Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 21

During expiration, the extent of the pressure drop between A and B is proportional to the flow rate. Clearly, with increased effort, the pressure at A will increase, the pressure difference between A & B will increase and flow rate will be increased … up to a point. Eventually, a critical flow rate will be reached, where the pressure gradient between A and B is sufficient to overcome the retractile force of the lung, the airway wall collapses and airflow ceases. Once there is no flow, the pressure inside the airway at point B quickly equilibrates with that at A. With no pressure difference forcing the airway wall to collapse, the retractile force of the lung reopens the airway and flow recommences. This brings us back to where we started and the cycle begins again. It will be apparent that this mechanism determines a maximum possible flow rate along the airway. Any attempt to increase flow rate (associated with a greater pressure difference A to B) will simply result in airway closure. As each route out of the lung will similarly have a maximal possible flow rate, the expiratory flow from the lung as a whole will have an absolute limit. It can be seen that this limit (to expiratory flow) is set by the internal mechanics of the lung, not by muscular strength or effort (above a certain level of effort). That is perhaps fortunate; if it were not the case then lung function tests such as peak expiratory flow rate (PEFR) would not be tests of lung function at all, but of muscular strength.