In a V/Q ratio, the V stands for ventilation, which is the air you breathe in. The oxygen goes into the alveoli and carbon dioxide exits. Alveoli are small air sacs at the end of your bronchioles, which are your smallest air tubes.

Q, meanwhile, stands for perfusion, which is blood flow. Deoxygenated blood from your heart goes to the pulmonary capillaries, which are tiny blood vessels. From there, the carbon dioxide exits your blood through the alveoli and the oxygen is absorbed.

The V/Q ratio is the amount of air that reaches your alveoli 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.

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 are low oxygen levels in your blood. Not having enough blood oxygen can lead to respiratory failure.

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. It affects more than 250 million people 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

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

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

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 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:

  • breathlessness when lying down that improves when you sit up
  • shortness of breath on exertion
  • wheezing
  • rapid weight gain, particularly in the legs
  • fatigue

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.

The following increase your risk for V/Q mismatch:

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 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.

Treatment for V/Q mismatch will involve treating the cause. This may include:

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.