Ventilation-Perfusion (V/Q) Scan: Uses and Key Findings

A V/Q scan, also called a ventilation-perfusion scan, is a diagnostic imaging test used to compare airflow and blood flow in the lungs. The “V” stands for ventilation, which refers to the movement of air into the alveoli. The “Q” stands for perfusion, which refers to blood flow through the pulmonary circulation.

This test is most commonly associated with the evaluation of pulmonary embolism, but it also helps explain important respiratory concepts such as V/Q mismatch, dead space ventilation, and impaired gas exchange.

What is a V/Q Scan?

A V/Q scan is a nuclear medicine imaging study that evaluates how well air and blood are distributed throughout the lungs. It is called a ventilation-perfusion scan because it has two major parts. The ventilation portion shows where air is moving in the lungs, while the perfusion portion shows where blood is flowing through the pulmonary vessels.

For gas exchange to occur efficiently, air must reach the alveoli and blood must reach the pulmonary capillaries surrounding those alveoli. When ventilation and perfusion are well matched, oxygen can move from the alveoli into the blood, and carbon dioxide can move from the blood into the alveoli to be exhaled.

A V/Q scan helps clinicians determine whether this relationship is normal or abnormal in different regions of the lung. The test is especially useful when clinicians suspect that a portion of the lung is receiving air but not enough blood flow. This is the classic pattern seen with pulmonary embolism.

In simple terms, a V/Q scan asks two questions:

• Is air reaching this part of the lung?
• Is blood reaching this part of the lung?

Note: When the answer to both questions is yes, ventilation and perfusion are matched. When the answer is different between the two, a V/Q mismatch is present.

Why Ventilation and Perfusion Matter

Ventilation and perfusion must work together for normal gas exchange. Ventilation brings oxygen into the alveoli and removes carbon dioxide from the lungs. Perfusion brings deoxygenated blood to the pulmonary capillaries so it can pick up oxygen and release carbon dioxide.

A healthy lung does not have perfectly equal ventilation and perfusion in every region. Even under normal conditions, the V/Q ratio varies from the top of the lungs to the bottom. In an upright person, the lung bases receive more ventilation and much more perfusion than the apices. Because blood flow increases more than ventilation in the bases, the V/Q ratio is lower there. The apices receive less blood flow relative to ventilation, so the V/Q ratio is higher.

The average V/Q ratio for the whole lung is approximately 0.8. This comes from normal alveolar ventilation of about 4 L/min and normal pulmonary capillary blood flow of about 5 L/min. However, this average does not mean every alveolus has the same ratio. Some lung regions have relatively high V/Q ratios, while others have relatively low V/Q ratios.

The problem occurs when the mismatch becomes severe enough to impair gas exchange. A V/Q scan helps identify where this is happening and whether the pattern suggests a vascular problem, an airway problem, or lung tissue disease.

Main Purpose of a V/Q Scan

The main clinical purpose of a V/Q scan is to help diagnose or rule out pulmonary embolism. A pulmonary embolism occurs when a clot or other obstruction blocks blood flow through part of the pulmonary arterial system. The clot often begins as a deep vein thrombosis in the lower extremities, then travels through the venous system, passes through the right side of the heart, and lodges in the pulmonary circulation.

When this happens, the affected alveoli may still receive air, but blood flow is reduced or absent. This creates a high V/Q mismatch. The region is ventilated, but it is not adequately perfused. As a result, ventilation is wasted because there is little or no blood available for gas exchange.

This is why the classic V/Q scan finding for pulmonary embolism is a perfusion defect without a matching ventilation defect. In other words, air is reaching the lung region, but blood flow is blocked.

A V/Q scan may also be used to assess regional pulmonary blood flow before lung resection surgery. If part of the lung needs to be removed, clinicians may want to know how much blood flow goes to different lung regions. This helps estimate how much functional pulmonary circulation will remain after surgery.

V/Q Scan and Pulmonary Embolism

Pulmonary embolism is one of the most important conditions associated with V/Q scanning. In PE, a clot blocks blood flow to part of the lung. Because the airways may remain open, ventilation to that region may continue. The problem is not that air cannot get in. The problem is that blood cannot reach the ventilated alveoli.

This pattern produces alveolar dead space. Dead space refers to ventilation that does not participate in gas exchange. In anatomic dead space, air remains in the conducting airways where gas exchange cannot occur. In alveolar dead space, air reaches the alveoli, but there is little or no perfusion available for gas exchange.

A pulmonary embolism increases alveolar dead space because ventilated alveoli are not receiving adequate blood flow. The patient may suddenly develop dyspnea, tachypnea, hypoxemia, chest discomfort, or signs of cardiopulmonary distress. The severity depends on the size of the embolus, the patient’s cardiopulmonary reserve, and the amount of pulmonary circulation affected.

On a V/Q scan, pulmonary embolism is suspected when one or more lung regions have reduced perfusion while ventilation is preserved. This is called a mismatched defect. Multiple segmental perfusion defects without corresponding ventilation defects are considered a high-probability pattern for pulmonary embolism.

Ventilation Portion of the Scan

The ventilation portion of a V/Q scan shows where air moves in the lungs. During this part of the test, the patient inhales a radiolabeled gas or aerosol. Common examples include xenon-133 or technetium-99m aerosol.

The patient breathes the substance through a mouthpiece or mask. The radioactive material distributes through the airways and alveoli, allowing a scanner to detect where ventilation is occurring. Areas that receive air normally will show normal ventilation. Areas that do not receive air normally may appear as ventilation defects.

The ventilation scan can help identify underventilated regions of the lung. Underventilation may occur due to airway obstruction, a tumor, a foreign body, mucus plugging, atelectasis, consolidation, emphysema, or other lung conditions that interfere with airflow.

Note: The ventilation scan shows where air is going. If a lung region does not receive air, the ventilation image helps reveal that abnormality.

Perfusion Portion of the Scan

The perfusion portion of a V/Q scan shows where blood flows through the pulmonary circulation. During this phase, the patient receives an intravenous injection of a radioactive tracer. A commonly used substance is technetium-99m-labeled macroaggregated albumin.

After injection, the tracer travels through the venous system to the right side of the heart and then into the pulmonary circulation. The particles become distributed in the pulmonary capillary bed in proportion to regional blood flow. A scanner then detects the radiation through the chest wall and creates an image of perfusion distribution.

Areas with normal blood flow show normal tracer distribution. Areas with decreased or absent blood flow appear as perfusion defects.

Note: The perfusion scan shows where blood is going. If a region of the lung is ventilated but lacks perfusion, pulmonary embolism becomes a major concern.

Matching vs. Mismatching

The most important concept in V/Q scan interpretation is the difference between matched and mismatched defects.

A matched defect occurs when ventilation and perfusion are both reduced in the same area. This means the affected lung region is not receiving normal air movement or normal blood flow. Matched defects are often associated with lung tissue disease, also called parenchymal lung disease. Examples may include pneumonia, atelectasis, consolidation, emphysema, or other conditions that interfere with both ventilation and perfusion.

A mismatched defect occurs when ventilation and perfusion do not match. The most important mismatch for pulmonary embolism is preserved ventilation with decreased perfusion. In this case, air reaches the alveoli, but blood flow is reduced or absent. This pattern suggests a blockage in the pulmonary circulation.

A mismatched defect is more suspicious for pulmonary embolism than a matched defect because PE is primarily a vascular problem. The airway may remain open, but blood flow is obstructed.

Note: The classic finding is multiple segmental perfusion defects without corresponding ventilation defects. This means multiple lung regions are receiving air but not enough blood flow. That pattern indicates a high probability of pulmonary embolism.

High V/Q Ratio

A high V/Q ratio occurs when ventilation is high compared with perfusion. This can happen when ventilation remains normal but perfusion decreases. Pulmonary embolism is the classic example.

In a high V/Q state, air enters the alveoli, but little or no blood flows past those alveoli. Because there is not enough blood available for gas exchange, ventilation is wasted. This creates a dead space effect.

As the V/Q ratio increases, alveolar oxygen tends to rise and alveolar carbon dioxide tends to fall. This happens because oxygen enters the alveolus but is not being removed efficiently by blood flow. Carbon dioxide decreases because there is less venous blood bringing carbon dioxide to the alveolus.

However, this does not mean the patient is oxygenating well overall. Even though the affected alveoli may contain oxygen, that oxygen cannot enter the bloodstream effectively if perfusion is absent. The result can be hypoxemia, especially when a large amount of pulmonary blood flow is affected.

Low V/Q Ratio

A low V/Q ratio occurs when ventilation is low compared with perfusion. In this situation, blood continues to flow through a region of the lung, but air movement is reduced or absent.

This may occur with airway obstruction, atelectasis, pneumonia, bronchitis, asthma, emphysema, pulmonary fibrosis, or hypoventilation. In these conditions, blood may pass through lung regions that are poorly ventilated. Because the blood is not exposed to enough fresh alveolar gas, oxygenation becomes impaired.

A low V/Q ratio creates a shunt-like effect. The blood is present, but ventilation is inadequate. Oxygen levels fall because alveolar oxygen is not being replaced effectively. Carbon dioxide may rise in the affected units because ventilation is not removing it adequately.

On a V/Q scan, low V/Q patterns may appear as ventilation defects with preserved or relatively greater perfusion. This is different from the pulmonary embolism pattern, where ventilation is preserved and perfusion is reduced.

How V/Q Scan Results Are Interpreted

V/Q scan results are often interpreted using probability categories rather than a simple positive or negative result. The scan may be reported as normal, low probability, intermediate probability, or high probability for pulmonary embolism.

A normal or near-normal V/Q scan makes clinically significant pulmonary embolism unlikely, especially when the patient also has a low clinical probability of PE.

A low-probability scan also lowers the suspicion for PE, particularly when clinical assessment does not strongly suggest embolism.

A high-probability scan strongly suggests PE, especially when the patient’s clinical probability is also high. For example, a patient with sudden dyspnea, pleuritic chest pain, tachycardia, hypoxemia, recent surgery, immobility, or known deep vein thrombosis would have a higher clinical suspicion.

An intermediate-probability scan is less helpful because it does not clearly confirm or exclude PE. In this situation, additional diagnostic testing may be needed.

This is why V/Q scan interpretation must be combined with clinical judgment. The scan result is important, but it should not be interpreted in isolation. The patient’s signs, symptoms, risk factors, physical findings, lab results, and other imaging studies all matter.

Clinical Probability and V/Q Scan Results

Clinical probability refers to how likely a patient is to have pulmonary embolism before imaging results are considered. This is based on the patient’s history, risk factors, symptoms, and overall presentation.

A high-probability V/Q scan in a patient with high clinical probability strongly supports the diagnosis of PE. A normal or near-normal scan in a patient with low clinical probability can help rule out clinically significant PE.

The challenge occurs when imaging and clinical probability do not align. For example, a high-probability scan in a patient with low clinical probability is less definitive than the same scan in a patient with high clinical probability. Similarly, an intermediate scan may not provide enough information to make a confident decision.

Note: This is an important test-taking concept. V/Q scan results are not interpreted in a vacuum. The best answer often depends on both the scan findings and the patient’s clinical picture.

V/Q Scan vs. CT Angiography

CT angiography (CTA) is now commonly used as the primary imaging test for suspected pulmonary embolism. CTA can directly visualize clots in the pulmonary arteries and may also reveal other causes of symptoms, such as pneumonia, pneumothorax, pleural disease, or other thoracic abnormalities.

CTA is widely available, fast, and useful for detecting emboli in the main and lobar pulmonary arteries. However, it requires iodinated contrast material. This can be a problem for patients with significant contrast allergy or renal failure.

A V/Q scan is often used when CTA is contraindicated or inconclusive. For example, if a patient cannot receive iodinated contrast because of kidney dysfunction or a serious contrast allergy, a V/Q scan may be a useful alternative.

Note: This does not mean the V/Q scan is outdated or unimportant. It remains clinically useful in selected patients and is still highly relevant for respiratory therapy students preparing for board exams.

V/Q Scan vs. Pulmonary Angiography

Pulmonary angiography is another imaging method used to evaluate pulmonary arterial blood flow. It involves catheter placement and injection of contrast dye into the pulmonary circulation. The contrast allows clinicians to visualize the pulmonary arteries and identify areas where blood flow is blocked.

Historically, pulmonary angiography was considered a definitive test for pulmonary embolism. However, it is more invasive than CTA or V/Q scanning. Modern CT angiography has replaced older approaches in many clinical settings.

If a V/Q scan is inconclusive, further testing may be required. Depending on the situation, this may include CT angiography, venous ultrasound, pulmonary angiography, or other diagnostic studies. The choice depends on the patient’s condition, contraindications, and the level of clinical suspicion.

V/Q SPECT

V/Q SPECT stands for ventilation-perfusion single-photon emission computed tomography. It is an advanced form of V/Q imaging that provides three-dimensional information rather than only traditional planar images.

Because SPECT imaging provides more detailed regional information, it may improve sensitivity and help estimate embolus size more accurately. This can make the scan more useful in certain clinical settings.

For basic respiratory therapy exam preparation, students should understand that V/Q SPECT is a more advanced version of V/Q imaging. The core principle remains the same: compare ventilation with perfusion to identify matched or mismatched defects.

Patient Requirements for a V/Q Scan

A patient must be able to cooperate with certain parts of the procedure. The ventilation portion often requires the patient to breathe through a mouthpiece or mask. The patient may also need to hold their breath briefly and remain still during image acquisition.

These requirements matter because poor cooperation can reduce image quality. A patient who is severely anxious, confused, unstable, or unable to follow instructions may not be an ideal candidate for the test.

For board exam purposes, this is important. A V/Q scan may be appropriate for evaluating suspected PE, but it may not be the best choice if the patient cannot cooperate with the procedure or cannot remain still.

What Respiratory Therapists Should Know

Respiratory therapists do not usually perform the full interpretation of a V/Q scan, but they should understand what the test evaluates and what the major findings mean.

A respiratory therapist may encounter V/Q scan results in the patient record, during clinical rounds, or in exam questions. Understanding the scan helps connect imaging findings with gas exchange abnormalities.

For example, a patient with PE may have sudden hypoxemia even though the airways are open. The reason is that part of the lung is ventilated but not perfused. Oxygen reaches the alveoli, but there is not enough blood flow to carry that oxygen away.

In contrast, a patient with atelectasis may have poor ventilation to a lung region while blood flow continues. This produces a low V/Q pattern and contributes to shunt-like physiology.

The respiratory therapist should be able to recognize that different V/Q patterns point toward different problems. High V/Q suggests wasted ventilation or dead space. Low V/Q suggests inadequate ventilation relative to perfusion.

Board Exam Tips for V/Q Scans

For board exam preparation, the most testable point is the relationship between V/Q scanning and pulmonary embolism.

A high probability of pulmonary embolism is suggested by multiple segmental perfusion defects without corresponding ventilation defects. This means the affected areas are being ventilated but not perfused.

Another important point is the difference between the ventilation and perfusion phases. The ventilation portion uses an inhaled radiolabeled gas or aerosol, such as xenon-133 or technetium aerosol. The perfusion portion uses an injected tracer, commonly technetium-99m-labeled macroaggregated albumin.

Students should also remember that a matched defect is less specific for PE and is more likely to suggest lung tissue disease. A mismatched defect, especially preserved ventilation with decreased perfusion, is more concerning for PE.

Note: A V/Q perfusion scan may appear as an information-gathering option when a pulmonary embolism is suspected. It should be selected when it fits the clinical situation and when the results would help confirm or rule out PE.

Common V/Q Scan Patterns

A normal V/Q scan shows ventilation and perfusion that are reasonably matched throughout the lungs. This makes clinically significant PE unlikely.

• A high-probability PE pattern shows one or more perfusion defects with preserved ventilation. This is a mismatched defect and suggests blocked pulmonary blood flow.
• A matched defect shows both reduced ventilation and reduced perfusion in the same region. This is more consistent with parenchymal lung disease than an isolated pulmonary embolism.
• A low V/Q pattern occurs when ventilation is reduced relative to perfusion. This may occur with airway obstruction, atelectasis, pneumonia, or other conditions that impair air movement.
• A high V/Q pattern occurs when perfusion is reduced relative to ventilation. This may occur with pulmonary embolism, pulmonary vascular obstruction, destruction of pulmonary vessels, or decreased cardiac output.

Clinical Examples

Consider a patient who suddenly becomes short of breath after surgery. The patient is tachycardic, hypoxemic, and has pleuritic chest pain. If a V/Q scan shows multiple segmental perfusion defects without ventilation defects, pulmonary embolism is highly suspected. The scan suggests that air is reaching those lung regions, but blood flow is blocked.

Now consider a patient with mucus plugging and atelectasis. In this case, a lung region may have reduced ventilation because the airway is obstructed. Blood flow may still continue through the area. This produces a low V/Q pattern and contributes to shunt-like physiology.

Finally, consider a patient with pneumonia. The affected lung region may have both poor ventilation and altered perfusion due to inflammation and consolidation. This may produce a matched defect rather than the classic mismatched pattern seen with PE.

Note: These examples show why V/Q interpretation depends on the pattern. The scan is not just about finding an abnormality. It is about comparing air movement with blood flow.

Limitations of a V/Q Scan

A V/Q scan can provide valuable information, but it has limitations. One major limitation is that intermediate-probability results may not clearly confirm or rule out PE. When this occurs, additional testing is often needed.

Another limitation is that abnormal lung tissue can make interpretation more difficult. Patients with COPD, pneumonia, atelectasis, or other lung diseases may have ventilation and perfusion abnormalities that complicate the scan results.

The test also requires patient cooperation. The patient must be able to breathe through a mouthpiece or mask, follow instructions, and remain still during imaging. Poor cooperation may reduce accuracy.

Finally, CT angiography is often preferred when it is safe and available because it can directly visualize emboli and identify alternative diagnoses. Still, the V/Q scan remains useful when CTA is contraindicated, especially in patients with contrast allergy or renal impairment.

V/Q Scan Practice Questions

1. What is a ventilation-perfusion scan commonly called?
A ventilation-perfusion scan is commonly called a V/Q scan or V̇/Q̇ scan.

2. What does the “V” stand for in a V/Q scan?
The “V” stands for ventilation, which refers to the movement of air into the lungs.

3. What does the “Q” stand for in a V/Q scan?
The “Q” stands for perfusion, which refers to blood flow through the pulmonary circulation.

4. What is the main purpose of a V/Q scan?
The main purpose of a V/Q scan is to compare ventilation and perfusion in the lungs, especially when evaluating for pulmonary embolism.

5. What condition is most commonly associated with the use of a V/Q scan?
Pulmonary embolism is the condition most commonly associated with the use of a V/Q scan.

6. What is a pulmonary embolism?
A pulmonary embolism is a blockage in the pulmonary circulation, usually caused by a blood clot that travels from a deep vein thrombosis.

7. Why can pulmonary embolism create a V/Q mismatch?
Pulmonary embolism can block blood flow to a lung region while ventilation remains present, creating a mismatch between air movement and perfusion.

8. What is the classic V/Q scan finding in pulmonary embolism?
The classic finding is ventilation without perfusion, meaning air reaches the lung region but blood flow is reduced or absent.

9. What type of defect suggests pulmonary embolism on a V/Q scan?
A mismatched defect suggests pulmonary embolism, especially when perfusion is reduced but ventilation is preserved.

10. What is a matched defect on a V/Q scan?
A matched defect occurs when both ventilation and perfusion are reduced in the same lung region.

11. What does a matched defect usually suggest?
A matched defect usually suggests lung tissue disease rather than an isolated pulmonary embolism.

12. What is a mismatched defect on a V/Q scan?
A mismatched defect occurs when ventilation and perfusion do not match, such as normal ventilation with decreased perfusion.

13. What does the ventilation portion of a V/Q scan evaluate?
The ventilation portion evaluates where air moves into the lungs during breathing.

14. What does the perfusion portion of a V/Q scan evaluate?
The perfusion portion evaluates where blood flows through the pulmonary circulation.

15. What radioactive gas may be used during the ventilation portion of a V/Q scan?
Xenon-133 may be used during the ventilation portion of a V/Q scan.

16. What radioactive material may be used during the perfusion portion of a V/Q scan?
Technetium-99m-labeled macroaggregated albumin may be used during the perfusion portion.

17. How is the ventilation tracer given to the patient?
The ventilation tracer is inhaled by the patient, usually through a mouthpiece or mask.

18. How is the perfusion tracer given to the patient?
The perfusion tracer is injected intravenously into the venous system.

19. Why are ventilation and perfusion images compared?
They are compared to determine whether the same lung regions are receiving both air and blood flow.

20. What does it mean if ventilation is normal but perfusion is decreased?
It means air is reaching the affected region, but blood flow is reduced, which may suggest pulmonary embolism.

21. What does it mean if ventilation is decreased but perfusion is preserved?
It means blood flow continues to a region that is poorly ventilated, which may create a low V/Q pattern.

22. What is a high V/Q ratio?
A high V/Q ratio occurs when ventilation is high compared with perfusion.

23. What condition is a classic example of a high V/Q ratio?
Pulmonary embolism is a classic example of a high V/Q ratio.

24. What is a low V/Q ratio?
A low V/Q ratio occurs when ventilation is low compared with perfusion.

25. What conditions may cause a low V/Q pattern?
Airway obstruction, atelectasis, pneumonia, bronchitis, asthma, emphysema, and hypoventilation may cause a low V/Q pattern.

26. What does a V/Q scan help clinicians compare?
A V/Q scan helps clinicians compare the distribution of air movement with the distribution of pulmonary blood flow.

27. Why is CT angiography often used before a V/Q scan for pulmonary embolism?
CT angiography is often used first because it is widely available, sensitive, specific, and can identify other causes of symptoms.

28. When may a V/Q scan be preferred over CT angiography?
A V/Q scan may be preferred when CT angiography is contraindicated or inconclusive.

29. Why might renal failure be a reason to avoid CT angiography?
Renal failure may be a reason to avoid CT angiography because the iodinated contrast dye can worsen kidney function.

30. Why might a contrast allergy make a V/Q scan useful?
A contrast allergy may make a V/Q scan useful because the V/Q scan does not require iodinated contrast material.

31. What does a perfusion defect mean on a V/Q scan?
A perfusion defect means that blood flow is reduced or absent in a specific region of the lung.

32. What does a ventilation defect mean on a V/Q scan?
A ventilation defect means that air movement is reduced or absent in a specific region of the lung.

33. What V/Q scan pattern has a high probability for pulmonary embolism?
Multiple segmental perfusion defects without corresponding ventilation defects indicate a high probability of pulmonary embolism.

34. Why does pulmonary embolism cause wasted ventilation?
Pulmonary embolism causes wasted ventilation because air reaches alveoli that receive little or no blood flow.

35. What is alveolar dead space?
Alveolar dead space occurs when alveoli are ventilated but not adequately perfused.

36. How does a pulmonary embolism affect alveolar dead space?
A pulmonary embolism increases alveolar dead space by blocking blood flow to ventilated alveoli.

37. What does a normal or near-normal V/Q scan suggest?
A normal or near-normal V/Q scan suggests that clinically significant pulmonary embolism is unlikely.

38. When can a low-probability V/Q scan help exclude pulmonary embolism?
A low-probability V/Q scan can help exclude pulmonary embolism when the patient also has a low clinical probability of PE.

39. Why is an intermediate-probability V/Q scan limited?
An intermediate-probability V/Q scan is limited because it cannot reliably confirm or exclude pulmonary embolism.

40. What may be needed after an intermediate-probability V/Q scan?
Additional testing may be needed, such as CT angiography, venous studies, pulmonary angiography, or other diagnostic procedures.

41. Why must V/Q scan results be interpreted with clinical assessment?
V/Q scan results must be interpreted with clinical assessment because the patient’s symptoms, risk factors, and overall probability of PE affect the meaning of the results.

42. What is clinical probability in relation to pulmonary embolism?
Clinical probability is the estimated likelihood that a patient has pulmonary embolism based on history, signs, symptoms, and risk factors.

43. What does a high-probability V/Q scan plus high clinical probability suggest?
A high-probability V/Q scan plus high clinical probability strongly suggests pulmonary embolism.

44. What does a high-probability V/Q scan with low clinical probability mean?
It is less definitive than when clinical probability is high, so the result must be interpreted carefully with the full clinical picture.

45. What does V/Q SPECT stand for?
V/Q SPECT stands for ventilation-perfusion single-photon emission computed tomography.

46. How is V/Q SPECT different from traditional planar V/Q imaging?
V/Q SPECT provides more detailed three-dimensional information than traditional planar V/Q imaging.

47. What is one advantage of V/Q SPECT?
One advantage of V/Q SPECT is improved sensitivity and better estimation of embolus size.

48. What patient ability is important for the ventilation portion of a V/Q scan?
The patient must be able to breathe through a mouthpiece or mask during the ventilation portion.

49. Why must the patient remain still during a V/Q scan?
The patient must remain still because movement can reduce image quality and make the scan harder to interpret.

50. Why may a V/Q scan be difficult for an unstable or uncooperative patient?
A V/Q scan may be difficult because the patient must follow breathing instructions, hold still, and cooperate during imaging.

51. What does a V/Q scan reveal about regional lung function?
A V/Q scan reveals how ventilation and perfusion are distributed in different regions of the lungs.

52. Why is the comparison between ventilation and perfusion important?
The comparison is important because gas exchange requires both air movement into the alveoli and blood flow through the pulmonary capillaries.

53. What does preserved ventilation with absent perfusion suggest?
Preserved ventilation with absent perfusion suggests that air is reaching the alveoli but pulmonary blood flow is blocked or severely reduced.

54. Why does pulmonary embolism often show normal ventilation on a V/Q scan?
Pulmonary embolism often shows normal ventilation because the airway to the affected lung region may remain open.

55. Why does pulmonary embolism show reduced perfusion on a V/Q scan?
Pulmonary embolism shows reduced perfusion because a clot blocks blood flow through part of the pulmonary arterial system.

56. What does the phrase “ventilated but not perfused” mean?
It means air reaches a lung region, but blood flow to that region is reduced or absent.

57. What does the phrase “perfused but not ventilated” mean?
It means blood reaches a lung region, but air movement into that region is reduced or absent.

58. What type of physiology is associated with ventilated but underperfused alveoli?
Ventilated but underperfused alveoli are associated with dead space-like physiology.

59. What type of physiology is associated with perfused but underventilated alveoli?
Perfused but underventilated alveoli are associated with shunt-like physiology.

60. How does airway obstruction affect the V/Q relationship?
Airway obstruction can reduce ventilation to a lung region while perfusion continues, creating a low V/Q pattern.

61. How can atelectasis affect a V/Q scan?
Atelectasis can cause reduced ventilation and may produce a matched defect or low V/Q pattern depending on the perfusion status.

62. How can pneumonia affect ventilation and perfusion?
Pneumonia can impair ventilation due to consolidation and inflammation, often contributing to low V/Q or matched abnormalities.

63. Why are multiple segmental perfusion defects important?
Multiple segmental perfusion defects are important because, when ventilation is preserved, they strongly suggest pulmonary embolism.

64. What does “segmental” mean in relation to a V/Q scan finding?
Segmental means the defect involves a specific lung segment, which can correspond to the distribution of pulmonary blood flow.

65. What does a high V/Q pattern indicate about perfusion?
A high V/Q pattern indicates that perfusion is reduced compared with ventilation.

66. What does a low V/Q pattern indicate about ventilation?
A low V/Q pattern indicates that ventilation is reduced compared with perfusion.

67. What happens to ventilation in alveolar dead space?
In alveolar dead space, ventilation reaches the alveoli but does not effectively participate in gas exchange due to poor perfusion.

68. Why can a patient with pulmonary embolism become hypoxemic?
A patient with pulmonary embolism can become hypoxemic because part of the lung is ventilated but not adequately perfused for gas exchange.

69. Why is V/Q mismatch important in respiratory care?
V/Q mismatch is important because it helps explain impaired oxygenation and abnormal gas exchange in many cardiopulmonary disorders.

70. What does a V/Q scan help determine in suspected pulmonary embolism?
A V/Q scan helps determine whether there are lung regions with preserved ventilation but reduced or absent perfusion.

71. What is the role of technetium-99m in the perfusion scan?
Technetium-99m is used as a radioactive tracer to help visualize pulmonary blood flow.

72. What is the role of xenon-133 in the ventilation scan?
Xenon-133 is an inhaled radioactive gas used to help visualize air movement into the lungs.

73. Why are ventilation and perfusion studies often performed together?
They are often performed together because the diagnostic value comes from comparing airflow with blood flow.

74. What does a V/Q scan show when ventilation and perfusion are matched?
It shows that the same lung regions are receiving both air and blood flow in a reasonably balanced way.

75. What is the most important V/Q scan concept for identifying PE?
The most important concept is a mismatch where ventilation is preserved but perfusion is decreased or absent.

76. What is the average normal V/Q ratio for the whole lung?
The average normal V/Q ratio for the whole lung is approximately 0.8.

77. Why is the average V/Q ratio about 0.8?
The average V/Q ratio is about 0.8 because normal alveolar ventilation is about 4 L/min and normal pulmonary blood flow is about 5 L/min.

78. Does every alveolus have the same V/Q ratio?
No. The V/Q ratio varies in different regions of the lung, especially from the apex to the base in an upright person.

79. How does gravity affect perfusion in the upright lung?
Gravity causes more blood flow to the lung bases than the apices.

80. Why is the V/Q ratio higher in the lung apices?
The V/Q ratio is higher in the lung apices because ventilation is greater relative to perfusion in that region.

81. Why is the V/Q ratio lower in the lung bases?
The V/Q ratio is lower in the lung bases because perfusion increases more than ventilation in that region.

82. What happens to alveolar oxygen when the V/Q ratio increases?
Alveolar oxygen pressure increases when the V/Q ratio increases.

83. What happens to alveolar carbon dioxide when the V/Q ratio increases?
Alveolar carbon dioxide pressure decreases when the V/Q ratio increases.

84. What happens to alveolar oxygen when the V/Q ratio decreases?
Alveolar oxygen pressure decreases when the V/Q ratio decreases.

85. What happens to alveolar carbon dioxide when the V/Q ratio decreases?
Alveolar carbon dioxide pressure increases when the V/Q ratio decreases.

86. Why does a high V/Q unit have increased alveolar oxygen?
A high V/Q unit has increased alveolar oxygen because oxygen enters the alveolus but is not removed normally by pulmonary blood flow.

87. Why does a high V/Q unit have decreased alveolar carbon dioxide?
A high V/Q unit has decreased alveolar carbon dioxide because less blood flow brings carbon dioxide to the alveolus.

88. Why does a low V/Q unit have decreased alveolar oxygen?
A low V/Q unit has decreased alveolar oxygen because oxygen is removed by blood faster than ventilation can replace it.

89. Why does a low V/Q unit have increased alveolar carbon dioxide?
A low V/Q unit has increased alveolar carbon dioxide because carbon dioxide enters from the blood faster than ventilation can remove it.

90. How do end-capillary blood gases relate to alveolar gas pressures?
End-capillary blood gases tend to reflect the oxygen and carbon dioxide pressures in the alveoli they pass through.

91. Why do arterial blood gases reflect mixed blood from different lung regions?
Arterial blood gases reflect mixed blood because blood from lung units with different V/Q ratios combines in the pulmonary veins.

92. What does decreased cardiac output do to the V/Q ratio?
Decreased cardiac output can increase the V/Q ratio because perfusion is reduced relative to ventilation.

93. How can destruction of pulmonary vessels affect the V/Q ratio?
Destruction of pulmonary vessels can increase the V/Q ratio by reducing perfusion to ventilated alveoli.

94. How can pulmonary hypertension affect perfusion findings?
Pulmonary hypertension can interfere with pulmonary blood flow and may contribute to abnormal perfusion findings.

95. Why can a tumor cause abnormal ventilation on a V/Q scan?
A tumor can obstruct an airway, preventing normal air movement to part of the lung.

96. Why can a tumor cause abnormal perfusion on a V/Q scan?
A tumor can affect pulmonary blood flow by compressing or invading vascular structures.

97. What should a respiratory therapist suspect if a PE candidate suddenly deteriorates?
The respiratory therapist should consider pulmonary embolism, especially if the patient develops sudden dyspnea, hypoxemia, or cardiopulmonary instability.

98. How may a V/Q scan be used before lung resection surgery?
A V/Q scan may be used to assess regional pulmonary blood flow before lung resection surgery.

99. Why is regional perfusion important before lung surgery?
Regional perfusion is important because it helps estimate how much functional pulmonary blood flow will remain after part of the lung is removed.

100. What is the key board-exam takeaway for V/Q scans?
The key takeaway is that multiple perfusion defects without matching ventilation defects indicate a high probability of pulmonary embolism.

Final Thoughts

A V/Q scan is an imaging test that compares ventilation and perfusion in different regions of the lungs. Its main clinical and exam-related use is evaluating suspected pulmonary embolism, especially when CT angiography cannot be performed or is inconclusive.

The classic PE pattern is ventilation without perfusion, often described as multiple segmental perfusion defects without matching ventilation defects. Matched defects are more likely to suggest lung tissue disease, while low V/Q patterns suggest poor ventilation relative to blood flow.

For respiratory therapy students, understanding V/Q scans helps connect imaging findings with gas exchange, dead space, shunting, and clinical decision-making.