Ventilation-perfusion mismatch pathophysiology

# 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 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.

# 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.

# Associated Conditions
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
- V/Q ratio= 0
- An area of no perfusion is termed a dead space
V/Q ratio is undefined
- V/Q ratio is undefined

# Genetics
The association between V/Q mismatch and genetics depends on the etiology of the mismatch. Some diseases with genetic components include:
- Asthma has over 100 genetic associations.
- Emphysema can be associated with a deficiency of alpha 1 antitrypsin 
- Cystic fibrosis is a genetic disorder affecting chloride transporters 

# Gross Pathology
The gross pathology depends on the exact reason for the V/Q mismatch.
V/Q mismatch can be seen on a Ventilation perfusion scan.

# 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.
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