Peak expiratory flow (PEF)

Peak expiratory flow is the greatest outward volumetric flow rate reached during a specified forced expiration. In an ideal continuous record,It has units of volume per time, commonly litres per second or litres per minute, whereas FEV1 and FVC are integrated volumes. The 2019 technical statement defines the peak from a maximal forced expiratory maneuver and specifies its reporting units.^[ref]

An original pair of triangular volumetric flow rate histories separates the quantities. Let numerical time be in seconds. One curve is $Q_1(t)=2(1-t)$ L/s over $0\le t\le 1$; the other is $Q_2(t)=4(1-2t)$ L/s over $0\le t\le 0.5$, followed by zero. Both expel 1 L in total and in the first second. Their peaks are nevertheless 2 and 4 L/s, or 120 and 240 L/min. Equal expired volume does not identify the highest volumetric flow rate.

The instrument history reflects the value of measuring that maximum directly. In 1959, Basil Martin Wright and C. B. McKerrow published a description of a portable instrument for maximum forced expiratory volumetric flow rate.^[ref] The Wood Library-Museum's original-object catalogue identifies its Wright peak flow meter, records the author as B. M. Wright (Basil Martin), and describes the instrument's portability.^[ref]

A peak flow meter and a full spirometer record different amounts of information. A retained peak does not reconstruct the preceding waveform or total volume. Conversely, numerical differentiation and finite sensor response can affect a peak inferred from a recorded volume trace. Adequate temporal response, effort and the starting lung volume belong to the measurement definition. The primary standard connects early volumetric flow rate and the peak with maneuver effort and requires complementary waveform displays.^[ref]

The triangular examples are mathematical constructions, not reference values or a diagnostic rule. Comparing peak measurements requires comparable units, maneuvers and instrument response rather than treating a maximum as a complete description of lung function.