An impedance pneumograph utilizing microprocessor-based instrumentation

For any great benefit to be realized from biological impedance measurement techniques, there must be an acceptable quantitative correlation between the impedance signal and the physiological parameter of interest. Carrying out an extensive experimental effort with the goal of maximizing this correlation requires versatile, yet easily operatable, instrumentation. This paper describes a microprocessor-based constant-current impedance pneumograph which provides the user with flexible control over several drive variables. Drive frequency, current magnitude, calibration, and other information is entered via a keyboard, and the microprocessor performs the otherwise tedious and time-consuming adjustments required during an experiment. In addition, there are provisions included for operations, i.e., variable sweeps, in which one set of variables is held constant and another is varied in a specified manner. Data can be formatted for analog output to a device such as a strip chart recorder or an oscilloscope, formatted for minicomputer processing, or selectively displayed on digital readouts. The flexibility afforded by the microprocessor should provide an improved definition of the relationships among drive frequency, drive amplitude, electrode type, and electrode placement required to achieve optimum correlation of the impedance signal to the specified physiological parameter.