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.