The peak expiratory flow (PEF) represents the maximal flow measured during a forceful expiration. Graphically, this is represented by the peak of the flow-volume loop and is usually reached within a tenth of a second. The magnitude of the PEF is ultimately limited by the characteristics of the central and large airways (i.e airways disease will reduce the PEF).
Thereafter, there are three additional markers of expiratory flow, which can aid spirometry interpretation when inspecting the shape of the flow-volume curve. These flow markers are defined by their relationship to the FVC:
The Mid-Expiratory Flow’s (MEF’s) represent the expiratory flow rates when a certain percentage (x%) of the FVC remains to be exhaled (MEFx)
Alternatively the Forced Expiratory Flow’s (FEF’s) may be used, which represent the expiratory flow rates when a certain percentage x% of the FVC has already been exhaled (FEFx)
The MEF75 is the flow rate when 75% of the FVC remains to be exhaled, and is equivalent to the FEF25 where 25% of the FVC has been exhaled.
The MEF or FEF values therefore reflect the flow capabilities of the peripheral airways; and consequently the cross sectional area of the bronchioles. Ultimately, these values represent the same features of the expiratory curve, albeit in reverse order.
The mid expiratory flow rates, particularly the MEF25, can vary between successive spirometry manoeuvres in the same patient; they are sensitive to insignificant fluctuations such as cough, expiratory effort and early termination of the manoeuvre. They depend on the patient performing a technically acceptable forced vital capacity since they are the flow rates in proportion to FVC. If a patient terminates the forced expiration early, the MEF values will be shifted to the left, higher up the flow volume loop.