Peak flow meter
= Peak flow meter
A \b[peak flow meter] is an instrument that measures <peak expiratory flow>, the greatest outward <volumetric flow rate> reached during a specified forced expiration. A portable instrument can retain that maximum without recording the whole <volume> versus time history. A <spirometer> that records a suitable flow trace can also determine the maximum, but the two outputs contain different information.
<Basil Martin Wright> and <C. B. McKerrow>'s 1959 primary paper described a portable instrument for maximum forced expiratory flow measurement.<Maximum Forced Expiratory Flow Rate as a Measure of Ventilatory Capacity by Basil Martin Wright and C. B. McKerrow>[BMJ 2(5159), pp. 1041–1046, primary instrument report]{ref} The Wood Library-Museum preserves an Airmed example: its collection record places the device in the practical transition from bulky laboratory measurements to portable measurements.<Wright Peak Flowmeter at the Wood Library-Museum of Anesthesiology>[Object description and catalogue, accession 1998-02-25-1]{ref}
A peak alone cannot determine expelled <volume>. For two original triangular flow pulses, stipulate the same maximum $Q_p=8\,\mathrm{L\,s^{-1}}$ but base durations 0.5 and 1.0 s. Their peaks both correspond to 480 L/min, while integrating their traces gives $Q_pD/2=2$ and 4 L. The second pulse expels twice as much air without a greater peak.
Finite instrument response introduces a separate issue. As an illustrative <first-order low-pass filter> model, suppose the indicated <volumetric flow rate> obeys
$$
\tau_m\frac{dq_m}{dt}+q_m=q(t),\qquad q_m(0)=0.
$$
For a rectangular flow pulse of height $Q$ lasting $D$, the indicated peak is $Q(1-e^{-D/\tau_m})$. Durations $D=\tau_m$ and $D=3\tau_m$ produce only about 63.2% and 95.0% of the input peak. This is a response model, not an assertion that the historical instrument has that exact mechanism or <time constant>.
A useful measurement therefore specifies the maneuver, units and instrument response as well as the displayed maximum. A retained peak does not establish <forced vital capacity>, <forced expiratory volume in one second> or <lung compliance>. Those require different information; <spirometry> standards distinguish equipment performance from maneuver quality.<Standardization of Spirometry 2019 Update by Brian L. Graham and others>[Equipment and FVC Maneuver sections]{ref}