Flow Limitation and its Determination

Abstract

During expiration, there is a maximum limit to the gas flow rate that can be achieved; once this limit is attained, greater muscular effort does not further augment flow. This phenomenon, known as expiratory flow limitation (EFL), has been identified from flow-volume curves. The key documentation was made by Fry and co-workers [1,2], who identified EFL from iso-volume pressure-flow relationships. To obtain such curves, flow, volume and oesophageal pressure (i.e. pleural pressure) were simultaneously measured in subjects seated in a volume displacement plethysmograph, which corrects for gas compression. The subjects were instructed to perform repeated vital capacity manoeuvres with varying amounts of effort. The highest flow obtained at each lung volume was then plotted against pleural pressure, as shown in Fig. 1 (left). It can be seen that at high lung volumes (e.g. 90% of vital capacity) expiratory flow is not limited; however, at volumes <80–85%, vital capacity plateaus occur, indicating maximum flow limitation. A maximum expiratory flow-volume curve (Fig. 1, right) can be easily constructed from the iso-volume flow-pressure curves depicted in the left panel of Fig. 1. After peak flow is achieved, flow decreases with volume but it always reflects the maximum attainable flow at that particular lung volume. If the expiratory flow generated during tidal respiration represents the maximal possible flow someone can generate at that volume, this subject is said to be flow limited [3].