Design of instrumentation and data-acquisition system for complex admittance measurement

Instantaneous left ventricular volume measurements have been made for many years using a tetrapolar conductance catheter. The main objective is to determine the efficiency of the beating heart, using a tetrapolar catheter inserted in the left ventricle of transgenic mice. The effect of the parallel myocardium contribution must be removed from the total measurement. A dual-frequency technique involving 1 kHz and 100 kHz was chosen because it has been established that the imaginary part (the capacitive reactance) of the complex admittance of the cardiac muscle is much smaller in the lower frequency than at the higher frequency. The design involves generation of an accurate frequency source for both the frequencies careful selection of operational amplifiers for the current conversion stage so that the current is not too large to kill the mouse and that it is capable of performing at high frequencies. The band pass filter stage involved careful design with minimal overlap of the pass bands of both the channels. The overall circuit was designed so that there is minimal shift in the phase due to the circuit elements alone. Work also involved design of GPIB--based data acquisition system using LabVIEW and a digital oscilloscope for effective data acquisition even at high frequencies, which are normally limited by the sampling frequency. This data acquisition system is currently being used in laboratory studies in vivo.