When you take a breath in, you may experience a mild, brief drop in blood pressure that’s unnoticeable. Pulsus paradoxus, sometimes called paradoxic pulse, refers to a blood pressure drop of at least 10 mm Hg with each breath in. This is enough of a difference to cause a noticeable change in the strength of your pulse.

Several things can cause pulsus paradoxus, especially conditions related to the heart or lungs.

When a person has a severe asthma attack, parts of their airways start to tighten and swell. The lungs start to overinflate in response, which puts extra pressure on the veins carrying unoxygenated blood from the heart to the lungs.

As a result, blood backs up in the right ventricle, which is the lower right part of the heart. This causes extra pressure to build up in the right side of the heart, which presses against the left side of the heart. All of this results in pulsus paradoxus.

In addition, asthma increases negative pressure in the lungs. This puts extra pressure on the left ventricle, which can also cause pulsus paradoxus.

In addition to a severe asthma attack, several heart and lung conditions can cause pulsus paradoxus. Hypovolemia can also cause pulsus paradoxus in situations where it’s severe. This occurs when a person doesn’t have enough blood in their body, usually due to dehydration, surgery, or an injury.

The following are the heart and lung conditions that may cause pulsus paradoxus:

Heart conditions:

Constrictive pericarditis

Constrictive pericarditis happens when the membrane surrounding the heart, called the pericardium, starts to thicken. As a result, when a person breathes in, the heart can’t open up as much as it usually does.

Pericardial tamponade

This condition, also known as cardiac tamponade, causes a person to build up extra fluid in the pericardium. Its symptoms include low blood pressure and large, noticeable neck veins. This is a medical emergency that requires quick treatment.

Lung conditions:

COPD exacerbations

Chronic obstructive pulmonary disease (COPD) is a condition that damages the lungs. When something, such as smoking cigarettes, causes its symptoms to suddenly get worse, it’s called a COPD exacerbation. COPD exacerbations have effects similar to those of asthma.

Massive pulmonary embolism

A pulmonary embolism is a blood clot in your lungs. This is a life-threatening condition that can affect someone’s ability to breath.

Obstructive sleep apnea

Sleep apnea causes some people to periodically stop breathing in their sleep. Obstructive sleep apnea involves blocked airways due to relaxed throat muscles.

Pectus excavatum

Pectus excavatum is the Latin term meaning “hollowed chest.” This condition causes a person’s breastbone to sink inward, which can increase pressure on the lungs and heart.

Large pleural effusion

It’s normal to have a little bit of fluid in the membranes that surround your lungs. However, people with pleural effusion have a buildup of extra fluid, which can make breathing difficult.

There are several ways to measure pulsus paradoxus, and some of them are more invasive than others.

The easiest way to check for it involves using a manual blood pressure cuff to listen for key differences in heart sounds while the cuff is deflating. Keep in mind that this won’t work with an automatic blood pressure cuff.

Another method involves inserting a catheter into an artery, usually the radial artery in the wrist or femoral artery in the groin. When hooked up to a machine called a transducer, the catheter can measure blood pressure beat to beat. This allows your doctor to see if there are any differences in your blood pressure when you breath in or out.

In cases of severe pulsus paradoxus, your doctor may be able to feel the difference in blood pressure just by feeling the pulse in your radial artery, just below your thumb. If they feel something unusual, they may ask you to take several slow, deep breaths to see if the pulse is weaker when you inhale.

Many things can cause pulsus paradoxus, which is a dip in blood pressure during inhalation. While it’s usually due to a heart or lung condition, such as asthma, it can also be the result of heavy blood loss.

If your doctor notices signs of pulsus paradoxus, they may run some additional tests, such as an echocardiogram, to check for any underlying conditions that might be causing it.