 Peak flow meter

A 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.^[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.^[ref]

A peak alone cannot determine expelled volume. For two original triangular flow pulses, stipulate the same maximum $Q_p=8\mathrm{L}{\mathrm{s}}^{-1}$ but base durations 0.5 and 1.0 s. Their peaks both correspond to 480 L/min, while integrating their traces gives $Q_{p}D/2=2$ 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 obeysFor 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.^[ref]