Ventilation-perfusion mismatch pathophysiology

Template:Ventilation-perfusion mismatch

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Aida Javanbakht, M.D.

Overview

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.

Pathogenesis

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 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:

• Chronic Bronchitis
• Asthma
• Foreign body aspiration
• Hepatopulmonary syndrome
• ARDS

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.