[Method for the evaluation of flow rate from pulmonary sounds].

We recorded the lung sound and flow rate from six normal subjects (3 male and 3 female). Sound was picked up at the trachea with a sensitive microphone held in a small probe. Flow rate was measured at the mouth using a Fleisch No. 3 pneumotachograph. Subjects were made to breath for 15 s, with an increasing peak flow rate starting from apnoea to around 2 l.s-1. Both sound and flow rate were directly digitized (i.e. without temporary analogic recording) at a sampling rate of 5120 Hz. Sound and flow were then divided in 128-sample blocks. For each block, the frequency spectrum was computed using the fast Fourier transform. Frequency spectrum depends on the flow rate in many ways. We computed the following formula on each spectrum: D = K.Fmean/(1 + A/Amean) where K and A are constant, Fmean and Amean are respectively the mean frequency and the mean amplitude of the spectrum computed on a 128-sample block. D may be considered as an evaluation of the flow rate every 50 ms. Plotted versus the measured values of the flow rate, D showed a linear relationship. This feature can be used as an almost instantaneous evaluation of the flow rate, or it is possible to compute the mean of D over consecutive 128-sample blocks. This has lead us to calculate the mean of the flow rate over 100, 200, ..., 800 ms. Of course, the longer the time window, the better the correlation between computed flow and real value. The values obtained for this correlation varied between 0.79 and 0.94.