V/Q Mismatch in COPD

Normal, Increased, and Decreased V/Q Results

X-ray image of pulmonary embolism
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V/Q mismatch is a defect that occurs in the lungs whereby ventilation (the exchange of air between the lungs and the environment) and perfusion (the passage of blood through the lungs) are not evenly matched, a symptom typical of chronic obstructive pulmonary disease (COPD).

What V/Q Means

Before discussing what happens in V/Q mismatch, it's important to state exactly what V/Q means:

  • V stands for ventilation, the air that is breathed in through the nose and mouth, down through the trachea, into the bronchi, and then down through smaller airways into the smallest airways known as the alveoli where the exchange of oxygen and carbon dioxide takes place.
  • Q stands for perfusion or blood flow. From the heart, deoxygenated ("blue blood") passes through the pulmonary arteries (which are more like veins because they have blood in need of oxygen) through smaller and smaller vessels and into capillaries (the smallest blood vessels) where oxygen and nutrient exchange takes place.

The alveoli are the smallest of airways, only one cell thick. Next to the alveoli lies a capillary, the smallest of blood vessels and also only one cell thick. This is the place where the exchange of oxygen and carbon takes place in the lungs, as oxygen passes from the alveoli to the capillaries, and carbon dioxide passes from the capillaries to the alveoli. 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.

Understanding Dead Space

It's important to next define "dead space." This refers to any area in which the alveoli and capillaries do not line up. This dead space can be:

  • Anatomical dead space, which is due to an obstruction of either the airways or the blood supply, such as with pulmonary emboli (obstructing a blood vessel) or a foreign body inhaled into an airway (obstructing an airway).
  • Physiological dead space, which occurs when the structures of the airways or blood vessels are normal, but there are other reasons why the alveoli and capillaries don't match up such as a shunting of blood away from one area of the lungs (physiological obstruction).

With dead space, you are essentially bringing blood to an area in which it is unable to pick up oxygen, or bringing air into a region that is unable to extract oxygen.

Measuring V/Q Ratio

Measuring the V/Q ratio is done via two separate tests. To measure ventilation, a person has a mask placed over her face and breathes in radioactive air. To measure perfusion, a radioactive protein is injected into a vein. Then the results of these two tests are compared.

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 passes through the lungs.

What a V/Q Mismatch Means

V/Q mismatch means that in some areas of the lungs, the alveoli and capillaries don't line up or there is dead space. This can mean that in some areas of the lung, there are more capillaries than alveoli, 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 problems.

V/Q Mismatch and Hypoxemia

V/Q mismatch is the most common cause of hypoxemia and a component of most causes of respiratory failure. Hypoxemia is a term that refers to a decreased level of oxygen in the blood.

When the cells of the body are deprived of oxygen (and become damaged and ultimately die) due to hypoxemia, it is then termed hypoxia. Symptoms of hypoxia may be faintness, shortness of breath, confusion, and if untreated, death.

Causes of V/Q Mismatch

V/Q mismatch can be caused by anything which increases or decreases ventilation of the lungs or increases or decreases perfusion of the lungs. In other words, 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.

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)

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 in the lungs which usually begins as a blood clot in the legs which breaks off and travels to the lungs. If it is large enough, a clot can clog a complete blood vessel resulting in no perfusion to some areas of the lung that are getting ventilation. Therefore V/Q increases because the ventilation (V) stays the same and the perfusion (Q) goes down. On the V/Q scan, there will be evidence that segments of the lung in the distribution of the affected blood are not being perfused.
  • 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.
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