stretch much. The heart gets squeezed by the lack of space left over as blood collects. This condition is known as cardiac tamponade, or sometimes as pericardial tamponade.

Tension Pneumothorax

Like blood collecting between the heart and the pericardium, air can leak from damaged lungs and become trapped between the lungs and the chest wall. In many cases, the leak is self-limiting. However, when enough air continues to leak and begins to put pressure on the lungs, heart, and major blood vessels, it’s known as a tension pneumothorax.

One hallmark of the tension pneumothorax is a significant and consistent drop in blood pressure. Pulsus paradoxus is an earlier sign that occurs as the tension pneumothorax is developing.


Besides cardiac tamponade, the pericardium can also cause problems if it becomes infected or inflamed. The stiffening and swelling of the pericardium during an episode of severe inflammation called pericarditis can lead to strain on the heart, interfering with an expansion of the ventricles during diastole.

Congestive Heart Failure

Decreased function in the ventricles usually following heart muscle damage from a heart attack can lead to a build-up of pressure in the circulatory system. This blood vessel traffic jam is known as congestive heart failure (CHF).

Left ventricular CHF, often simply referred to as left-sided heart failure, leads to pressure building up in the circulation of the lungs.

Right ventricular CHF, on the other hand, can sometimes precipitate the opposite effect of pulsus paradoxus, which is known as reverse pulsus paradoxus, during positive pressure ventilation. Instead of the increased thoracic pressure leading to a decrease in blood return to the left ventricle, it can instead improve blood flow from the right side into the left ventricle, which leads to better cardiac output and higher blood pressures.

Acute Asthma

Difficulty breathing from bronchospasm during acute asthma causes the patient to increase negative pressure in the thorax to compensate. The increased effort has an effect similar to that of backward pressure on circulation from the circulatory conditions listed above. Or, it could be seen as similar to the effect of positive pressure ventilation as described.

Chronic Obstructive Pulmonary Disease

Respiratory conditions that lead to increased work of breathing have similar effects on blood pressure. COPD causes the alveoli in the lungs to become delicate and lose their elasticity. Instead of a million rubber balloons, the alveoli become a million sandwich bags that won’t push air out during expiration.

Air trapped in the lungs from COPD can lead to a similar type of increased pressure on the circulatory system as CHF creates. And, like acute asthma, chronic obstructive pulmonary disease (COPD) can lead to more work of breathing.

A Word From Verywell

Even with modern medicine, there isn’t much consensus on what causes pulsus paradoxus. All we know so far is that it has to do with pressure in the thoracic cavity. All of the conditions that cause pulsus paradoxus affect the pressure inside the chest and breathing always affects that pressure gradient. Indeed, pressure is what keeps blood going round and round, and air going in and out.

Officially, pulsus paradoxus starts with a blood pressure drop of 10 mmHg, but the body doesn’t really follow hard and fast rules. We are organic beings that live on a continuum, a sliding scale of presentations, conditions, symptoms, signs, and oddities. Pulsus paradoxus happens because a constantly changing combination of factors leads to more pressure than expected pressing on the heart when we inhale. If we—the caregivers—pay close attention to what our monitors are telling us, pulsus paradoxus can be a great tool for identifying a very sick patient before her condition becomes dire.

Mechanism of Pulsus Paradoxus

How Pulsus Paradoxus Is Measured

Recognizing pulsus paradoxus requires the ability to measure systolic blood pressure while the patient is inhaling and exhaling. It can be done with a sphygmomanometer (the device that your healthcare provider uses to measure your blood pressure) but it is best to use an arterial line. An arterial line is a catheter inserted into an artery. It can be used to take blood samples of oxygenated blood or to measure the blood pressure in the arteries continuously and directly.

One of the reasons to regularly measure blood pressure at consistent intervals is to identify trends. A typical question caregivers get from both patients and newer care providers is why the blood pressure will fluctuate. It is a dynamic, organic process that has many factors affecting it. That’s why an arterial line is so much better as a blood pressure monitoring device of sick patients if it is available.

Mechanism of Pulsus Paradoxus

Blood Pressure During Positive Pressure Ventilation

Caregivers who treat patients on ventilators are aware of the effect of positive pressure ventilation on blood return to the heart. In cases of patients with low blood flow conditions, such as those who have just been resuscitated from cardiac arrest, positive pressure ventilation can have a profound effect on blood pressure.

Mechanism of Pulsus Paradoxus

Effect on Blood Pressure

When the heart is beating and everything is working the way it’s supposed to, expansion of the chest changes the pressure enough to have a small effect on systolic blood pressure (the top number).

Imagine if you were walking up an escalator and someone kept flipping the switch between up and down. Your progress up the escalator is like blood flowing through the arteries. Each step is a little push just like systole pushes blood. Each time the escalator is going up, it’s like exhaling and pushing you a little farther up. Each time the escalator is going down, it’s like inhaling and you really just stay put even though your legs are moving.

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.

Mechanism of Pulsus Paradoxus

Thoracic Pressure

Understanding how pulsus paradoxus happens and why it’s significant requires an understanding of how pressure in the chest cavity affects both breathing and circulation. The chest cavity (thorax) is a closed container that can expand and contract with the help of muscles in the chest wall, back, and floor of the chest cavity (diaphragm).

The lungs, airways, heart, and the largest blood vessels (often called the great vessels) share space inside the thorax. When the chest is expanded by muscles separating the ribs and by the diaphragm dropping down toward the abdomen, the pressure inside the chest falls. This causes air to rush into the airways because the atmosphere around the body now has a higher pressure than inside the chest and inside the lungs.

This is the normal way that humans take a breath. It’s called negative pressure ventilation and it can be compared to the bulb of a turkey baster. When you let go of the baster, the bulb expands and air rushes in.

The whole thing works in reverse as well. If you contract the chest wall and raise the diaphragm, the pressure inside the thorax becomes higher than the surrounding atmosphere and air is blown out. That works with the turkey baster as well. It’s as if you were to take the little rubber bulb from the turkey baster and put it on a bicycle horn to honk it.

Mechanism of Pulsus Paradoxus

Pulsus Paradoxus

Pulsus paradoxus is a drop in blood pressure of more than 10 mmHg (millimeters of mercury) when taking a breath. It’s a very specific sign that can only be adequately recognized when monitoring pressure with an arterial catheter. Pulsus paradoxus is a sign of some other condition and by itself is not a medical condition.

When you breathe in (called inspiration or inhalation), your blood pressure drops a little. However, when the amount of change in blood pressure is big enough, this is called pulsus paradoxus and could be a sign of one of several significant medical conditions.

How much of a drop is too much? A doctor named Adolf Kussmaul arbitrarily picked 10 mmHg over 100 years ago (which translates into 10 points using the standard mercury pressure gauge) and medical professionals have been using that number ever since.