The role of equal pressure points in understanding pulmonary diseases.

THE RESPIRATORY SYSTEM is composed of a conducting part and a respiratory part. The conducting airways, i.e., the trachea, bronchi, and bronchioles, differ in architecture. Both trachea and bronchi are surrounded by rings of hyaline cartilage for support, whereas the bronchiolar wall does not contain any cartilage and therefore tends to collapse in response to changes in intrapleural or airway pressure. As a result of airflow resistance, pressure that is generated in the alveoli drops along the airways during expiration (1, 3). This is also known as friction loss. When airway pressure has dropped to a level where it equals intrapleural pressure during forced expiration, an equal pressure point (EPP) is reached (4). At the EPP, airways that are not supported by cartilage will collapse (Fig. 1). In healthy lungs, the EPP will be reached in cartilaginous airways as a result of sufficient alveolar driving pressure and only a gradual drop in pressure due to minimal airway resistance (4). In the case of (severe) airway obstruction, resistance to airflow will be much greater, and the pressure drop will be much steeper. The EPP will move upstream toward the alveoli and will be reached in the thin-walled bronchioles, causing airway collapse and the typical depression in the flow-volume curve (2, 3). Although the clinical importance of EPPs, especially in obstructive airway diseases, is unquestionable, relatively little effort has been put into a comprehensible description of the dynamics involved in this phenomenon. This is unfortunate, because correctly applying complex physics in a clinical context can be a difficult task. Therefore, this report aimed to provide a straightforward mathematical approach to the concept of EPPs in the airways by combining important principles of airflow dynamics in a conceivable way. This results in an equation (Eq. 1) that can be used to get a better understanding of how airflow and airway properties alter the tendency of airways to collapse: