Cardiotocography


Cardiotocography is a technical means of recording the fetal heartbeat and the uterine contractions during pregnancy. The machine used to perform the monitoring is called a cardiotocograph, more commonly known as an electronic fetal monitor.
Fetal monitoring was invented by Doctors Alan Bradfield, Orvan Hess and Edward Hon. A refined version was later developed for Hewlett Packard by Konrad Hammacher.
CTG monitoring is widely used to assess fetal wellbeing. A review found that in the antenatal period 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. The same review found that computerised CTG machines resulted in lower numbers of baby deaths than the traditional CTG machines. More up-to-date research is needed to provide more information around this practice.
CTG monitoring can sometimes lead to medical interventions which are not necessarily needed. Fetal vibroacoustic stimulation has been used to provoke the baby into being more active. This can improve their CTG monitoring so that the mother does not have to be monitored for as long. However the safety of this technique has not been fully assessed; hearing impairment, stress reactions and other effects should be investigated before this technique is used widely.

Method

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. Doppler ultrasound provides the information which is recorded on a paper strip known as a cardiotocograph. 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 have ruptured either spontaneously or artificially, and the cervix is open. To gauge the strength of contractions, a small catheter 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.

Interpretation

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, the American College of Obstetricians and Gynecologists, 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:
There are several factors used in assessing uterine activity.
The NICHD nomenclature defines uterine activity by quantifying the number of contractions present in a 10-minute window, averaged over 30 minutes. Uterine activity may be defined as:
The NICHD nomenclature defines baseline fetal heart rate as:
The baseline FHR is determined by approximating the mean FHR rounded to increments of 5 beats per minute during a 10-minute window, excluding accelerations and decelerations and periods of marked FHR variability. There must be at least 2 minutes of identifiable baseline segments 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 than 160 bpm.

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. In contrast, the presence of mild baseline FHR variability, or an absence of FHR variability does not reliably predict fetal acidemia or hypoxia; lack of moderate baseline FHR variability may be a result of the fetal sleep cycle, or a result of medications, extreme prematurity, congenital anomalies, or pre-existing neurological injury.
The NICHD nomenclature defines baseline FHR variability as:
Baseline FHR variability is determined in a 10-minute window, excluding accelerations and decelerations. Baseline FHR variability is defined as fluctuations in the baseline FHR that are irregular in amplitude and frequency. The fluctuations are visually quantitated as the amplitude of the peak-to-trough in beats per minute. Using this definition, the baseline FHR variability is categorized by the quantitated amplitude as:
Saltatory pattern of fetal heart rate is defined in cardiotocography guidelines by FIGO and NICE as fetal heart rate baseline amplitude changes of more than 25 beats per minute.
In a 1992 study, the saltatory pattern FHR was defined by O'Brien-Abel and Benedetti as:
Fetal heart baseline amplitude changes of greater than 25 bpm with an oscillatory frequency of greater the 6 per minutes for a minimum duration of 1 minute.
The pathophysiology of saltatory pattern is not well-known. It has been linked with rapidly progressing hypoxia for example due to an umbilical cord compression, and it is presumed to be caused by an instability of fetal central nervous system.
In a study by Nunes et al., four saltatory patterns exceeding in CTG more than 20 minutes in the last 30 minutes before birth were associated with fetal metabolic acidosis. According to this study, saltatory pattern is relatively rare condition, thus only four cases were found from three large databases.
In a recent study by Tarvonen et al., it was demonstrated that the occurrence of saltatory pattern, already with minimum duration of 2 minutes, in CTG tracings during labor was associated with fetal hypoxia indicated by high umbilical vein blood erythropoietin levels and umbilical artery blood acidosis at birth in human fetuses. As saltatory patterns preceded late decelerations of fetal heart rate 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 defines an acceleration as a visually apparent abrupt increase in fetal heart rate. An abrupt increase is defined as an increase from the onset of acceleration to the peak in less than or equal to 30 seconds. To be called an acceleration, the peak must be greater than or equal to 15 bpm, and the acceleration must last greater than or equal to 15 seconds from the onset to return to baseline.
A prolonged acceleration is greater than or equal to 2 minutes but less than 10 minutes in duration.
An acceleration lasting greater than or equal to 10 minutes is defined as a baseline change.
Before 32 weeks of gestation, accelerations are defined as having a peak greater than or equal to 10 bpm and a duration of greater than or equal to 10 seconds.

Periodic or episodic decelerations

Periodic refers to decelerations that are associated with contractions; episodic refers to those not associated with contractions. There are four types of decelerations as defined by the NICHD nomenclature, all of which are visually assessed.
Additionally, decelerations can be recurrent or intermittent based on their frequency within a 20 min window.

FHR pattern classification

Before 2008, fetal heart rate was classified as either "reassuring" or "nonreassuring". The NICHD workgroup proposed terminology of a three-tiered system to replace the older, undefined terms.
has recently modified the guidelines on intrapartum fetal monitoring, proposing following interpretation:
According to the Cochrane review from the 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 cerebral palsy, infant mortality or other standard measures of neonatal wellbeing, neither on any meaningful 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.