Cardiotocography

In medicine (obstetrics), cardiotocography (CTG) is a technical means of recording (-graphy) the fetal heartbeat (cardio-) and the uterine contractions (-toco-) 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 or external fetal monitor (EFM). CTG can be used to identify signs of fetal distress.

The cardiotocograph was invented by Doctors Orvan Hess and Edward Hon. A refined (antepartal, non-invasive, beat-to-beat) version was later developed for Hewlett Packard by Dr. Konrad Hammacher.

Method

Simultaneous recordings are done 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, overlays 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 child's head to adequately measure the pulse. 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 nurse, midwife, or physician can watch the CTG traces of multiple patients simultaneously, via a computer station.

Interpretation

Cardiotocography is used to monitor several different measures: uterine contractions and four fetal heart rate features - baseline heart rate, variability, accelerations, and decelerations.

Uterine contractions - time between contractions, which reduces as childbirth progresses.
Baseline heart rate - average baseline fetal heart rate (normal 110–160).
Variability - fetal heart rate variability per minute (normal ≥5).
Accelerations - increases in fetal heart rate (normally present).
Decelerations - decreases in fetal heart rate (normally minimal). There are three types of decelerations, depending on their relationship with uterine contraction:

Early - begin at start of contraction and end with conclusion of contraction; a sign of increased vagal tone due to fetal head compression.
Variable - occur at any time; a sign of umbilical cord compression.
Late - begin at the peak of a contraction; a sign of fetal hypoxia due to uterus or placental insufficiency - the most worrisome deceleration.

Types of tests

Use of CTG during the third trimester to monitor fetal wellbeing is called a nonstress test. A positive (good) result is indicated by a reactive non-stress test. This means that the fetal heart rate increased (acceleration) by at least 15 beats per minute for at least 15 seconds at least twice during a 20 minute interval.

Use of this machine during labor is called a stress test. When introduced, this practice was expected to reduce the incidence of fetal demise in labor 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 fetal distress and hence increased (and possibly unnecessary) cesarean deliveries.

Biophysical profile is another test associated with CTG. It is often done when the non stress test is non reactive.

Effect on 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, 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 caesarian section.