Anatomy and physiology of collateral respiratory pathways

When complete obstruction of an airway occurs, ventilation and gas exchange distal to the obstruction can be preserved if "collateral ventilation" exists. Three collateral pathways have been described: the interalveolar, the bronchioloalveolar and the interbronchiolar communications. The various techniques used to measure collateral flow, resistance, compliance and time constant are described, with particular attention to those trying to separate the resistances of intrasegmental and collateral pathways. The various techniques used to measure collateral flow, resistance, compliance and time constant are described, with particular attention to those trying to separate the resistances of intrasegmental and collateral pathways. The relative importance of collateral ventilation in humans and animals is shown; factors affecting this ventilation are described: type of flow, lung volume, surface tension, alveolar CO2, pulmonary circulation and oedema, cholinergic and adrenergic receptors, histamine, interdependence. Parenchymal diseases disturb collateral flow; on the other hand, collateral channels can provide alternative pathways in various human pathologies.