Recording flow in the first second of a maximal forced expiratory manoeuvre: influence of frequency content.

The frequency content of the first second of the maximum forced expiratory manoeuvre (MFEM) was measured to determine if the currently accepted frequency limit of 20 Hz for MFEM is adequate for recording peak expiratory flow (PEF). The frequency response of a Fleisch pneumotachograph (PT) was measured and used to record MFEM from 24 patients attending a lung-function laboratory and 26 normal volunteers. The first 1.024 s of the signal recorded at 1,000 Hz for that blow with maximum PEF, underwent fast Fourier transformation using a triangular window function, applied after 0.75 s to reduce flow linearly to zero. All the frequencies above a set limit were removed, followed by inverse transformation to reconstitute the blow. The limits for this frequency cut-off were progressively varied from 100 Hz down to 15 Hz, with the resulting PEF being compared with the PEF from the reconstituted blow with no frequency reduction. The average+/-SD age for the group was 47+/-18 yrs and the average PEF was 450+/-187 L x min(-1), which, when expressed as a standardized residual, was 0.1+/-2.1, with a range from -4.5-3.9 indicating a good spread around normal values. Average rise time to PEF was 83+/-38 ms and dwell time >90% PEF was 45+/-25 ms. Cut-off >20 Hz reduced the mean PEF of the group by 8.5 L x min(-1) (95% confidence limit 5.5-11.4 L x min(-1)), whereas cut-off >30 Hz reduced mean PEF by 4.4 L x min(-1) (2.6-6.2). In the present study subjects, 30 Hz was on the 95th percentile of frequencies for defining the upper limit for 98% of the power spectrum for the first second of the blow. It has been shown that there are frequencies >20 Hz that contribute to peak expiratory flow enough to influence readings made using conventional hand-held peak expiratory flow meters, such as the mini-Wright. Devices used for recording flow from a maximum forced expiratory manoeuvre should therefore have an adequate frequency response of up to 30 Hz.