Ventilation-perfusion mismatch: Difference between revisions
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== Overview == | == Overview == | ||
Ventilation-Perfusion (V/Q) mismatch occurs when there is a there is defect in alveolar gas exchange in the lung either due to poor perfusion or reduced air entry. It is a valuable tool in both understanding the normal physiology of lung and for diagnosing some common lung pathologies. | Ventilation-Perfusion (V/Q) mismatch occurs when there is a there is defect in alveolar gas exchange in the lung either due to poor perfusion or reduced air entry. It is a valuable tool in both understanding the normal physiology of lung and for diagnosing some common lung pathologies. Ventilation of air and perfusion of blood is not distributed equally in the 400 million alveoli of a normal lung. This varies based on the effects of gravity, how patent the airways are and any pathological process that affect various parts of the lung. In ideal conditions, V/Q ratio should be 1, with 100 percent of alveoli participating in gas exchange and 100 percent perfusion of pulmonary capillaries. However this does not happen even in healthy individuals as not all the alveoli in a healthy lung are recruited. Gravity plays an important role in perfusion of the lungs. The apex of the lung receives less perfusion than the base of the lung. The normal V/Q ratio is estimated to be 0.8. An easy way to interpret this value of 0.8 would be that 80 percent of the alveoli in the lung have good air ventilation and blood perfusion. Any discrepancy between pulmonary blood flow and [[ventilation]] is called V/Q mismatch. Ideally [[Ventilation (physiology)|ventilation]] and [[perfusion]] should be equal with a V/Q ratio of 1, but the normal lung varies due to a higher perfusion at the [[Base of lung|base of the lung]] than the [[Apex of lung|apex of the lung]]. This causes a higher V/Q ratio at the apex compared to the base. The average V/Q ratio in a normal lung is about 0.8, with about 4 liters of oxygen and 5 liters of blood entering the lung per minute. Diseased lung can cause a V/Q mismatch due to decreased blood flow or oxygenation. This results in [[hypoxemia]], and there are many causes of it. | ||
Ventilation of air and perfusion of blood is not distributed equally in the 400 million alveoli of a normal lung. This varies based on the effects of gravity, how patent the airways are and any pathological process that affect various parts of the lung. In ideal conditions, V/Q ratio should be 1, with 100 percent of alveoli participating in gas exchange and 100 percent perfusion of pulmonary capillaries. However this does not happen even in healthy individuals as not all the alveoli in a healthy lung are recruited. Gravity plays an important role in perfusion of the lungs. The apex of the lung receives less perfusion than the base of the lung. | |||
The normal V/Q ratio is estimated to be 0.8. | |||
An easy way to interpret this value of 0.8 would be that 80 percent of the alveoli in the lung have good air ventilation and blood perfusion. | |||
== Causes == | == Causes == | ||
V/Q mismatch occurs when there is either and increase or decrease in V/Q ratio. | V/Q mismatch occurs when there is either and increase or decrease in V/Q ratio. | ||
Causes of increased V/Q ratio: | Causes of increased V/Q ratio: | ||
*Pulmonary embolism | *Pulmonary embolism | ||
| Line 19: | Line 13: | ||
*Increased age | *Increased age | ||
Causes | Causes of decreased V/Q ratio: | ||
*Chronic Bronchitis | *Chronic Bronchitis | ||
*Asthma | *Asthma | ||
| Line 28: | Line 22: | ||
*Respiratory distress syndrome | *Respiratory distress syndrome | ||
== | ==Pathogenesis== | ||
V/Q mismatch is one of the most common reasons of hypoxemia in patients with [[lung]] diseases like [[Obstructive lung disease|obstructive lung]] diseases, pulmonary vascular diseases, and [[Interstitial lung disease|interstitial diseases]]. An increased V/Q mismatch is caused by a decrease in blood flow to the lung, for example a [[pulmonary embolism]]. A decreased V/Q mismatch is caused by a decrease in ventilation or an airway obstruction, for example [[Asthma]]. A V/Q mismatch due to a perfusion defect will improve with 100% [[Oxygen therapy|oxygen therapy.]] | |||
In the normal condition when there is a low ventilation, the body tries to keep this ratio in a normal range by restricting the perfusion in that specific area of the lung. This unique mechanism is called hypoxic pulmonary vasoconstriction. If this process continues for a long time it can cause pulmonary hypertension. | |||
==Associated Conditions== | |||
Some conditions that cause decrease in V/Q are: | |||
* [[Bronchitis]] | |||
* [[Asthma]] | |||
* [[Foreign body aspiration]] | |||
* [[Hepatopulmonary syndrome]], | |||
Some conditions that cause increase in V/Q are: | |||
* [[Pulmonary embolism]] | |||
* [[Emphysema]] | |||
==Genetics== | |||
The association between V/Q mismatch and genetic depends on the etiology of the mismatch. For example ''ORMDL3'' and ''GSDML'' genes play a role in causing [[asthma]] . | |||
== Gross Pathology == | |||
The gross pathology depends on the exact reason for the V/Q mismatch. | |||
== Microscopic Pathology == | |||
The microscopic pathology depends on the exact reason for the V/Q mismatch. For example in [[asthma]] there are extracellular Charcot-Leyden crystals and increased mucosal [[Goblet cell|goblet]] cells. | |||
== Understanding V/Q mismatch in the context of hypoxia == | |||
There are two causes of V/Q mismatch: | There are two causes of V/Q mismatch: | ||
# Decreased Ventilation | # Decreased Ventilation | ||
# Decreased Perfusion | # Decreased Perfusion | ||
=== Shunts === | |||
Pulmonary shunts are formed when there is decreased ventilation in one part of the lung with normal perfusion. This deoxygenated blood enters arterial circulation without getting oxygenated in the lung. Absorptive or compressive pulmonary atelectasis is the major reason for shunt formation. Pulmonary AV malformation, hepatopulmonary syndrome are the less common causes. | Pulmonary shunts are formed when there is decreased ventilation in one part of the lung with normal perfusion. This deoxygenated blood enters arterial circulation without getting oxygenated in the lung. Absorptive or compressive pulmonary atelectasis is the major reason for shunt formation. Pulmonary AV malformation, hepatopulmonary syndrome are the less common causes. | ||
=== Dead space ventilation === | |||
When blood supply to part of lung is cut off, oxygen in the ventilated atmospheric air is not able to enter the blood stream leading to lesser overall efficiency of alveolar oxygenation mechanism. Pulmonary Embolism is the most common cause of dead space ventilation | When blood supply to part of lung is cut off, oxygen in the ventilated atmospheric air is not able to enter the blood stream leading to lesser overall efficiency of alveolar oxygenation mechanism. Pulmonary Embolism is the most common cause of dead space ventilation | ||
Revision as of 21:21, 17 November 2018
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Ventilation-Perfusion (V/Q) mismatch occurs when there is a there is defect in alveolar gas exchange in the lung either due to poor perfusion or reduced air entry. It is a valuable tool in both understanding the normal physiology of lung and for diagnosing some common lung pathologies. Ventilation of air and perfusion of blood is not distributed equally in the 400 million alveoli of a normal lung. This varies based on the effects of gravity, how patent the airways are and any pathological process that affect various parts of the lung. In ideal conditions, V/Q ratio should be 1, with 100 percent of alveoli participating in gas exchange and 100 percent perfusion of pulmonary capillaries. However this does not happen even in healthy individuals as not all the alveoli in a healthy lung are recruited. Gravity plays an important role in perfusion of the lungs. The apex of the lung receives less perfusion than the base of the lung. The normal V/Q ratio is estimated to be 0.8. An easy way to interpret this value of 0.8 would be that 80 percent of the alveoli in the lung have good air ventilation and blood perfusion. Any discrepancy between pulmonary blood flow and ventilation is called V/Q mismatch. Ideally ventilation and perfusion should be equal with a V/Q ratio of 1, but the normal lung varies due to a higher perfusion at the base of the lung than the apex of the lung. This causes a higher V/Q ratio at the apex compared to the base. The average V/Q ratio in a normal lung is about 0.8, with about 4 liters of oxygen and 5 liters of blood entering the lung per minute. Diseased lung can cause a V/Q mismatch due to decreased blood flow or oxygenation. This results in hypoxemia, and there are many causes of it.
Causes
V/Q mismatch occurs when there is either and increase or decrease in V/Q ratio. Causes of increased V/Q ratio:
- Pulmonary embolism
- Emphysema
- Increased age
Causes of decreased V/Q ratio:
- Chronic Bronchitis
- Asthma
- Pneumonia
- Pulmonary edema
- Airway obstruction (ex. foreign body aspiration)
- Idiopathic pulmonary fibrosis
- Respiratory distress syndrome
Pathogenesis
V/Q mismatch is one of the most common reasons of hypoxemia in patients with lung diseases like obstructive lung diseases, pulmonary vascular diseases, and interstitial diseases. An increased V/Q mismatch is caused by a decrease in blood flow to the lung, for example a pulmonary embolism. A decreased V/Q mismatch is caused by a decrease in ventilation or an airway obstruction, for example Asthma. A V/Q mismatch due to a perfusion defect will improve with 100% oxygen therapy.
In the normal condition when there is a low ventilation, the body tries to keep this ratio in a normal range by restricting the perfusion in that specific area of the lung. This unique mechanism is called hypoxic pulmonary vasoconstriction. If this process continues for a long time it can cause pulmonary hypertension.
Associated Conditions
Some conditions that cause decrease in V/Q are:
Some conditions that cause increase in V/Q are:
Genetics
The association between V/Q mismatch and genetic depends on the etiology of the mismatch. For example ORMDL3 and GSDML genes play a role in causing asthma .
Gross Pathology
The gross pathology depends on the exact reason for the V/Q mismatch.
Microscopic Pathology
The microscopic pathology depends on the exact reason for the V/Q mismatch. For example in asthma there are extracellular Charcot-Leyden crystals and increased mucosal goblet cells.
Understanding V/Q mismatch in the context of hypoxia
There are two causes of V/Q mismatch:
- Decreased Ventilation
- Decreased Perfusion
Shunts
Pulmonary shunts are formed when there is decreased ventilation in one part of the lung with normal perfusion. This deoxygenated blood enters arterial circulation without getting oxygenated in the lung. Absorptive or compressive pulmonary atelectasis is the major reason for shunt formation. Pulmonary AV malformation, hepatopulmonary syndrome are the less common causes.
Dead space ventilation
When blood supply to part of lung is cut off, oxygen in the ventilated atmospheric air is not able to enter the blood stream leading to lesser overall efficiency of alveolar oxygenation mechanism. Pulmonary Embolism is the most common cause of dead space ventilation
Natural History and Complications
History, Symptoms, and Physical Exam
Differential Diagnosis
V/Q mismatch is finding that can be indicative of a serious respiratory disease. The differential diagnosis for V/Q mismatch includes:
- Pulmonary embolism
- Emphysema
- Chronic Bronchitis
- Asthma
- Pneumonia
- Pulmonary edema
- Airway obstruction (ex. foreign body aspiration)
- Idiopathic pulmonary fibrosis
- Respiratory distress syndrome
A work up must be done to diagnose and treat the underlying illness
Work up
V/Q mismatch can be caused by various diseases and a work up must be done for diagnosis and treatment.
- Labs:
- Arterial Blood Gas
- PAO2
- PaO2
- PaCo2
- Bicarbonate levels
- DLCO2
- Spirometry
- Imaging
- Chest X-Ray
- Ventilation Perfusion scan
Calculations using measurements from Arterial Blood Gas (ABG) and the response of those measures to supplemental oxygen are used to investigate the cause of hypoxia.
| Cause | P(Alv)O2 | A-a gradient | Response to
supplemental oxygen |
|---|---|---|---|
| Diffusion limitation | Normal | Increased | Improved PaO2 |
| Hypoventilation | Reduced | Normal | Improved PaO2 |
| Reduced PiO2 | Reduced | Normal | Improved PaO2 |
| Shunt formation | Reduced in local areas of lung | Increased | Improved PaO2 |
| Dead space formation | Normal | Increased | Minimal to no improvement |
PiO2 - partial pressure of oxygen in inspired air
P(Alv)O2 - partial pressure of oxygen in alveolar air
PaO2 - partial pressure of oxygen in arterial air
A-a gradient - P(Alv)O2 - PaO2