thumb|Footage of a CTG test

Cardiotocography (CTG) is a technique used to monitor the fetal heartbeat and uterine contractions during pregnancy and labour. The machine used to perform the monitoring is called a cardiotocograph.

Fetal heart sounds were described as early as 350 years ago and approximately 200 years ago mechanical stethoscopes, such as the Pinard horn, were introduced in clinical practice.

Modern-day CTG was developed and introduced in the 1950s and early 1960s by Edward Hon, Roberto Caldeyro-Barcia and Konrad Hammacher. The first commercial fetal monitor (Hewlett-Packard 8020A) was released in 1968.

CTG monitoring is widely used to assess fetal well-being by identifying babies at risk of hypoxia (lack of oxygen). CTG is mainly used during labour. A review found that in the antenatal period (before labour), there is no evidence to suggest that monitoring women with high-risk pregnancies benefits the mother or baby, although research around this is old and should be interpreted with caution. Up-to-date research is needed to provide more information surrounding this practice. External tocometry is useful for showing the beginning and end of contractions as well as their frequency, but not the strength of the contractions. The absolute values of pressure readings on an external tocometer are dependent on position and are not sensitive in people who are obese. Internal tocometry can only be used if the amniotic sac is ruptured (either spontaneously or artificially) and the cervix is open. To gauge the strength of contractions, a small catheter (called an 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 may be stored on a computer for later reference. The plotting speed (paper feed) is set at 3 cm/min in the U.S. and 1 cm/min in Europe. A variety of systems for centralized viewing of CTG have been installed in 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.

Interpretation

thumb|A typical CTG output for a woman not in labour. A: Fetal heartbeat; B: Indicator showing movements felt by mother (triggered by pressing a button); C: Fetal movement; D: Uterine contractions|500px

In the US, the Eunice Kennedy Shriver National Institute of Child Health and Human Development sponsored a workshop to develop a standardized nomenclature for use in interpreting Intrapartum fetal heart rate and uterine contraction patterns. This nomenclature has been adopted by the Association of Women's Health, Obstetric and Neonatal Nurses (AWHONN), the American College of Obstetricians and Gynecologists (ACOG), and the Society for Maternal-Fetal Medicine.

The Royal College of Obstetricians and Gynaecologists and the Society of Obstetricians and Gynaecologists of Canada have also published consensus statements on standardized nomenclature for fetal heart rate patterns.

Interpretation of a CTG tracing requires both qualitative and quantitative description of several factors. This is commonly summed up in the following acronym, DR C BRAVADO:

  • DR: Define Risk
  • C: Contractions (uterine activity)
  • BRA: Baseline fetal heart rate (FHR)
  • V: Baseline FHR variability
  • A: Presence of accelerations
  • D: Periodic or episodic decelerations
  • O: Changes or trends of FHR patterns over time

Uterine activity

There are several factors used in assessing uterine activity.

  • Frequency: the number of contractions per unit time.
  • Duration: the amount of time from the start of a contraction to the end of the same contraction.
  • Resting tone: a measure of how relaxed the uterus is between contractions. With external monitoring, this necessitates the use of palpation to determine relative strength. With an IUPC, this is determined by assessing actual pressures as graphed on the paper.
  • Interval: the amount of time between the end of one contraction to the beginning of the next contraction.

The NICHD nomenclature

Baseline FHR variability

Moderate baseline fetal heart rate variability reflects the delivery of oxygen to the fetal central nervous system. Its presence is reassuring in predicting an absence of metabolic acidemia and hypoxic injury to the fetus at the time it is observed. Based on the duration of the change, increased (i.e. marked) baseline variability is divided into two terms: zigzag pattern and saltatory pattern of FHR. The NICHD nomenclature defines baseline FHR variability as:

Zigzag pattern of fetal heart rate

A Zigzag pattern of fetal heart rate (FHR) is defined as FHR baseline amplitude changes of more than 25 beats per minute (bpm) with a minimum duration of 2 minutes and maximum of 30 minutes. Despite the similarities in the shape of the FHR patterns, the zigzag pattern is distinguished from the saltatory pattern by its duration. According to the International Federation of Gynaecology and Obstetrics (FIGO), a saltatory pattern is defined as FHR baseline amplitude changes of more than 25bpm with durations of >30 minutes.

Saltatory pattern of fetal heart rate

A saltatory pattern of fetal heart rate is defined in cardiotocography (CTG) guidelines by FIGO as fetal heart rate (FHR) baseline amplitude changes of more than 25 beats per minute (bpm) with a duration of >30 minutes.

In a 1992 study, the saltatory pattern FHR was defined by O'Brien-Abel and Benedetti as "[f]etal heart baseline amplitude changes of greater than 25bpm with an oscillatory frequency of greater than 6 per minutes for a minimum duration of 1 minute". The pathophysiology of the saltatory pattern is not well-known. for example due to an umbilical cord compression, and it is presumed to be caused by an instability of the fetal central nervous system. four saltatory patterns in CTG exceeding 20 minutes in the last 30 minutes before birth were associated with fetal metabolic acidosis. According to this study, saltatory pattern is a relatively rare condition; only four cases were found from three large databases.

In a study by Tarvonen et al. (2019), it was demonstrated that the occurrence of saltatory pattern (already with the minimum duration of 2 minutes) in CTG tracings during labor was associated with fetal hypoxia indicated by high umbilical vein (UV) blood erythropoietin (EPO) levels and umbilical artery (UA) blood acidosis at birth in human fetuses. As saltatory patterns preceded late decelerations of fetal heart rate (FHR) in the majority of cases, saltatory pattern seems to be an early sign of fetal hypoxia. According to the authors, awareness on this gives obstetricians and midwives time to intensify electronic fetal monitoring and to plan possible interventions before fetal asphyxia occurs.

Accelerations

The NICHD nomenclature

A prolonged acceleration is greater than 2 minutes but less than 10 minutes in duration, while an acceleration lasting 10 minutes or more is defined as a baseline change.

Before 32 weeks of gestation, accelerations are defined as having a peak of at least 10bpm and a duration of at least 10 seconds.

  • Late decelerations: a result of placental insufficiency, which can result in fetal distress. Monitoring usually shows symmetrical gradual decrease and return to baseline of the fetal heart rate in association with a uterine contraction. A 'gradual' deceleration has an onset to nadir of 30 seconds or more. In contrast to early deceleration, the low point of fetal heart rate occurs after the peak of the contraction, and returns to baseline after the contraction is complete.
  • Normal: No hypoxia or acidosis; no intervention necessary to improve fetal oxygenation state.
  • Baseline 110–160bpm
  • Variability 5–25bpm
  • No repetitive decelerations (decelerations are defined as repetitive when associated with >50% contractions)
  • Suspicious: Low probability of hypoxia/acidosis, warrants action to correct reversible causes if identified, close monitoring or adjunctive methods.
  • Lacking at least one characteristic of normality, but with no pathological features.
  • Pathological: High probability of hypoxia/acidosis, requires immediate action to correct reversible causes, adjunctive methods, or if this is not possible expedite delivery. In acute situations, delivery should happen immediately.
  • Baseline <100bpm
  • Reduced or increased variability or sinusoidal pattern
  • Repetitive late or prolonged decelerations for >30 min, or >20 min if reduced variability (decelerations are defined as repetitive when associated with >50% contractions)
  • Deceleration >5 minutes

Benefits

According to the Cochrane review from February 2017, CTG was associated with fewer neonatal seizures but it is unclear if it had any impact on long-term neurodevelopmental outcomes. No clear differences in incidence of cerebral palsy, infant mortality, other standard measures of neonatal wellbeing, or any meaningful differences in long-term outcomes could be shown. Continuous CTG was associated with the higher rates of caesarean sections and instrumental vaginal births. The authors see the challenge in how to discuss these results with women to enable them to make an informed decision without compromising the normality of labour. Future research should focus on events that happen in pregnancy and labour that could be the cause of long-term problems for the baby.