A NOVEL TECHNIQUE FOR ASSESSMENT OF FETAL AUTONOMIC NERVOUS SYSTEM ACTIVITY FROM DOPPLER ULTRASOUND SIGNAL

摘要

An apparatus and method of an abdominal beat-to-beat fetal heart rate (FHR) signal registration with Doppler ultrasound technique is provided. Fetal heart rate signals are recorded with ultrasound transducer which continuously emits 2 MHz ultrasound wave of a very low power 1.5 mw/ cm2. The wave reflected from moving parts of fetal heart (walls or valves) returns to the transducer, which has receiving elements. Frequency shift between emitted and reflected waves is caused by the Doppler effect and provides information on the speed of moving object on which the ultrasound beam is focused.Doppler ultrasound transducer is held on patient abdomen in the direction such that ultrasound waves emitted will pass the fetal heart movement. The reflected waves from moving fetal heart rate are received by receiving element in the transducer.This signal is fed to the RF amplifier (2 MHz) and FM demodulator to detect the movement of the fetal heart. Demodulated detected waveform has definite events relating to contraction and relaxations of fetal heart. Each event is a combination of different frequency components relating to motion of fetal heart and angle of incidence of the ultrasound wave on it. This signal is then passed through envelope filter (Band Pass Filter 22 Hz) with centre frequency of 22 Hz which results in generating two simple peaks per cardiac events. This signal is then passed through a variable threshold* detector where threshold is kept at half the peak value of incoming signal. Two separately detected pulses then pass through a non retriggerable monostable multivibrator for avoiding double triggering of a single cardiac event. The adjustable pulse width for this monostable multivibrator is 50 ms to 200 ms giving fetal heart rate range up to 300 BPM. At the same time, Doppler signal related to heart movements and contained in the audio frequency range (from 0.2 to 1 kHz) is fed to the speaker, which helps in correct positioning of transducer on maternal abdomen. The maternal and per abdomen ECG is also monitored during the process for separate filtering and evaluation studies. This output is given to signal conditioning board to personal computer USB port for HRV analysis. The received data is then processed for estimation of distribution parameters and diagnostic index calculation, describing activity of sympathetic and parasympathetic nervous systems of fetus.