In vivo comparison of electromagnetic and Doppler flowmeters: with special attention to the processing of the analogue Doppler flow signal

In acute experiments, instantaneous arterial flow tracings were obtained simultaneously with an electromagnetic (square-wave and pulsed field) and a non-directional Doppler flowmeter at rest, during reactive hyperaemia and during isoprenaline infusion. A wide band Doppler flow signal was processed with a zero-crossing meter preceded with and without a differentiator. The performance of both high and low threshold zero-crossing meters was also examined. The combination of a differentiator with a high threshold zero-crossing meter gave the best results. In this way, the Doppler flowmeter followed the electromagnetic flowmeter without loss of high frequency information. Since the differentiator weights the amplitude with respect to frequency, the low amplitude high frequency components, which normally are riding on the high amplitude low frequencies, can be counted in the zero-crossing meter. The peak velocity readings in the femoral artery, however, were significantly higher than those calculated from the electromagnetic flowmeter readings, both with the differentiator and the band-pass filter preceding the zero-crossing meter, necessitating in situ calibration when absolute flow values are required. Possible causes of this discrepancy are discussed.