= Peak expiratory flow
{title2=PEF}
{wiki}
\b[Peak expiratory flow] is the greatest outward <volumetric flow rate> reached during a specified forced expiration. In an ideal continuous record,
$$
\mathrm{PEF}=\max_t Q(t).
$$
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.<Standardization of Spirometry 2019 Update by Brian L. Graham and others>[Reported Values and Table 9]{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 $0\le t\le1$; the other is $Q_2(t)=4(1-2t)$ L/s over $0\le t\le0.5$, 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>.<Maximum Forced Expiratory Flow Rate as a Measure of Ventilatory Capacity by Basil Martin Wright and C. B. McKerrow>[BMJ 2, 1041–1046; PMID 13846051, published bylines B. M. Wright and C. B. McKerrow]{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.<Wright Peak Flowmeter at the Wood Library-Museum of Anesthesiology>[Catalogue record, accession 1998-02-25-1, author identification and object description]{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.<Standardization of Spirometry 2019 Update by Brian L. Graham and others>[Equipment, Display; start of forced expiration]{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.
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