Fetal Monitors - Electronic and Doppler
Electronic fetal monitor
In medicine, cardiotocography (CTG) is a technical means of recording the fetal
heartbeat and the uterine contractions during pregnancy, typically in the third
trimester. The machine used to perform the monitoring is called a
cardiotocograph, more commonly known as an electronic fetal monitor (EFM).
The invasive fetal monitoring was invented by Doctors Orvan Hess and Edward Hon.
A refined (antepartal, non-invasive, beat-to-beat) version (cardiotocograph) was
later developed for Hewlett Packard by Dr. Konrad Hammacher.
Schematic explanation of cardiotocography: heart rate (A) is calculated from
fetal heart motion determined by ultrasound, and uterine contractions are
measured by a pressure transducer (B). These numbers are represented on a time
scale with the help of a running piece of paper, producing a graphical
Simultaneous recordings are performed by two separate transducers, one for the
measurement of the fetal heart rate and a second one for the uterine
contractions. Each of the transducers may be either external or internal.
External measurement means taping or strapping the two sensors to the abdominal
wall. The heart ultrasonic sensor, similar to a Doppler fetal monitor, detects
motion of the fetal heart. The pressure-sensitive contraction transducer, called
a tocodynamometer (toco), measures the tension of the maternal abdominal wall -
an indirect measure of the intrauterine pressure.
Internal measurement requires a certain degree of cervical dilatation, as it
involves inserting a pressure catheter into the uterine cavity, as well as
attaching a scalp electrode to the fetal head to adequately measure the electric
activity of the fetal heart. Internal measurement is more precise, and might be
preferable when a complicated childbirth is expected.
A typical CTG reading is printed on paper and/or stored on a computer for later
reference. Use of CTG and a computer network, allows continual remote
surveillance: a single obstetrical nurse, midwife, or obstetrician can watch the
CTG traces of multiple patients simultaneously, via a computer station.
Doppler fetal monitor
What is a doppler?
Invented in 1958 by Dr. Edward H. Hon a Doppler fetal monitor or Doppler fetal
heart rate monitor is a hand-held ultrasound transducer used to detect the heart
beat of a fetus for prenatal care. It uses the Doppler effect to provide an
audible simulation of the heart beat. Some models also display the heart rate in
beats per minute. Use of this monitor is sometimes known as Doppler
auscultation. Doppler fetal monitors are commonly referred to simply as "Dopplers".
Doppler fetal monitors provide information about the fetus similar that provided
by a fetal stethoscope. One advantage of the Doppler fetal monitor over a
(purely acoustic) fetal stethoscope is the electronic audio output, which allows
people other than the user to hear the heartbeat. One disadvantage is the
greater complexity and cost and the lower reliability of an electronic device.
Originally intended for use by health care professionals, this device is
becoming popular for personal use.
Dopplers for home or hospital use differ in the following ways:
Manufacturer: popular manufacturers are Nicolet, Huntleigh, Summit Doppler,
EchoHeart, Ultrasound Technologies (Seward / Wakeling), Parks Medical
Electronics (as Obstetrical Dopplers),and Sunray.
Probe type: waterproof or not. Waterproof probes are used for water births.
Frequency: 2- or 3-MHz probes. Most practitioners can find the heart rate with
either probe. A 3-MHz probe is recommended to detect a heart rate in early
pregnancy (8–10 weeks gestation). A 2-MHz probe is recommended for pregnant
women who are overweight. A new 5-MHz EchoHeart Transvaginal Fetal Doppler Probe
aids in the detection of fetal heart tones (FHT) early in pregnancy (6–8 weeks)
and for patients who have a retroverted uterus or throughout pregnancy for FHT
detection for women who are obese.
Heart rate display: some Dopplers automatically display the heart rate; for
others the fetal heart rate must be counted and timed by the practitioner.
The generic use of the word "Sonicaid" for Doppler fetal monitors comes from the
products of the UK company Sonicaid Ltd. Sonicaid products included the D205/206
portable fetal Dopplers and FM2/3/4 series of fetal monitors. The company was
acquired by Oxford Instruments in 1987 to form Oxford Sonicaid.
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