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 representation.
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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