Many of the posts that are going to appear here in future are about CTG monitoring (sometimes referred to, somewhat inappropriately, as EFM or electronic fetal monitoring). While many of you will no doubt know everything I have written below – it is nonetheless a good idea to make sure we are all talking about the same thing!
The cardiotocograph or CTG is a recording device that generates a graphical recording of two sets of information. The first, the cardio- part of the CTG, is the fetal heart-rate. The second set of information, the toco- part of the CTG, relates to the activity of the pregnant woman’s uterus. The changes in each of these are plotted over time so the relationships between changes in one can be compared with changes in the other. The process of using a CTG is known as cardiotocography and the output generated is often referred to as a CTG trace. The regular interpretation of a CTG trace by a skilled practitioner is called CTG monitoring. (It is important to be aware that one or more practitioners must actively work to generate an interpretable CTG trace and apply knowledge to interpret the CTG trace. The CTG machine itself does not monitor the fetus, it merely generates a recording. A clinician does the monitoring.)
There are several different ways to obtain information about the fetal heart-rate and the woman’s uterine activity, and new methods of doing so are beginning to enter clinical use. A distinction is often made between “external” recording (where the sensors are placed on the outside of the woman’s body) and “internal” recording (where the sensors pass through the woman’s vagina to attach to the fetus or sit within the uterine cavity). It is possible to use any one method to record the fetal heart-rate combined with any one method to record uterine activity, with the resulting output still considered as a CTG trace.
Ways to record the fetal heart-rate
1. FSE. A fetal spiral (or scalp) electrode (FSE) is attached to the part of the fetus that can be felt through the cervix. This records the electrical signal from the fetal heart, known as the fetal electrocardiogram or ECG.
2. Doppler. Perhaps the most common way to record the heart rate is to use a Doppler monitor, usually a circular disc, which is placed on the woman’s abdomen over where the chest of the fetus lies. This uses Doppler technology to sense the movement of blood through the fetal heart.
3. External fetal ECG recording. New technology has been developed, and continues to be refined, which records the fetal ECG from a sensor placed externally on the woman’s abdomen.
Ways to record uterine activity
1. Intrauterine pressure monitoring. A thin tube is passed through the woman’s cervix, past the fetus, into the fluid filled space within the uterus.
2. Tocodynamometry. A circular disc, the same size as the Doppler monitor of the CTG, is worn on the woman’s abdomen. It senses changes in pressure in the underlying tissues.
3. Electrohysterography. This is a new approach to recording uterine activity. A sensor worn externally on the woman’s abdomen records electrical activity generated within the uterine muscle as it contracts.
Originally the recording device was connected directly to the CTG machine via wires. Telemetry systems now record the output from sensors wirelessly so that the woman does not needed to be physically connected to the CTG machine. Many telemetry systems are also waterproof so the sensors can be worn in the shower or bath.
Generating a trace
Originally the CTG trace was printed directly onto a moving piece of paper using ink or thermal printing. The paper speed was set to either 1 cm per minute (which has become the standard in most countries where CTG use has become commonplace), or 3 cm per minute. Improvements in computer technology lead to digital CTG monitoring becoming possible. Data collected from the fetus and the pregnant woman can be converted into a digital electronic signal for display on a computer screen, making it possible to save the CTG permanently as an electronic record.
Other information that might be present on a CTG trace
- Second twin. It is possible to monitor more than one fetus simultaneously on most CTG machines.
- Fetal movement recording. Some tocodynamometer sensors are also designed to detect the presence of fetal movements.
- Maternal heart rate. Some sensors detect this automatically, other systems rely on the use of a pulse oximeter worn on the woman’s finger.
- Maternal blood pressure. Many CTG machines have a plug that allows a blood pressure cuff to be attached. The machine can be set to automatically detect the woman’s blood pressure at regular intervals.
- Annotations. These are brief notes written contemporaneously on the CTG which provide information of use in interpreting the CTG trace. Depending on the degree of sophistication of the CTG machine, maternity professionals might hand write on the paper print out, or enter data using a computer interface such as a touch screen for display on a digital CTG.
When the CTG might be used
Research refers to the use of the CTG at three different times:
- Antenatal: refers to the use of CTG monitoring prior to the onset of labour. This typically takes 20 to 30 minutes but may require longer periods of time for sufficient information to be present to decide whether it is normal or not.
- Admission: refers to the generation of a CTG trace at the time that a woman presents to a maternity service in labour. Like antenatal CTG monitoring, the trace typically runs for 20 to 30 minutes.
- Intrapartum: refers to the generation of a CTG trace during a woman’s labour. This may be continuous (running until the birth of the baby), or intermittent (for example 20 minutes of tracing performed every second hour.