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Fetal Doppler

1. Fetal heart sound in clinic
Marsar, a Frenchman, claimed in 1950 that there was fetal heart sound when the fetus was in the uterine. In 1818, Surgeon Mayor detected fetal heart sound directly by the ear. A Frenchman named Laennec contrived wooden bell stethoscope in 1819 which was applied to clinical practices in 1821. Although there was only a limited knowledge about fetal heart sound at that time, it laid a foundation for the development of fetal monitoring.

Kergaradec published a study in 1822 to expatiating on the use of stethoscope to diagnose pregnancy and to monitor abnormality of fetus. As a result, auscultation of fetal heart sound became popular in Europe and America. Auscultation of Fetal Heart Sound of Kennedy, which later served as an important guidance of clinical practice of obstetrics for a long period, was released in 1833.
Fetal Doppler detects the fetal heart sound and calculates the fetal heart rate (FHR).

2. The Principle and Application of Fetal Doppler
The Fetal Doppler consists of essentially two components:(i) a hand-held probe containing one or more transducers or transducer arrays for generating and detecting ultrasonic waves; and (ii) an electronic base unit (hereafter referred to as a base unit) capable of converting the electrical signal from the probe to an audible response meaningful to the human ear. In use, the hand-held probe is held against the abdomen of a woman suspected of being pregnant, with the transducer end of the probe facing in the direction of the suspected fetus. The probe is then activated, resulting in an ultrasonic wave stream being directed through the abdominal wall. A portion of this wave stream is reflected back to the probe. If a live fetus is present, the movement of its heart (and of blood through the heart chambers) results in a frequency shift ("Doppler shift") in the waves reflected from that region. The magnitude and sign of the Doppler shift varies with the instantaneous velocity of the sound-wave-reflecting surface and hence, if this surface is that of the fetal heart, the motion of the heart chambers. An audible signal is generated by the base unit from the varying Doppler shift. The base unit may also display a visual readout, e.g. a digital display, of fetal heart rate.

In a typical pregnancy, the Fetal Doppler is incapable of reliably detecting the fetal heartbeat until about 12 weeks gestation. This is owing primarily to the low level of ultrasonic energy reflected from the first trimester fetal heart and to the high degree of dampening of that energy (ultrasonic impedance) by the abdominal wall of the mother.

At present Fetal Doppler are manufactured in a large number of countries, and the functions are being improved continuously. Many domestic hospitals are equipped one fetal monitor for each bed in gynecologic department, and it is getting popularized in China.

Fetal Doppler is effected in multi-pulse fashion. Continuous emit-receive mode is changed into pulse emit-receive manner, viz. emit-stop-receive-stop-re-emit…, which greatly enhances safety, and which reduces negative effects on pregnant woman and the fetus. Ultrasonic frequencies applied to fetal monitoring at present are mainly: 1MHz、1.5MHz、2MHz、2.5MHz. 1MHz and 2MHz are usually applied to single-fetal monitoring, and 1.5MHz and 2.5MHz are often used for twin-fetal monitoring. Repetition frequency of pulse of products made by different companies which commonly is over 1 KHz varies. Ultrasonic pulse-echo Doppler technology for fetal monitor has been well developed nowadays, and its clinical importance has also been widely recognized.

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