Fetal Heart Rate Measurement Using Doppler Ultrasound TechniquePublished on 14/8/2018 & updated on 14/8/2018
There are two types of ultrasonic transducers namely narrow beam and wide beam types. Narrow beam type uses only one ultrasound transmitter / receiver piezoelectric crystal pair. It is very sensitive but produces good trigger signal to measure heart rate immediately. Wide beam transducers operate in many configurations. It has multiple piezoelectric crystals mounted on it to detect fetal movement on a wider area.
The block diagram of ultrasonic Doppler shift based fetal heart rate is shown below. The setup uses both narrow beam and wide-angle beam transducer. The transmitted signal goes to the receiving path. It acts as local oscillator signal to mix diodes in the demodulator. The value of the reflected signal is 90 to 130 dB lesser than transmitted signal. The receiving signal requires special measures to reduce the effect of interference, low noise distortion, filters to drives amplifier into non-linear operating region. Magnetic and Electrical shields surround the high frequency section of the circuits. Depending on the type of transducer used filters are selected to match the Doppler shifted frequency components.
Monitoring Labour ActivityThe uterus muscles start contracting with high intensity during labour. During the expulsion period, the intrauterine pressure reaches 150 mmHg. During spontaneous active labour, a normal patient has uterine contractions at an interval for three to five minutes, 30 to 70 seconds with a peak intensity of 50 to 75 mmHg. Labour activity is recorded either in terms of intra-uterine pressure using catheter or using external transducer. Transducer has a tip that presses against the mother’s abdomen for effective coupling. When the tension in uterus increases, transducer surface is displaced. Inside the transducer housing, a strain gauge converts this displacement into electrical signal. The above diagram shows the setup for measuring labour activity externally. In an AC amplifier, the output of transducer is amplified. Few low frequency labour activity signals comes from the synchronous detector and reached the DC amplifier. Finally, the activity can be recorded on a chart or displayed in a meter. Passive transducers are used to drive the circuits to obtain electrical indication of pressure.
In internal method, a fluid filled catheter is used to measure intra-uterine pressure (IUP). Tube is inserted into the uterus without rupturing fetal membrane. Amniotic fluid is let to flow in freely to ensure correct placement. Catheter is fitted with a pressure transducer. Any change in the amniotic pressure is transmitted to the gauge using in compressible fluid in catheter. Finally, pressure transducer converts the catheter pressure into electrical signal and displays the result on strip recorder.
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