Ventilation-perfusion mismatch

# 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.

# 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
- V/Q ratio= 0
- An area of no perfusion is termed a dead space
V/Q ratio is undefined
- 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. Anything that affects the blood flow and oxygenation in the lung can cause a V/Q mismatch.  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. For example:
- Pulmonary fibrosis can present with a honeycomb appearance of the lung 
- Pulmonary infarct can present as a wedge shaped infarct 

# 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
- In Interstitial lung disease caused by asbestosis ferruginous bodies may be present 

# Understanding V/Q mismatch in the context of hypoxia
There are two causes of V/Q mismatch:
- Decreased Ventilation
- Decreased Perfusion

## Shunts
Pulmonary shunts occur when the V/Q ratio is zero. It is formed when there is absent 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 , ARDS, and pulmonary edema are less common causes. Shunting is characterized by it poor response to 100% oxygen therapy.

## Dead space ventilation
Dead space ventilation occurs when V/Q ratio is undefined. 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 the alveolar oxygenation mechanism.  Pulmonary Embolism is the most common cause of dead space ventilation. Hypoxemia caused by dead space ventilation will improve with 100% oxygen therapy.

# Natural History and Complications
If left untreated patients with V/Q mismatch may progress to develop pulmonary hypertension , respiratory failure, and even death.   In patients with an decreased V/Q ratio, 100% oxygen can be used as acute treatment while additional diagnostic studies are necessary to determine the underlying cause. Prognosis, mortality and survival rate of patients with V/Q mismatch depend on the underlying cause and early treatment.

# History, Symptoms, and Physical Exam
Symptoms of V/Q mismatch depend on the underlying cause but in general a majority of cases with V/Q mismatch include:
- Dyspnea
- Exercise intolerance
- Chest pain
- Dizziness
- Altered mental status
- Cough
Patients with V/Q mismatch usually appear distressed. Physical examination of patients with V/Q mismatch depends on the underlying cause. 
Some common physical examination findings of V/Q mismatch include:
- Tachypnea or Bradypnea
- Abnormal or decreased Breath sounds
- Rapid pulse.
If the patient is in respiratory failure they may present with:
- Visible internal jugular venous pulse
- Intercostal retractions
- Pallor
- Cyanosis
- Nasal flaring

# Differential Diagnosis
V/Q mismatch is finding that can be indicative of a serious respiratory disease. The differential diagnosis for V/Q mismatch includes:
- Chronic Bronchitis
- Asthma
- Foreign body aspiration
- Hepatopulmonary syndrome
- ARDS
- Bronchiectasis
- Interstitial Lung disease
- Cystic Fibrosis
- Pulmonary Hypertension
- Pulmonary embolism
- Emphysema
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
- Arterial Blood Gas
- PAO2
- PaO2
- PaCo2
- Bicarbonate levels
- DLCO2
- Spirometry
- Imaging
Chest X-Ray
Ventilation-Perfusion scan
- 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.
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