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)
• Idiopathic pulmonary fibrosis
• Respiratory distress syndrome

Pathophysiology

Understanding V/Q mismatch in the context of hypoxia

There are five causes of hypoxia

1. Diffusion limitation
2. Hypoventilation
3. Low partial pressure of oxygen in inspired air
4. Shunt formation
5. Dead space formation

The first three causes result in a normal V/Q and the last two result in an abnormal V/Q.

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

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.

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