From the Guidelines
The V/Q mismatch mechanism in asthma is primarily caused by ventilation-perfusion inequality due to airway obstruction, leading to hypoxemia, and can be partially reversed with bronchodilators and supplemental oxygen therapy as needed, as supported by the BTS guideline for oxygen use in adults in healthcare and emergency settings 1. The V/Q mismatch in asthma occurs when airways become narrowed and inflamed, causing areas of the lung to receive normal blood flow but inadequate air, resulting in low V/Q ratios and hypoxemia.
- Key factors contributing to V/Q mismatch in asthma include:
- Bronchoconstriction and airway hyperresponsiveness, which quickly narrow the airways in response to various stimuli 1
- Airway edema, mucus hypersecretion, and formation of inspissated mucus plugs, which further limit airflow 1
- Remodeling of airways, including sub-basement fibrosis, smooth muscle hypertrophy, and angiogenesis, which can lead to persistent changes in airway structure 1 Bronchodilators like albuterol and ipratropium bromide can help partially reverse V/Q mismatch by dilating airways and improving ventilation, while corticosteroids such as prednisone reduce underlying inflammation, and supplemental oxygen therapy may be necessary during severe exacerbations to correct hypoxemia 1. Understanding the V/Q mismatch mechanism in asthma is crucial for effective management, including pulse oximetry monitoring and tailored treatment approaches to address the underlying inflammation and airway obstruction.
From the Research
V/Q Mismatch Mechanism in Asthma
- The ventilation/perfusion (V/Q) mismatch is a key mechanism in asthma, particularly in acute severe asthma, where it leads to life-threatening hypoxemia 2.
- Studies have shown that V/Q mismatch is characterized by a bimodal blood flow pattern, with a marked deterioration of the dispersion of pulmonary blood flow, resulting in numerous alveolar units with low V/A/Q ratios 2, 3.
- The V/Q mismatch is thought to be caused by the narrowing of airways, which reduces ventilation to certain areas of the lung, while perfusion remains unaffected, leading to areas with low V/A/Q ratios 2.
- Inhaled platelet-activating factor (PAF) has been shown to induce moderate-to-severe disturbance of V/A/Q status, possibly related to altered microvascular permeability within the airway wall 2.
- Salbutamol, a bronchodilator, has been shown to prevent PAF-induced systemic and lung function abnormalities, possibly by antagonizing venoconstriction in the bronchial circulation 2.
- The administration of 100% oxygen has been shown to worsen V/Q mismatch, possibly due to the increase in pulmonary vascular reactivity 3.
- V/Q mismatch has been correlated with the severity of asthma symptoms, but not with the severity of reduced maximal airflow rates 2, 3.
Treatment and Management
- Inhaled corticosteroids (ICS) and long-acting beta-agonists (LABA) are commonly used to treat asthma, with the combination of ICS and LABA being more effective than ICS alone in reducing exacerbations and improving lung function 4, 5.
- The addition of LABA to ICS has been shown to be more effective than adding anti-leukotrienes (LTRA) in reducing oral steroid-treated exacerbations and improving lung function 5.
- Salbutamol, either inhaled or intravenous, has been shown to improve airflow rates, but its effect on V/Q mismatch varies depending on the route of administration, with intravenous salbutamol worsening V/Q mismatch and inhaled salbutamol having minimal effects 3.