The role of increased airway microvascular permeability and plasma exudation in asthma.

Airway oedema and inflammation are recognized as cardinal features of asthma, resulting from increase microvascular permeability of the bronchial circulation with the exudation of plasma and inflammatory cells into the airway lumen. Resistance to airflow is increased and the epithelium is disrupted either directly or by cytotoxic proteins derived from migrating inflammatory cells. Such mediators include bradykinin, platelet-activating factor (PAF), leukotrienes and histamine. Antigen-induced and neurogenic inflammation, generated by immunoglobulin E (IgE) and neuropeptides respectively, may also contribute to oedema generation. Assessment of increased bronchial vascular permeability in animals has largely involved measurement of the extravasation of radiolabelled albumin or protein-bound dyes. Non-invasive techniques are less reliable in humans, but measurement of the rate of clearance of inhaled particles labelled with isotope may prove successful. Airway oedema appears to be an important feature of asthma and future research may be aimed at developing drugs that specifically prevent airway microvascular leakage.