When you breathe, a certain fraction of your air enters the lungs and blood flows through the capillaries. When this ratio is above or below 0.8, you may have ventilation/perfusion (V/Q) mismatch.

A V/Q mismatch happens when part of your lung receives oxygen without blood flow or blood flow without oxygen. This happens if you have an obstructed airway, such as when you’re choking, or if you have an obstructed blood vessel, such as a blood clot in your lung.

It can also happen when a medical condition causes you to bring in air but not extract oxygen, or bring in blood but not pick up oxygen.

A V/Q mismatch can cause hypoxemia, which is low oxygen levels in your blood. Not having enough blood oxygen can lead to respiratory failure.

In a V/Q ratio:

  • The V stands for ventilation, which is the air you breathe in.
  • The Q, somewhat confusingly, stands for perfusion, which is blood flow.

Essentially, the V/Q ratio is the amount of air that reaches your lungs divided by the amount of blood flow in the capillaries in your lungs.

When your lungs are functioning properly, 4 liters of air enter your respiratory tract while 5 liters of blood go through your capillaries every minute for a V/Q ratio of 0.8. A number that’s higher or lower is called a V/Q mismatch.

The exact symptoms of V/Q mismatch can vary depending on the underlying cause.

Common symptoms include:

Symptoms can also worsen as the underlying condition progresses. For example, if you typically experience wheezing or shortness of breath with COPD, a V/Q mismatch can increase the severity of both.

When to see a doctor

V/Q mismatch can interfere with your quality of life. It can reduce your energy level and make it harder to breathe. Treating the underlying cause of a mismatch can improve ventilation and perfusion.

See a doctor if you experience heart or respiratory symptoms like:

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There are two types of V/Q mismatch: dead space and shunt.

Dead space

Dead space is an area with ventilation but inadequate perfusion, in which oxygen can’t enter the bloodstream. A pulmonary embolism is a common cause of dead space, and this type of mismatch is treatable with oxygen therapy.

Additionally, there are two different types of dead space mismatches:

  • anatomic, in which there are areas in the respiratory tract with oxygen and no blood flow
  • physiologic, when the alveoli has oxygen but not enough blood flow to carry this oxygen


Shunt, on the other hand, is the opposite of dead space. This occurs when there’s adequate blood flow or perfusion, but not enough ventilation.

A shunt can be:

  • absolute, with zero ventilation
  • relative, with a small amount of ventilation

Conditions that can cause a shunt include pneumonia and pulmonary edema.

Anything that affects your body’s ability to deliver enough oxygen to your blood can cause a V/Q mismatch.

Chronic obstructive pulmonary disease (COPD)

COPD is a chronic inflammatory lung disease that obstructs airflow to your lungs. In 2019, it caused 3.23 million deaths worldwide.

Emphysema and chronic bronchitis are the most common conditions associated with COPD. Many people with COPD have both. The most common cause of COPD is cigarette smoke. Long-term exposure to chemical irritants can also cause COPD.

COPD increases your risk for other conditions affecting the lungs and heart, such as lung cancer and heart disease.

Some symptoms include:

  • difficulty breathing
  • chronic cough
  • wheezing
  • excess mucus production


Asthma is a condition that causes your airways to swell and narrow. It’s a common condition that affects approximately 1 in 13 people in the United States.

Experts aren’t sure what causes some people to develop asthma, but environmental factors and genetics appear to play a role. Asthma can be triggered by a number of things, including common allergens, such as:

Symptoms can vary from mild to severe and may include:

  • shortness of breath
  • chest tightness
  • coughing
  • wheezing


Pneumonia is a lung infection that can be caused by bacteria, a virus, or fungus. It can cause alveoli to fill with fluid or pus, making it difficult for you to breathe.

The condition can vary from mild to severe, depending on the cause and factors, such as your age and overall health. People over the age of 65, those with heart conditions, and people with a compromised immune system have a higher risk for severe pneumonia.

Pneumonia symptoms include:

Chronic bronchitis

Bronchitis is the inflammation of the lining of your bronchial tubes. The bronchial tubes carry air to and from your lungs.

Unlike acute bronchitis that comes on suddenly, chronic bronchitis develops over time and causes recurrent episodes that can last months or even years. The chronic inflammation results in excessive mucus buildup in your airways, which resists airflow in and out of your lungs and continues to worsen. Many people with chronic bronchitis eventually develop emphysema and COPD.

Symptoms of chronic bronchitis include:

  • chronic cough
  • thick, discolored mucus
  • shortness of breath
  • wheezing
  • chest pain

Pulmonary edema

Pulmonary edema, also known as pulmonary congestion or lung congestion, is a condition caused by excess fluid in the lungs. The fluid interferes with your body’s ability to get enough oxygen to your bloodstream.

It’s often caused by heart problems, such as congestive heart failure, but can also be caused by trauma to the chest, pneumonia, and exposure to toxins or high altitudes.

Symptoms include:

Airway obstruction

Airway obstruction is a blockage of any part of your airway. It can be caused by swallowing or inhaling a foreign object, or by:

An airway blockage can be mild, blocking only some airflow, to severe enough to cause a complete blockage, which is a medical emergency.

Pulmonary embolism

A pulmonary embolism is a blood clot in the lungs. A blood clot restricts blood flow, which can damage the lung and other organs.

They are most often caused by deep vein thrombosis, which are blood clots that begin in veins in other parts of the body, often the legs. Blood clots can be caused by injuries or damage to blood vessels, medical conditions, and being inactive for long periods.

Shortness of breath, chest pain, and irregular heartbeat are common symptoms.

Obstructive sleep apnea

Obstructive sleep apnea is characterized by a decrease in airflow while asleep. It affects at least 3 percent of the total population, but the risk is much higher for people with obesity, according to the American Sleep Apnea Association.

Common symptoms for those with sleep apnea include:


Cigarette smoke is another risk factor for a V/Q mismatch and other conditions because prolonged exposure to smoke can cause airway inflammation and airspace enlargement. Both can restrict airflow.

Smoking is also a risk factor for COPD, but quitting has been shown to slow the progression of this disease and improve breathing.

Measuring the V/Q ratio

V/Q ratio is measured using a test called a pulmonary ventilation/perfusion scan. It involves a series of two scans: one to measure how well air flows through your lungs and the other to show where blood is flowing in your lungs.

The test involves an injection of a mildly radioactive substance that gathers in areas of abnormal airflow or blood flow. This will then show in the images produced by a special type of scanner.

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The main goal of treatment for V/Q mismatch is to improve blood flood and airflow in the lungs, which involves treating the underlying cause. This may include:


Bronchodilators are a type of medication to improve breathing. They work by either relaxing the muscles in your lungs, or widening your airways. This treatment can improve breathing problems caused by COPD, asthma, and other lung conditions.

Inhaled corticosteroids

Your doctor might prescribe an inhaled steroid to help improve lung function. These can help asthma, COPD, and other lung conditions that cause inflammation and shortness of breath. Steroids can also reduce mucus in your airways, which can improve airflow to your lungs.

Oxygen therapy

Depending on the severity of your condition, you might require oxygen therapy. This is supplemental oxygen to improve your blood oxygen level, making it easier to breathe. This therapy can treat hypoxemia (low oxygen).

Oral steroids

In addition to an inhaled steroid, your doctor might also prescribe an oral steroid, such as prednisone. Similarly, this medication helps calm airway inflammation associated with asthma, COPD, and similar conditions.


Treating a V/Q mismatch can also involve the use of antibiotics. This is necessary when a bacterial infection like pneumonia lowers the oxygen level in your blood and makes it harder to breathe. Antibiotics work by fighting infections and preventing the reproduction of bacteria.

Pulmonary rehabilitation therapy

Pulmonary rehabilitation therapy is a type of therapy to improve lung function and reduce symptoms associated with serious lung diseases like COPD. It involves exercising your lungs to reduce shortness of breath, which can make it easier to perform daily activities. Therapy can also increase oxygen levels.

Blood thinners

Blood thinners help prevent the formation of new blood clots. Your doctor will likely prescribe this medication if you have a history of pulmonary embolism.


Sometimes, surgery is necessary to treat a V/Q mismatch. For example, you might need surgery to remove a large blood clot from inside a vein. This helps restore blood flow and can improve oxygen levels.

Additionally, surgery might improve poor blood flow caused by heart problems like congestive heart failure. Procedures include a coronary artery bypass, which diverts blood around a damaged artery, or valve surgery to repair or replace a diseased heart valve.

You need the right amount of oxygen and blood flow to breathe. Anything that interferes with this balance can cause a V/Q mismatch.

Shortness of breath, even if mild, should be evaluated by a doctor. Most causes of V/Q mismatch can be managed or treated, though timely treatment is important.

If you or someone else experiences sudden or severe shortness of breath or chest pain, get emergency medical care right away.