Analogue device for measurement of area under the maximum expiratory flow-volume curve
The generation of the maximum expiratory flow-volume (m. e. f. v. ) curve forms the basis of a test extensively employed in almost all lung-function laboratories. The area under the m. e. f. v. curve (a. f. v. ) has recently been shown to be a very sensitive index of lung-function impairment. A prerequisite for the routine employment of the a. f. v. measurement in conjunction with the m. e. f. v. curve is that the data should be immediately accessible. The planimetry of areas is both time-consuming and subject to human error. Alternatively, the use of digital equipment to measure a. f. v. , while avoiding these disadvantages, could prove relatively expensive. The authors have developed an inexpensive analog device for the direct measurement of a. v. f. based on a mathematical derivation which states that by squaring the flow signal and integrating with respect to time the area under the m. e. f. v. curve is obtained. The circuit diagram of the measuring device is shown. It comprises, basically, a multiplier module which squares the input signal (flowrate) prior to integration. The integrator amplifier is balanced by means of an offset adjustment network, while the calibration amplifier is used to obtain a readout on the digital voltmeter (d. v. m. ) display in units of square litres per second. The sample-and-hold module is activated on completion of the manoeuvre and maintains the d. v. m. display.