From the Guidelines
In asthma, ventilation is decreased, perfusion remains normal, airway resistance is increased, lung compliance is often normal or increased, minute ventilation initially increases but may decrease in severe cases, and pCO2 on ABG is typically normal or low but rises in severe attacks, as supported by the most recent guidelines 1.
Pathophysiology of Asthma
Asthma is a complex disorder characterized by variable and recurring symptoms, airflow obstruction, bronchial hyperresponsiveness, and an underlying inflammation, as described in the expert panel report 3 (EPR-3) guidelines for the diagnosis and management of asthma 1. The interaction of these features determines the clinical manifestations and severity of asthma.
Key Physiological Changes
- Ventilation: Decreased due to bronchoconstriction, inflammation, and mucus plugging that obstruct airflow.
- Perfusion: Typically remains normal, creating ventilation-perfusion (V/Q) mismatch where poorly ventilated areas continue to receive normal blood flow.
- Airway Resistance: Significantly increased due to bronchospasm, mucosal edema, and mucus secretion, making it harder to move air through the airways.
- Lung Compliance: Often normal or increased due to air trapping, though severe attacks may reduce compliance from hyperinflation.
- Minute Ventilation: Initially increases as patients try to compensate for airflow limitation, but may decrease in severe exacerbations when respiratory muscles fatigue.
- pCO2 on ABG: Typically normal or low in mild to moderate asthma due to hyperventilation, but rises (hypercapnia) in severe attacks when the patient can no longer maintain adequate ventilation, signaling respiratory failure and need for immediate intervention, as indicated by guidelines for managing asthma exacerbations 1.
Clinical Implications
The progression from hypocapnia to normocapnia to hypercapnia serves as an important clinical indicator of worsening asthma severity. Guidelines emphasize the importance of classifying the severity of asthma exacerbations based on symptoms, signs, and functional lung assessment to determine treatment 1. In severe cases, immediate treatment may include oxygen therapy, bronchodilators, and corticosteroids, as recommended by guidelines on the management of asthma 1.
From the Research
Asthma Pathophysiology
- Ventilation: refers to the process of moving air in and out of the lungs 2
- Perfusion: refers to the process of blood flowing through the lungs 2
- Ventilation-perfusion mismatch: occurs when there is a mismatch between ventilation and perfusion, leading to inefficient gas exchange 2, 3
- Airway resistance: not directly discussed in the provided studies, but it refers to the opposition to airflow in the airways
- Compliance: not directly discussed in the provided studies, but it refers to the ability of the lungs to expand and fill with air
- Minute ventilation: not directly discussed in the provided studies, but it refers to the total amount of air moved in and out of the lungs per minute
- pCO2 on ABG (Arterial Blood Gas): not directly discussed in the provided studies, but it refers to the level of carbon dioxide in the blood, which can be affected by ventilation-perfusion mismatch 2, 3