The generation and display of signals from electro-mechanical spirometers.

We have investigated how faithfully the electronic circuitry in two commonly used electro-mechanical spirometers records maximal expiratory flow-time and flow-volume curves according to the criteria of the American Thoracic Society. We have made an assessment of the speed of response necessary for X-Y plotters to record such curves accurately. Using an instrumentation recorder to record and subsequently replay flow-time and flow-volume curves onto a general purpose X-Y plotter, we have found that faithful reproduction could be obtained with a four-fold reduction in the replay speed compared to the recording speed. Consideration should be given to the influence of the spirometer and recording device used when interpreting the results of maximal expiratory manoeuvres.