Cardiotocographs (CTG) or fetal monitor (EFM) are devices for recording the fetal heartbeat and the uterine contractions during pregnancy.
External cardiotocography can be used for continuous or intermittent monitoring. The fetal heart rate and the activity of the uterine muscle are detected by two transducers placed on the mother’s abdomen (one above the fetal heart, to monitor heart rate and the other at the fundus of the uterus to measure frequency of contractions). Doppler ultrasound provides the information which is recorded on a paper strip known as a cardiotocograph (CTG).External tocometry is useful in showing the beginning and end of contractions, as well as frequency, but not the strength of contractions. The absolute values of pressure readings on an external tocometer are dependent on position, and are not sensitive in people who are obese. In cases where information on the strength, or precise timing, of contractions is needed, an internal tocometer is more appropriate.
Internal cardiotocography uses an electronic transducer connected directly to the fetal scalp. A wire electrode is attached to the fetal scalp through the cervical opening and is connected to the monitor. This type of electrode is sometimes called a spiral or scalp electrode. Internal monitoring provides a more accurate and consistent transmission of the fetal heart rate than external monitoring because factors such as movement do not affect it. Internal monitoring may be used when external monitoring of the fetal heart rate is inadequate, or closer surveillance is needed. Internal tocometry can only be used if membranes (fore-waters) have ruptured either spontaneously or artificially, and the cervix is open. To gauge the strength of contractions, a small catheter (Intrauterine pressure catheter or IUPC) is passed into the uterus, past the fetus. Combined with an internal fetal monitor, an IUPC may give a more precise reading of the baby’s heart rate and the strength of contractions.
A typical CTG reading is printed on paper and/or stored on a computer for later reference. A variety of systems for centralized viewing of CTG have been installed in a large number of maternity hospitals in industrialised countries, allowing simultaneous monitoring of multiple tracings in one or more locations. Display of maternal vital signs, ST signals and an electronic partogram are available in the majority of these systems. A few of them have incorporated computer analysis of cardiotocographic signals or combined cardiotocographic and ST data analysis.