DOPPLER FOETAL HEART DETECTOR/MONITOR

Movement of the foetal heart and blood in the foetal and placental circulation can be detected with an ultrasonic doppler device which has a transducer placed on the skin containing two ultrasonic crystals. One transmits a continuous wave ultrasonic beam into the body and the other detects returning echoes. The echoes from moving structures are shifted in frequency and this shift is detected and delivered to a loudspeaker or applied to a pattern recognition circuit which identifies the beats of the heart.

In its simplest form (the foetal heart detector) the doppler frequencies are applied to a loudspeaker and give an indication of foetal life and the foetal heart rate. These are used extensively in the antenatal clinic and sometimes in the labour ward. These devices comprise the transducer, an oscillator operating at the ultrasonic frequency (usually around 2 MHz), a tuned amplifier, and a simple demodulation circuit followed by an audio amplifier and loudspeaker.

The foetal heart monitor is used during labour to record the foetal heart rate on a chart alongside a second trace showing the uterine contractions. Well-known patterns of foetal heart rate variation occur in response to the uterine contractions and abnormalities in these patterns indicate foetal distress. The transducer used for the foetal heart rate monitor is different from those used on the simple detectors in that the transmitted beam has a wide angle to guarantee that ultrasonic waves pass through the foetal heart even when the foetus has moved, and there are a number of receiving crystals facing in different directions to ensure detection of returning echoes from the foetus at all times. Not all foetal heart rate monitors employ ultrasound since the preferred method is to detect the foetal heart rate from the foetal ECG.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details