Cardiotocographic (CTG) monitors are complex electronic devices developed to acquire, process and display foetal heart rate (FHR) and uterine contraction (UC) signals. This chapter describes the main characteristics of current CTG monitors, in order to allow a better understanding of the technology. An ultrasound transducer is used for the external monitoring of FHR signals, whereas a tocodynamometer is used for the external monitoring of UCs. These technologies are recommended for routine clinical use in both the antepartum and intrapartum periods. Foetal electrode and intrauterine pressure sensors provide internal monitoring of FHR and UC signals, respectively, which are more precise than external signals. They are only applicable during labour, after cervical dilatation and ruptured membranes, and they have established contraindications. The registration of foetal movements, simultaneous monitoring of twins and triplets, continuous maternal heart rate monitoring, monitoring of other maternal parameters, alarms, digital outputs and telemetry are other available characteristics in some CTG monitors.
Highlights
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Several technical developments were incorporated into CTG monitors over the years.
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Knowledge of monitor characteristics is needed for a better use of the technology.
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External CTG monitoring is recommended for routine clinical use.
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Internal CTG monitoring is more precise but has established contraindications.
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Other fetal and maternal parameters are available in some CTG monitors.
Introduction
In the late 1950s and early 1960s, Hon in the United States , Caldeyro-Barcia in Uruguay and Hammacher in Germany independently developed different parts of the equipment and basic principles necessary for the continuous monitoring of foetal heart rate (FHR) and maternal uterine contraction (UC) signals, in a technique that became known as cardiotocography (CTG). The first commercial monitor was commercialised in 1967 , and in high-resource countries, the technology rapidly gained a prominent place in routine obstetric practice.
CTG monitors are complex electronic devices developed to acquire, process and display FHR and UC signals. Although there have been substantial technical advances since the initial models were developed, particularly in system software and in the ultrasound (US) sensor, the basic principles of the technology have remained unaltered. The main aim of this chapter is to describe the principal characteristics of current CTG monitors, in order to allow a better understanding and use of the technology.
The core features of the CTG monitor
All software features required for the acquisition, processing and display of FHR and UC signals, as well as that of other additional signals (see subsequent text), are incorporated in the central unit of the CTG monitor. This unit usually has a digital display showing currently acquired FHR and UC values, FHR signal quality displays and imbedded speakers to provide an audible representation of the FHR. The central unit usually also incorporates a thermal printer to provide a paper output of the acquired CTG tracing.
The horizontal scale of CTG tracings is commonly called ‘paper speed’, and the options that are usually available are 1, 2 or 3 cm/min. In most countries, 1 cm/min is selected, whereas in North America and Japan it is almost exclusively 3 cm/min. Some clinicians feel that 1 cm/min provides records of sufficient detail for clinical analysis, and this option has the advantage of reducing paper costs. Others feel that the small details of CTG tracings are better evaluated using higher paper speeds . The vertical scale of CTG tracings may also be different, and the available alternatives are 20 or 30 beats per minute (bpm)/cm. The selected scales should be the ones with which health-care professionals are most familiar, because different options produce slightly different CTG patterns. The inadvertent use of paper scales, which staff is unaccustomed to, may lead to an erroneous interpretation of CTG features .
The core features of the CTG monitor
All software features required for the acquisition, processing and display of FHR and UC signals, as well as that of other additional signals (see subsequent text), are incorporated in the central unit of the CTG monitor. This unit usually has a digital display showing currently acquired FHR and UC values, FHR signal quality displays and imbedded speakers to provide an audible representation of the FHR. The central unit usually also incorporates a thermal printer to provide a paper output of the acquired CTG tracing.
The horizontal scale of CTG tracings is commonly called ‘paper speed’, and the options that are usually available are 1, 2 or 3 cm/min. In most countries, 1 cm/min is selected, whereas in North America and Japan it is almost exclusively 3 cm/min. Some clinicians feel that 1 cm/min provides records of sufficient detail for clinical analysis, and this option has the advantage of reducing paper costs. Others feel that the small details of CTG tracings are better evaluated using higher paper speeds . The vertical scale of CTG tracings may also be different, and the available alternatives are 20 or 30 beats per minute (bpm)/cm. The selected scales should be the ones with which health-care professionals are most familiar, because different options produce slightly different CTG patterns. The inadvertent use of paper scales, which staff is unaccustomed to, may lead to an erroneous interpretation of CTG features .
External monitoring of the FHR – the US transducer
The US transducer ( Fig. 1 ) is used for the external monitoring of the FHR, in both the antepartum and intrapartum periods. This transducer contains piezoelectric effect crystals that convert electrical energy into US waves, and that use the Doppler effect to detect movements of the cardiac structures. It is based on the principle that US waves inciding on a moving object are reflected with an altered frequency, which can be detected by the emitting transducer. The signal is thus constituted based on the movement of intracardiac structures, such as the cardiac valves and the interventricular septum. Both continuous and pulsed Doppler may be used for this purpose . The transducer is placed on the maternal abdomen, with the support of an elastic band encircling the abdomen, and it is directed at the foetal heart. For an adequate transmission of sound waves, conductive gel needs to be placed between the sensor and the abdomen.
Doppler US was first described for the evaluation of the FHR in 1964 . Due to the almost systematic acquisition of good-quality signals from 26–28 weeks of gestation onwards, it was rapidly incorporated into commercial models. However, the resulting signal can be affected by movement of other intra-abdominal structures, such as maternal vessels and foetal extremities, causing artefacts. This problem was initially addressed by filtering the signals obtained beyond a certain depth of the US beam, but this solution was found to be insufficient . An additional problem is that the signal does not always have an unequivocal peak to allow a rigorous evaluation of interbeat intervals ( Fig. 2 ). The first models, also known as first-generation CTG monitors, incorporated intensity detectors to identify signal peaks. Although tracings were generally of acceptable quality, they presented many artefacts and occasionally showed large signal noise. Second-generation models were introduced a few years later, with a widened US beam and the signal-processing functions of ‘spike removal’, ‘signal modulation’ ( Fig. 2 ) and ‘autocorrelation’ ( Fig. 3 ), made possible by the development of microprocessors .
After the signal has been modulated, the ‘autocorrelation’ function is applied. This consists of an evaluation of the overlap in the area between incoming signals and a sample of recently acquired signals (usually encompassing around three to five heart beats), so as to identify periods of greatest overlap, and the interval between these periods is used to calculate the heart rate ( Fig. 3 ).
Tracings obtained with second-generation CTG monitors have a lower signal loss , and they are visually more similar to those acquired with internal monitoring . However, the ‘autocorrelation’ function results in an evaluation of interbeat intervals that are only an approximation of true heart cycles . The technology is also affected by signal loss, monitoring of the maternal heart rate (MHR) and incorrect evaluations due to the counting of half the number of beats (‘ half counting ’) or twice their real number (‘ double counting ’) . All these problems are more frequent during the second stage of labour. Moreover, the signal-processing methods incorporated into second-generation CTG monitors usually lead to an inaccurate recording of foetal cardiac arrhythmias.
External FHR monitoring is the recommended method for routine CTG monitoring in both the antepartum and intrapartum periods, provided a tracing of acceptable quality is obtained . However, careful repositioning of the probe is frequently needed during the second stage of labour, and when atypical FHR patterns occur, MHR monitoring should be ruled out by evaluating the radial pulse. If any doubt remains as to the nature of the signal, US evaluation or internal FHR monitoring should be performed, the latter in the absence of contraindications (see subsequent text). Internal FHR monitoring should also be preferred when an acceptable external recording cannot be obtained or when a cardiac arrhythmia is suspected .
Internal monitoring of the FHR – the foetal electrode
Internal FHR monitoring is carried out with a disposable foetal electrode ( Fig. 4 ), applied to the foetal presenting part, whether it may be scalp or breech. For the presenting part to be accessible, about 2–4 cm of cervical dilatation and ruptured membranes are required, so the technique can only be used during labour.
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