From the Guidelines
In asthma, ventilation-perfusion mismatch, increased airway resistance, and altered lung compliance lead to compromised gas exchange, with ABG findings indicating hypoxemia and potential hypercapnia in severe cases, necessitating prompt intervention to prevent respiratory failure. In asthma, ventilation-perfusion mismatch occurs as airways narrow unevenly, leading to areas of the lung receiving air but inadequate blood flow 1. Airway resistance increases significantly due to bronchospasm, mucosal edema, and mucus plugging, making it harder to move air through the bronchial tubes. Lung compliance often increases (hyperinflation) as air becomes trapped behind obstructed airways, causing the lungs to become more distended and less elastic. Some key points to consider in asthma management include:
- Minute ventilation typically increases as patients breathe faster to compensate for poor gas exchange, though severe attacks may reduce overall ventilation.
- ABG findings vary with asthma severity: in mild to moderate attacks, pO2 is often decreased while pCO2 remains normal or low due to hyperventilation; in severe, life-threatening asthma, pCO2 rises (>40 mmHg) indicating ventilatory failure, while pO2 drops significantly (<60 mmHg) 1.
- The progression from hypoxemia with hypocapnia to hypoxemia with hypercapnia represents a critical deterioration requiring immediate intervention, as it signals respiratory muscle fatigue and impending respiratory failure.
- The severity of asthma exacerbations can be classified based on symptoms, lung function, and ABG results, guiding treatment and management decisions 1.
From the Research
Asthma Pathophysiology
- Ventilation: The process of moving air in and out of the lungs, which is impaired in asthma due to airway obstruction and inflammation 2.
- Perfusion: The process of supplying oxygen to the body's tissues, which can be affected in asthma due to ventilation-perfusion mismatch 2.
- Airway resistance: The opposition to airflow in the airways, which is increased in asthma due to bronchoconstriction and airway inflammation 3.
- Compliance: The ability of the lungs to expand and fill with air, which can be decreased in asthma due to airway inflammation and remodeling 4.
Respiratory Parameters
- Minute ventilation: The volume of air breathed in or out of the lungs per minute, which can be increased in asthma due to hyperventilation 2.
- pCO2: The partial pressure of carbon dioxide in arterial blood, which can be decreased in asthma due to hyperventilation 2.
- pO2: The partial pressure of oxygen in arterial blood, which can be decreased in asthma due to ventilation-perfusion mismatch and airway obstruction 2.
Arterial Blood Gas (ABG) Analysis
- ABG analysis can help assess the severity of asthma and guide treatment decisions, with parameters such as pCO2, pO2, and pH providing valuable information about respiratory function 2.
- In asthma, ABG analysis may show a decrease in pO2 and an increase in pCO2, indicating hypoxemia and hypercapnia, respectively 2.