Ventilation-perfusion mismatch
Template:Ventilation-perfusion mismatch 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 exhange 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
Increased V/Q ratio:
- Pulmonary embolism
- Emphysema
- Increased age
Decreased V/Q ratio:
- Chronic Bronchitis
- Asthma
- Pneumonia
- Pulmonary edema
- Airway obstruction (ex. foreign body aspiration)
Pathophysiology
Understanding V/Q mismatch in the context of hypoxia
There are five causes of hypoxia
- Diffusion limitation
- Hypoventilation
- Low partial pressure of oxygen in inspired air
- Shunt formation
- Dead space formation
The first three causes result in a normal V/Q and the last two result in an abnormal V/Q.
Shunts
Shunts are formed when the air entry is cut off from part of the lung and 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
Work up
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