What is the mechanism of ventilation-perfusion (V/Q) mismatch in asthma?

From the Guidelines

V/Q mismatch in asthma occurs due to airflow obstruction, creating areas of the lung that are ventilated but not adequately perfused with blood, leading to hypoxemia despite increased respiratory effort. During an asthma attack, bronchospasm, inflammation, and mucus plugging cause narrowing of airways, resulting in regional air trapping and hyperinflation, as described in the expert panel report 3 (EPR-3) guidelines for the diagnosis and management of asthma 1. This creates areas with high ventilation-to-perfusion ratios (high V/Q) where air reaches alveoli but blood flow is reduced due to hypoxic vasoconstriction or mechanical compression of blood vessels by hyperinflated alveoli.

Some key points to consider in the mechanism of V/Q mismatch in asthma include:

• Airflow limitation caused by bronchoconstriction, airway hyperresponsiveness, and airway edema, as outlined in the EPR-3 guidelines 1
• Remodeling of airways, including sub-basement fibrosis, mucus hypersecretion, injury to epithelial cells, smooth muscle hypertrophy, and angiogenesis, which can contribute to persistent changes in airway structure and function 1
• The use of oxygen therapy, which can affect V/Q matching by reversing hypoxic pulmonary vasoconstriction and increasing blood flow to poorly ventilated areas, as discussed in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1

The most effective treatment for V/Q mismatch in asthma focuses on bronchodilators like albuterol and anti-inflammatory medications like corticosteroids to reduce airway obstruction and restore more uniform ventilation throughout the lungs, thereby improving V/Q matching and oxygen delivery. According to the BTS guideline for oxygen use in adults in healthcare and emergency settings, oxygen should be used as the driving gas for nebulised bronchodilators in patients with acute severe asthma, and supplemental oxygen should be provided to maintain an appropriate oxygen saturation level 1.

From the Research

Mechanism of V/Q Mismatch in Asthma

• The ventilation-perfusion (V/Q) mismatch in asthma is characterized by a broad unimodal or bimodal distribution of blood flow with respect to V/Q, with a mean of 10.5% of the blood flow associated with low V/Q units without any appreciable shunt 2.
• The second moment of the perfusion distribution on a log scale (log SD Q) is a descriptor of V/Q inequality, and it is moderately high in patients with acute severe asthma, with a mean of 1.18 (SEM 0.08) 2.
• The administration of salbutamol, a bronchodilator, can worsen V/Q relationships, particularly when given intravenously, due to increased heart rate, cardiac output, and oxygen consumption (VO2) 2.
• Inhaled corticosteroids (ICS) and long-acting β-agonists (LABAs) are commonly used treatments for asthma, and their combination can improve lung function and symptom control 3, 4.
• However, the effect of ICS on long-term lung function in asthma is still debated, with some studies suggesting modest improvements in pre-bronchodilator forced expiratory volume in 1 s (FEV1) 5.

Factors Contributing to V/Q Mismatch

• Pulmonary vascular reactivity is high in patients with acute severe asthma, which can contribute to V/Q mismatch 2.
• The use of 100% oxygen can increase log SD Q, indicating worsened V/Q relationships 2.
• The pathophysiology of V/Q mismatch in asthma is complex and involves disturbances in the physiological matching of alveolar ventilation and pulmonary perfusion, leading to intrapulmonary shunting and arterial hypoxemia 6.

Clinical Implications

• V/Q mismatch is a predictor of adverse outcomes in patients with asthma, and its role in ventilation-induced lung injury and worsening lung edema has been described 6.
• The use of single maintenance and reliever therapy (SMART) with ICS and LABAs can reduce the risk of asthma exacerbations compared to traditional controller and reliever therapies 4.
• The long-term benefits of ICS on lung function in asthma are still being investigated, with some studies suggesting modest improvements in lung function 5.