In obstetrics, cardiotocography (CTG) is a technical method for recording (-graphy) the foetal heartbeat using ultrasound (cardio-) and the uterine contractions (-toco-) during pregnancy, typically in the third trimester. The machine which is used to perform the monitoring is called a cardiotocograph, commonly known as a foetal monitor.
Sensors are placed against the mother's abdomen and are connected to a heart rate monitor, which produces a record of the baby's heartbeat. Cardiotocography records changes in the foetal heart rate and their temporal relationship to uterine contractions. The aim is to identify babies who may be short of oxygen (hypoxic), so additional assessments of foetal well-being may be used, or the baby delivered by caesarean section or instrumental vaginal birth.
CTG is used both antenatally (before birth) and during labour to monitor the baby for any signs of distress. By looking at various different aspects of the baby's heart rate, doctors and midwives can see how the baby is coping.
CTG is most commonly carried out externally. This means that the equipment used to monitor baby's heart is placed on the abdomen of the mother. An elastic belt is placed around the mother's abdomen. It has two round plates about the size of a tennis ball which make contact with the skin. One of these plates uses ultrasound to measure the baby's heart rate. The other measures the pressure in the abdomen and the mother's contractions.
The CTG belt is connected to a machine which interprets the signal coming from the plates. The baby's heart rate can be heard as a beating or pulsing sound which the machine produces. Some mothers can find this distracting or worrying, but it is possible to turn the volume down if the sounds cause distress. The machine also provides a printout which shows the baby's heart rate over a certain length of time. It also shows how the heart rate changes with contractions.
Before labour the mother may be asked to press a button on the machine every time the baby moves. At this time there may not be any contractions so the CTG will only monitor the baby's heart rate. Occasionally, if a signal can't be found using the external monitor, or when monitoring is more important, internal monitoring can be used. For internal monitoring, a small device called an electrode is inserted through the vagina and placed on the baby's scalp. This device records the heart rate.
Cardiotocography uses ultrasound to detect the baby's heart rate. Ultrasound travels freely through fluid and soft tissues. However, ultrasound is reflected back (it bounces back as 'echoes') when it hits a more solid (dense) surface. For example, the ultrasound will travel freely though blood in a heart chamber. But, when it hits a solid valve, a lot of the ultrasound echoes back. Another example is that when ultrasound travels though bile in a gallbladder it will echo back strongly if it hits a solid gallstone. So, as ultrasound 'hits' different structures in the body, of different density, it sends back echoes of varying strength. In CTG monitoring a special type of ultrasound, called Doppler, is used. This type of ultrasound is used to measure structures that are moving, making it useful for monitoring heart rate. The other plate on the CTG measures how tense the mother's abdomen is. This measurement is used to show when the uterus is contracting.
Baby's heart rate
It is normal for a baby's heart rate to vary between 110 and 160 beats a minute. This is much faster than an adult heart rate, which is about 60-100 beats per minute. A heart rate in a baby that doesn't vary or is too low or too high may signal a problem. Changes in the baby's heart rate that occur along with contractions form a pattern. Certain changes in this pattern may suggest a problem. If test results suggest a baby has a problem, the doctor may decide to deliver the baby right away. This may mean a need to have a Caesarean section or an assisted delivery using forceps.
The pressure-sensitive contraction transducer, called a tocodynamometer (toco) has a flat area that is fixated to the skin by a band around the belly. The pressure required to flatten a section of the wall correlates with the internal pressure, thereby providing an estimate of it.
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 foetal head to adequately measure the electric activity of the foetal 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.
Baseline fetal heart rate
The baseline FHR is determined by approximating the mean FHR rounded to increments of 5 beats per minute (bpm) during a 10-minute window, excluding accelerations and decelerations and periods of marked FHR variability (greater than 25 bpm). There must be at least 2 minutes of identifiable baseline segments (not necessarily contiguous) in any 10-minute window, or the baseline for that period is indeterminate. In such cases, it may be necessary to refer to the previous 10-minute window for determination of the baseline. Abnormal baseline is termed bradycardia when the baseline FHR is less than 110 bpm; it is termed tachycardia when the baseline FHR is greater than160 bpm.
Effect of CTG management
A Cochrane Collaboration review has shown that use of cardiotocography reduces the rate of seizures in the newborn, but there is no clear benefit in the prevention of cerebral palsy, perinatal death and other complications of labour. In contrast, labour monitored by CTG is slightly more likely to result in instrumental delivery (forceps or vacuum extraction) or caesarean section. The introduction of additional methods of intrapartum assessment has given mixed results.
When introduced, this practice was expected to reduce the incidence of foetal demise in labour and make for a reduction in cerebral palsy (CP). Its use became almost universal for hospital births in the U.S. In recent years there has been some controversy as to the utility of the cardiotocograph in low-risk pregnancies, and the related belief that over-reliance on the test has led to increased misdiagnoses of foetal distress and hence increased (and possibly unnecessary) caesarean deliveries.
Compiled and edited by John Sandham