Ventilation-Perfusion Ratio Overview Illustration

Ventilation-Perfusion Ratio and V/Q Mismatch (2024)

by | Updated: Apr 19, 2024

The ventilation-perfusion (V/Q) ratio is a concept that describes the relationship between the amount of air reaching the alveoli (ventilation) and the flow of blood in the surrounding capillaries (perfusion).

An optimal V/Q ratio ensures that oxygen entering the lungs is effectively transferred to the bloodstream while simultaneously facilitating the removal of carbon dioxide.

Imbalances in this ratio can lead to critical respiratory conditions, highlighting its importance in both clinical and research settings.

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What is the Ventilation-Perfusion Ratio?

The ventilation-perfusion ratio (V/Q ratio) represents the balance between the amount of air reaching the alveoli (ventilation) and the amount of blood flow in the capillaries surrounding the alveoli (perfusion). An optimal V/Q ratio ensures efficient gas exchange in the lungs, and imbalances can lead to respiratory conditions or inefficiencies in oxygen delivery.

Ventilation-Perfusion Ratio Alveoli Vector Illustration

What is Ventilation?

Ventilation (V) refers to the process of moving air in and out of the lungs and encompasses two main phases:

  1. Inspiration: During this phase, air is drawn into the lungs as the diaphragm contracts and the thoracic cavity expands.
  2. Expiration: During this phase, air is expelled from the lungs as the diaphragm relaxes and the thoracic cavity reduces in size.

Proper ventilation is essential for maintaining adequate levels of oxygen in the blood and removing carbon dioxide, a waste product of metabolism.

The process ensures that fresh oxygen-rich air reaches the alveoli for efficient gas exchange.

What is Perfusion?

Perfusion (Q) refers to the passage of blood through the capillaries of organs or tissues, ensuring that they receive oxygen and nutrients for their metabolic needs.

Pulmonary perfusion involves the flow of blood through the pulmonary capillaries surrounding the alveoli.

Here, oxygen from the inhaled air diffuses into the blood, while carbon dioxide, a metabolic waste product, diffuses from the blood into the alveoli to be exhaled.

Adequate perfusion is vital for effective oxygenation of blood and removal of waste products. Inadequate perfusion can lead to tissue hypoxia or other complications.

What is a V/Q mismatch?

A V/Q mismatch refers to an imbalance between ventilation (V) and perfusion (Q) in the lungs. It means that certain regions of the lungs are not optimally exchanging gases due to either inadequate ventilation relative to blood flow or inadequate blood flow relative to ventilation.

There are two main types of V/Q mismatch:

  1. Low V/Q (shunt): This occurs when alveoli are ventilated less than the blood perfusing them, leading to poorly oxygenated blood. Causes can include conditions like pneumonia, atelectasis, or pulmonary edema.
  2. High V/Q (dead space): This occurs when alveoli are well-ventilated but poorly perfused, meaning the oxygen doesn’t efficiently transfer to the blood. Causes can include conditions like pulmonary embolism or certain types of chronic obstructive pulmonary diseases.

Note: A V/Q mismatch can compromise the oxygenation of blood and lead to hypoxemia, making it a critical consideration in respiratory medicine.


V/Q mismatch can lead to impaired gas exchange in the lungs, and its symptoms are primarily related to the reduction in arterial oxygen levels (hypoxemia).

Here are the common symptoms associated with a V/Q mismatch:

  • Dyspnea (Shortness of Breath): This is the most common symptom. Patients may feel breathless at rest or during exertion.
  • Cyanosis: A bluish discoloration of the skin and mucous membranes due to decreased oxygen in the blood.
  • Tachycardia: An increased heart rate, as the heart tries to compensate for decreased oxygen in the blood.
  • Tachypnea: An increased respiratory rate, reflecting the body’s attempt to increase oxygen intake and expel carbon dioxide.
  • Decreased Exercise Tolerance: Patients may feel easily fatigued or winded during physical activity.
  • Confusion or Altered Mental Status: Severe hypoxemia can affect brain function, leading to confusion, dizziness, or even loss of consciousness.
  • Chest Pain: Especially if the V/Q mismatch is caused by conditions like a pulmonary embolism.
  • Palpitations: Irregular heartbeat sensations.
  • Wheezing or Cough: Especially if the underlying cause is related to an airway obstruction or disease.

Note: The severity and presentation of symptoms can vary based on the cause and extent of the V/Q mismatch. Some conditions might present subtly, while others can lead to acute respiratory distress.


The treatment for a V/Q mismatch primarily focuses on addressing the underlying cause, improving oxygenation, and ensuring adequate gas exchange.

Here are some general treatment approaches:

  • Supplemental Oxygen: Providing extra oxygen can help raise the oxygen levels in the blood, especially in cases where hypoxemia is present.
  • Bronchodilators: Medications like albuterol can be used to open up the airways, especially in conditions like asthma or COPD, where bronchoconstriction contributes to the mismatch.
  • Inhaled Steroids: Used to reduce inflammation in the airways, especially in chronic conditions like asthma or COPD.
  • Anticoagulation: In cases where a pulmonary embolism is the cause of the V/Q mismatch, anticoagulant medications are essential to prevent further clot formation and facilitate the resolution of existing clots.
  • Diuretics: In conditions like pulmonary edema, diuretics can help remove excess fluid from the body, including the lungs.
  • Positive Pressure Ventilation: In severe cases or where patients can’t maintain adequate oxygenation, mechanical ventilation using positive pressure can be employed to ensure adequate ventilation and oxygenation.
  • Antibiotics: If an infection like pneumonia is causing the V/Q mismatch, appropriate antibiotics are essential.
  • Treatment of Underlying Cardiac Conditions: In cases where heart conditions lead to pulmonary issues, management of the cardiac condition is crucial.
  • Pulmonary Vasodilators: In certain conditions like pulmonary hypertension, medications that dilate the pulmonary blood vessels can be beneficial.
  • Lung Volume Reduction Surgery: For patients with severe COPD, this surgical procedure can help improve the ventilation-perfusion ratio by removing non-functioning lung tissue.
  • Lung Transplant: In extreme cases where lung function is severely compromised, and other treatments are ineffective, a lung transplant might be considered.

Note: Regular monitoring, including arterial blood gases, pulse oximetry, and pulmonary function tests, can guide treatment decisions. It’s essential always to address the root cause of the V/Q mismatch to effectively treat it.

What is a Shunt?

A shunt is a pathological condition where blood passes from the right to the left side of the heart without being adequately oxygenated in the lungs.

In a shunt, certain portions of the lung receive blood flow (perfusion) but no corresponding airflow (ventilation).


There are various causes for shunting, including:

  • Intrinsic lung diseases such as pneumonia, atelectasis, or pulmonary edema. These conditions fill alveoli with fluid or collapse them, preventing effective ventilation in affected areas.
  • Congenital heart diseases such as tetralogy of Fallot or patent ductus arteriosus can result in shunts at the cardiac level, causing deoxygenated blood to mix with oxygenated blood.

Note: When a significant portion of blood bypasses the normal gas exchange process in the lungs, it can lead to hypoxemia (low oxygen levels in the blood). The degree of hypoxemia depends on the extent of the shunt and its impact on overall lung function.

What is Dead Space?

Dead space refers to the portion of each breath that doesn’t participate in the gas exchange process because it either doesn’t reach the alveoli or reaches alveoli that are not perfused.

In essence, it’s the volume of air that’s inhaled but doesn’t contribute to oxygenating the blood.


There are three primary types of dead space:

  1. Anatomical Dead Space: This is the volume of the conducting airways, such as the trachea and bronchi, where no gas exchange occurs. It’s a normal component of the respiratory system.
  2. Alveolar Dead Space: Refers to the volume of air that ventilates alveoli which, for some reason (like a blood clot or reduced blood flow), don’t have a good blood supply and, therefore, no gas exchange occurs.
  3. Physiological Dead Space: This is the sum of anatomical and alveolar dead spaces. It represents the total volume of a breath that doesn’t participate in gas exchange.

Note: An increase in dead space can decrease the efficiency of ventilation and may require an individual to breathe more frequently or deeply to meet their oxygen requirements.

Ventilation-Perfusion Practice Questions

1. What is a normal V/Q ratio?
4/5 or 0.8

2. How is the V/Q ratio calculated?
By dividing alveolar ventilation by pulmonary capillary blood flow.

3. What is the V/Q ratio in the upper lung?
Greater than 0.8

4. What is the V/Q ratio in the lower lung?
Less than 0.8

5. What happens to the V/Q ratio when a patient has an obstructed airway?
It decreases.

6. What happens to the PAO2 when a patient has an obstructed airway?
It decreases.

7. What happens to the PACO2 when a patient has an obstructed airway?
It increases.

8. When the V/Q ratio decreases, what happens to the PAO2 and PACO2?
PAO2 decreases, and PACO2 increases.

9. When the V/Q ratio increases, what happens to PAO2 and PACO2?
PAO2 increases, and PACO2 decreases.

10. What does V/Q stand for?
Ventilation (V) and Perfusion (Q)

11. What is the meaning of the V/Q ratio?
It is the relationship of the overall alveolar ventilation (L/min) to the overall pulmonary blood flow (L/min).

12. What is the respiratory quotient?
The ratio between the volume of oxygen consumed and the volume of carbon dioxide produced.

13. What is the average respiratory quotient?

14. What is the respiratory exchange ratio?
The quantity of oxygen and carbon dioxide exchanged during a 1-minute period.

15. What is ETCO2?
A measurement of exhaled CO2

16. What is the normal range of ETCO2?
35-40 mmHg or about 5%

17. When will ETCO2 increase and decrease?
It increases during hypoventilation and decreases during hyperventilation.

18. When will the ETCO2 remain at zero?
During esophageal intubation or in cardiac arrest.

19. What is a capnogram?
The waveform produced during capnography.

20. What is a capnography?
A rapid, continuous, noninvasive monitoring and graphic display of the patient’s inhaled and exhaled CO2 plotted against time.

21. What three disease states would cause an increased V/Q ratio?
Pulmonary embolism, complete or partial pulmonary artery obstruction, and decreased cardiac output.

22. What three diseases would cause a decreased V/Q ratio?
Asthma, pneumonia, and hypoventilation.

23. How will a capnogram change during bronchospasm?
It will produce a “shark-fin” type pattern due to uneven exhalation. Then, as a bronchodilator treatment starts to become effective, the exhalation line will become more normal.

24. How will a capnogram change during esophageal intubation?
The capnogram will read zero if the ETT is placed in the esophagus.

25. What is the V/Q if your patient’s respiratory rate is 15, their tidal volume is 450, their weight is 160 pounds, and their cardiac output is 5.6 liters?

26. What are the normal values for ventilation and capillary blood flow?
Normal ventilation is 4 L/min; normal capillary blood flow is 5 L/min.

27. What is the normal overall V/Q ratio for the lung, and how is it determined?
A normal V/Q ratio is 0.8, which is determined by dividing the normal ventilation of 4 by the normal capillary blood flow which of 5 (i.e., 4/5 = 0.8).

28. How does the upper lung region V/Q compare to the normal V/Q ratio?
The upper lung region is higher than 0.8 because ventilation is best at the top of the lung.

29. How does the lower lung region V/Q compare with the normal V/Q ratio?
The lower lung region is lower than 0.8 because ventilation progressively decreases from top to bottom in an upright lung, and perfusion is best at the bottom of the lung.

30. What is the cause of an increased V/Q ratio?
An increase in ventilation or a decrease in perfusion.

31. What is the cause of a decreased V/Q ratio?
A decrease in ventilation or an increase in perfusion.

32. What is a classic dead space condition?
Pulmonary embolism

33. How to determine the PAO2 balance?
It is the amount of O2 entering the alveoli and the removal of O2 by capillary blood flow.

34. How to determine the PACO2 balance?
It is the amount of CO2 diffusing into the alveoli from the capillary and the removal of CO2 from the alveoli by ventilation.

35. What is the respiratory zone of the lungs?
It’s where gas exchange takes place, including the bronchioles, alveoli, alveolar ducts, and alveolar sacs.

36. What is the respiratory exchange ratio?
The quantity of O2 and CO2 exchanged in one minute.

37. What is the relationship between RQ and RR under normal conditions?
They are equal.

38. What type of V/Q ratio is associated with dead space ventilation?
An increase in the V/Q ratio leads to wasted ventilation (i.e., dead space).

39. What type of V/Q ratio is associated with shunting?
A decrease in the V/Q ratio leads to a shunt.

40. What is the number one cause of hypoxemia?
V/Q mismatch

41. What type of disorder always affects the V/Q ratio?
Pulmonary disorders

42. What disorders are associated with an increased V/Q ratio?
Pulmonary embolism, partial or complete obstruction of the pulmonary artery or arterioles, atherosclerosis, collagen disease, extrinsic pressure on the pulmonary vessels, tumor compressing vessels, destruction of pulmonary vessels, and decreased cardiac output.

43. What disorders are associated with a decreased V/Q ratio?
Obstructive lung disease, emphysema, bronchitis, asthma, restrictive lung disease, atelectasis, pneumonia, silicosis, pulmonary fibrosis, and hypoventilation.

44. What are the signs of diminished pulmonary perfusion?
Little or no blood flow in relation to ventilation, ventilation without perfusion, increased V/Q ratio, increased dead space ventilation, increased PAO2, and decreased PACO2.

45. What is a quick description of dead space?
Ventilation without perfusion

46. Where does pulmonary circulation begin?
At the main pulmonary artery

47. What artery follows the path of the branching airways to the level of terminal bronchioles?
The main pulmonary artery

48. When does the pulmonary artery break up to supply the pulmonary capillary bed?
Beyond the terminal bronchioles

49. What percentage of the alveolar surface area do the pulmonary capillaries cover?

50. Which is higher, pulmonary or systemic circulation?
Systemic circulation is much higher.

51. What is the difference between the walls of the aorta and the walls of the pulmonary artery?
The pulmonary artery walls are much thinner and have less smooth muscle and elastin than the walls of the aorta.

52. What does it mean for pulmonary vasculature resistance if the cardiac output is high and the pulmonary circulation pressure is low?
High cardiac output + low pulmonary pressure = low pulmonary vasculature resistance

53. What is the equation for vascular resistance?
(input pressure – output pressure) / blood flow

54. How can you measure the distribution of pulmonary blood flow?
By using radioactive xenon dissolved in saline and injected into the peripheral vein, and xenon is counted by radiation detectors placed over the chest.

55. Which part of the lung receives more blood flow?
The base of the lung receives more blood flow per unit volume than the apex of the lung.

56. Why does the base of the lung receive more blood flow than the apex?
Gravity produces a gradient of pulmonary blood flow because of hydrostatic pressures within the pulmonary vessels.

57. What is another name for pulmonary capillaries?
Alveolar vessels

58. What is another name for large pulmonary vessels?
Extra-alveolar vessels

59. Where are extra-alveolar vessels located?
They run through the lung parenchyma.

60. Does the sympathetic or parasympathetic nervous system supply the pulmonary blood vessels?
Both; however, the autonomic nervous system probably does not have a major function in the normal control of pulmonary blood flow.

61. What is the benefit of hypoxic pulmonary vasoconstriction?
It diverts mixed venous blood away from poorly ventilated areas of the lung that have a low PAO2 by locally increasing vascular resistance, and mixed venous blood is sent to better ventilated areas of the lung.

62. How is the hypoxic reaction in pulmonary circulation different from the hypoxic reaction in systemic circulation?
In pulmonary circulation, the mixed venous blood is sent away from the hypoxic area, while in systemic circulation, there is localized vasodilation to the hypoxic area, increasing the blood flow to that area.

63. When is gas exchange maximally efficient?
When ventilation and perfusion are appropriately matched in all regions of the lungs.

64. What will happen if ventilation stays the same in the alveoli but perfusion decreases?
Less CO2 will be brought to the alveoli from the blood, and less O2 will move from the alveoli into the blood. Also, the PACO2 will decrease, and PAO2 will increase.

65. What is the expected PO2 and PCO2 of blood entering the pulmonary capillary in a normal alveolus?
PO2 = 40 mmHg; PCO2 = 46 mmHg

66. Normally, what pressures will the blood leaving the pulmonary capillaries have?
PO2 = 100 mmHg; PCO2 = 40 mmHg

67. What two things need to happen for gas exchange to occur?
Ventilation and perfusion

68. Would a pulmonary embolism result in a high or low V/Q ratio?
It would result in a high V/Q ratio.

69. Would atelectasis result in a high or low V/Q ratio?
It would result in a low V/Q ratio.

70. What is the most common cause of hypoxemia in patients with a respiratory condition?
V/Q imbalance

Final Thoughts

The ventilation-perfusion ratio measures the lung’s functional capacity to oxygenate the blood while expelling carbon dioxide.

A deeper understanding of this ratio and its potential imbalances (i.e., V/Q mismatch) is crucial for diagnosing and treating respiratory conditions.

As advancements in pulmonary medicine continue, maintaining a clear grasp of this fundamental concept ensures a solid foundation for addressing complex challenges while providing optimal patient care.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


  • Egan’s Fundamentals of Respiratory Care. Mosby, 2020.
  • Jardins, Des Terry. Cardiopulmonary Anatomy & Physiology: Essentials of Respiratory Care. 7th ed., Cengage Learning, 2019.
  • Hopkins SR. Ventilation/Perfusion Relations.hips and Gas Exchange: Measurement Approaches. Compr Physiol. 2020

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