Ventilation-perfusion mismatch

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

In normal lung physiology the V/Q ratio is a measurement used to determine the efficacy and adequacy of ventilation and perfusion of the lung. Ventilation is the amount of air that reaches the lungs and Perfusion is the amount of blood flow to the lung. 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 multiple factors such as gravity, size and patency of airways, and positioning. There is 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, which is a decreased oxygen concentration of blood.

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

Some conditions that cause decrease in V/Q are:

• Chronic Bronchitis
• Asthma
• Foreign body aspiration
• Hepatopulmonary syndrome
• ARDS
• Bronchiectasis
• Interstitial Lung disease
• Cystic Fibrosis
• Pulmonary Hypertension

Some conditions that cause increase in V/Q are:

• Pulmonary embolism
• Emphysema

Extreme conditions:

• An area of no ventilation is termed a shunt
  • V/Q ratio= 0
• An area of no perfusion is termed a dead space
  • V/Q ratio is undefined

Pathogenesis

V/Q mismatch is one of the most common causes of hypoxemia. It can be caused by 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 normal conditions 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.

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:

• 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 cells.

Understanding V/Q mismatch in the context of hypoxia

There are two causes of V/Q mismatch:

1. Decreased Ventilation
2. 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 is the less common causes.

Dead space ventilation

When blood supply to part of the lung is cut off, oxygen in the ventilated atmospheric air is not able to enter the bloodstream leading to lesser overall efficiency of alveolar oxygenation mechanism. Pulmonary Embolism is the most common cause of dead space ventilation

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 workup must be done to diagnose and treat the underlying illness

Work up

V/Q mismatch can be caused by various diseases and a workup 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.

Physiological changes in Alveolar and Blood gas in various causes 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