The Peak Flow Working Group: physiological determinants of peak expiratory flow.

Peak expiratory flow (PEF) can be defined as the maximum flow at the mouth achieved during an expiration, delivered with maximum force starting from the level of maximum lung inflation. There is evidence that PEF may be determined by the wave speed flow-limiting mechanism, in which case a high PEF is consistent with a low resistance upstream to the flow-limiting segment (the choke point), a large cross-sectional area here, and/or a small compliance of the airway wall. PEF is obtained when the expiration is initiated with maximal inflation of the lungs, because this will make the elastic recoil pressure maximal and the upstream frictional pressure loss minimal. PEF is obtained with maximum expiratory effort, because high acceleration of flow will cause wave speed limitation at a higher lung volume. If PEF is not flow-limited, the effort-dependence will be marked. Flow transients of PEF beyond the maximum flow-volume perimeter may be related to effort-dependent contribution of flow from the collapsing airway, but time-dependent factors, instability of the airway, and inhomogeneities of the lungs may contribute. For the purpose of standardization, it is recommended that forced expiration is initiated with open glottis and immediately after a maximal inspiration. The position of the head should be neutral, because hyperextension may increase PEF and flexion may decrease PEF by changing the stiffness of the larger airways.