What is ventilation perfusion

There actually is a spectrum of ventilation-perfusion relationships throughout the lung, created by normal physiologic relationships that dictate regional perfusion and ventilation.

Figure 4. The ventilation, perfusion, and the ventilation-perfusion ratio spectrums throughout the lungs, created by normal physiology that dictate regional perfusion and ventilation. In the upright lung, more ventilation goes to the lung base than the lung apex.

This arises because there are more alveoli at the larger bases. Figure 5. Ventilation of the lung decreases as the rib number decreases to the apex lung. This arises because there are more alveoli at the larger bases, and basilar alveoli have larger inflation.

In the upright lung, more perfusion goes to the lung base than the lung apex because there are more alveoli and pulmonary blood vessels in the larger bases, and because gravitational effects on pulmonary blood flow favor perfusion to the bases. Figure 6. Blood flow in the lung decreases as the rib number decreases to the apex lung.

This arises because there are more alveoli and pulmonary blood vessels in the larger bases. Although the apical-basal gradients for ventilation and perfusion are in the same direction, the magnitudes of changes in each from apex to base are different. The slope of the perfusion curve is steeper than that for ventilation. As a result, the ventilation-perfusion ratio decreases from apex to base.

Figure 7. The slope of the ventilation-perfusion ratio decreases from apex to base. This arises from the slope of the perfusion curve being steeper than that of the ventilation slope. In disease states, ventilation-perfusion relationships throughout the lung are altered, creating abnormal gas exchange, especially for oxygen. In particular, regions of the lung characterized by ventilation-perfusion ratios of less than one contributes to hypoxemia and widening of the alveolar-arterial oxygen gradient.

If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account , which will give you access to free videos and downloads. Blood Gas Analysis Clinical Guide. What is the ventilation-perfusion ratio? Written by Michael A. Grippi, MD. Edited by Shelley Jacobs, PhD. Equal ventilation and perfusion When breathing room air at an F I O 2 of 0. Perfused, not ventilated In one extreme of ventilation-perfusion mismatch, an alveolus is perfused, but not ventilated; in other words, it has a ventilation-perfusion ratio of zero.

Ventilated, not perfused In another extreme case of ventilation-perfusion mismatch, the alveolus is ventilated, but not perfused; in other words, the ventilation-perfusion ratio is infinity. A chest x-ray is usually done before or after a ventilation and perfusion scan. The table may feel hard or cold. You may feel a sharp prick when the IV is placed in the vein in your arm for the perfusion part of the scan. The mask used during the ventilation scan may make you feel nervous about being in a small space claustrophobia.

You must lie still during the scan. The radioisotope injection usually does not cause discomfort. The ventilation scan is used to see how well air moves and blood flows through the lungs.

The perfusion scan measures the blood supply through the lungs. A ventilation and perfusion scan is most often done to detect a pulmonary embolus blood clot in the lungs. It is also used to:. The provider should take a ventilation and perfusion scan and then evaluate it with a chest x-ray. All parts of both lungs should take up the radioisotope evenly.

If the lungs take up lower than normal amounts of radioisotope during a ventilation or perfusion scan, it may be due to any of the following:. No radiation is released from the scanner. Instead, it detects radiation and converts it into an image. There is a small exposure to radiation from the radioisotope.

The radioisotopes used during scans are short-lived. All of the radiation leaves the body in a few days. However, as with any radiation exposure, caution is advised for pregnant or breastfeeding women. There is a slight risk for infection or bleeding at the site where the needle is inserted. The risk with perfusion scan is the same as with inserting an intravenous needle for any other purpose.

In rare cases, a person may develop an allergy to the radioisotope. This may include a serious anaphylactic reaction. A pulmonary ventilation and perfusion scan may be a lower-risk alternative to pulmonary angiography for evaluating disorders of the lung blood supply.

This test may not provide a definite diagnosis, particularly in people with lung disease. Other tests may be needed to confirm or rule out the findings of a pulmonary ventilation and perfusion scan. This test has largely been replaced by CT pulmonary angiography for diagnosing pulmonary embolism. However, people with kidney problems or an allergy to contrast dye can more safely have this test.

Laboratory Tests and Diagnostic Procedures.