An Overview of V/Q Mismatch

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V/Q mismatch is a defect that occurs in some areas of the lungs when the alveoli and capillaries don't line up or there is dead space, which means that part of your lung isn't receiving sufficient blood or oxygen flow. V stands for ventilation, the exchange of air between the lungs and the environment, while Q stands for perfusion, the passage of blood through the lungs. When V/Q ratio is out of balance, it can result in low oxygen flow in the blood (hypoxemia), a sign of respiratory failure.

V/Q mismatch may be caused by chronic obstructive pulmonary disease (COPD) or other lung conditions.

Symptoms

While you may not have any overt symptoms of V/Q mismatch outside of trouble breathing or catching your breath, symptoms of hypoxia may include:

  • Faintness or dizziness
  • Shortness of breath
  • Confusion
  • Greyish or bluish tint to the skin and mucus membranes (cyanosis)

V/Q mismatch is the most common cause of hypoxemia and a component of most causes of respiratory failure.

When the cells of the body are deprived of oxygen (and become damaged and ultimately die) due to hypoxemia, the condition advances to hypoxia—when there is not enough oxygen in your body to support its needs.

Causes

V/Q mismatch means that air exchange and blood flow are not evenly matched. It may be caused by anything that interferes with the ability of fresh air to get to the alveoli or anything that prevents blood flow to the capillaries. Both of these can disrupt the balance between ventilation and perfusion.

This can occur when:

  • In some areas of the lung, there are more capillaries (thin-walled blood vessels) than alveoli (the site where gas exchange happens in the lungs), so they are better perfused by blood than ventilated.
  • In other areas, there may be more alveoli with fresh oxygen than capillaries to pick up that oxygen, so they are better ventilated than perfused.

Some degree of mismatch occurs normally, but when it increases beyond a certain point, it causes the concerns outlined above.

Respiratory infections, heart conditions, and sleep apnea also increase your risk factor for V/Q mismatch.

Decreased V/Q Ratio

A decreased V/Q ratio occurs when either there is decreased ventilation in the lungs or increased perfusion (more blood flow to the lungs). Conditions which may result in a decreased V/Q ratio include:

  • Chronic bronchitis: The combination of bronchospasm, mucus plugs, inflammation, and airway obstruction all add up to worsen ventilation and, thus, decrease the ratio of ventilation to perfusion.
  • Asthma
  • Pulmonary edema
  • Airway obstruction (foreign body aspiration)

Increased V/Q Ratio

An increased V/Q ratio occurs when there is either increased ventilation or decreased perfusion (blood flow to the lungs). An increased V/Q ratio may be seen with:

  • Pulmonary embolism: A pulmonary embolism is a blood clot that usually starts in the legs, breaks off, and travels to the lungs. If it is large enough, a clot can completely clog a blood vessel, resulting in no perfusion to some areas of the lung that are getting ventilation. As a result, V/Q increases because the ventilation (V) stays the same and the perfusion (Q) goes down.
  • Emphysema: Emphysema can also cause an elevated V/Q ratio. Due to damaged and destroyed alveoli in the lungs, the surface area available for the transfer of oxygen and carbon dioxide goes down. Ventilation, therefore, goes up, leading to an increased V/Q ratio.

Diagnosis

Measuring V/Q ratio is done via two separate tests. To measure ventilation, your physician will place a mask over your face and you'll breathe in radioactive air. To measure perfusion, your physician will inject a radioactive protein into a vein. The results of these two tests are then compared to get the V/Q ratio.

V/Q or ventilation/perfusion refers to the amount of air that reaches the alveoli divided by the amount of blood that flows through the capillaries in the lungs.

A normal V/Q ratio is around 0.80. This means that roughly 4 liters of oxygen pass through the lungs each minute while 5 liters of blood pass through the lungs. An abnormal V/Q ratio would then be above or below 0.80.

Treatment

Treatment of a V/Q mismatch is designed around the root cause of the issue, whether that's COPD, asthma, blood clot, or other condition, with the main goal being to increase blood flow or oxygen flow and reduce or prevent hypoxemia.

Therapies may range from medication to supplemental oxygen to surgery, if necessary.

Treatment options include any of the options below:

  • Corticosteroid inhalers
  • Bronchodilators
  • Antibiotics
  • Oral steroids
  • Supplemental oxygen therapy
  • Blood thinner medications
  • Surgery

A Word From Verywell

While some V/Q mismatch is normal, an increased or decreased ratio may signify that your lungs aren't functioning properly. It should be taken as a warning sign that oxygen flow and blood flow aren't matching up—and intervention is required. It's important that your doctor checks your ventilation and perfusion ratio if you're having trouble breathing or shortness of breath, as V/Q mismatch is one of the primary causes of hypoxia, or low blood oxygen levels, which can be dangerous. Be sure to pay attention to any symptoms of dizziness or lightheadedness, and communicate these signs to your doctor right away.

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