Obesity on obstetrics: new challenges and solutions using abdominal fetal ECG

Midwives online Dec 2008/Jan 2009


Abstract

Obesity is now an important health problem and pregnancy coupled with obesity can result in the pregnancy being classified as high risk. Careful and close monitoring is therefore necessary. This article highlights some of the problems with Doppler ultrasound (cardiotocogram (CTG)) in monitoring obese mothers, and how by using the technology of abdominal fetal electrocardiogram (ECG) monitoring, the quality of care in relation to fetal heart rate (FHR) monitoring to this cohort can be improved. A study of 120 pregnancies, ranging from a body mass index (BMI) of 18 to 44, showed that obesity did not affect the success rate of the FHR data.

Introduction

Obesity is emerging as an important global health problem. Obese pregnant women are at high risk throughout the antenatal, intrapartum and postpartum period. Confidential Enquiries into Maternal and Child Health (CEMACH) Perinatal Mortality 2006 report reveals that ‘of the women who had a stillbirth and a recorded BMI, 26% were obese (BMI >30)’. Obesity in pregnancy has been selected as CEMACH’s principle project with a maternal health focus for 2008 to 2011. There is currently no national clinical guideline available in the UK with regard to clinical care in obesity in pregnancy.


The antenatal period

During the antenatal period, it is considered that obese mothers are at a higher risk of hypertension, diabetes, and stillbirths. Therefore they may require more monitoring in their pregnancies. In clinical practice, abdominal palpation for fetal assessment of growth, fetal presentation and position can be very difficult in obese mothers. The current monitoring method using Doppler ultrasound has a poor penetration in fat tissue. At the moment, there is not enough evidence to evaluate the effectiveness of this method in obese pregnant women.
Ramsey et al (2006) reported that external electronic fetal monitoring can be problematic and that women may have reduced awareness of fetal movements. Ultrasound to measure growth, arterial Doppler and CTG are difficult to perform successfully in obese women.

Technology

In this article, the authors discuss two techniques of monitoring the FHR in the antenatal period, Doppler ultrasound (CTG) (see Figure 1) and abdominal fetal ECG (see Figure 2). 

Doppler ultrasound monitoring

Determining the FHR using Doppler ultrasound dates back to the 1970s, it uses high frequency sound waves directed to the fetal heart and relies on the Doppler effect of the echo to calculate the FHR, (as ultrasound energy is directed towards the fetus, it is classified as an invasive technique). The abdominal transducer of the monitor recognises each heartbeat from a series of echoes and uses the strongest to measure the heart rate. The strongest echo may not occur at the same time in every heartbeat, making it difficult to establish accurate measurements of the beat-to-beat fatal heart rate variability.


Abdominal fetal ECG monitoring

The concept of using the fetal ECGs to establish the FHR is not new. It was first initiated in 1906, when Cremer discovered that by applying electrodes to the maternal abdomen, it was possible to obtain the maternal and fetal ECG. (cited in Cicinelli et al, 1994). Abdominal fetal ECG is a technology that is entirely non-invasive. It detects the electrical activity from the fetal heart that is already present on the mother’s abdomen using multiple surface electrodes. There is no potential harm to mother or fetus from the monitoring and therefore is classified as non-invasive. As adipose tissue is not a barrier to the conduction of electrical signals, this method can provide a useful and valuable tool to clinicians in monitoring fetal wellbeing in obese mothers.

The abdominal fetal ECG monitor uses the R-R interval from the fetal ECG to establish the FHR, therefore providing a very accurate measurement of the beat-to-beat variability. This is demonstrated in Figure 3.

The abdominal fetal ECG monitor is lightweight and portable, providing mothers the freedom of movement. There are no belts that can be uncomfortable for some women as well (see Figure 2). The woman can wear the monitor around her neck using a neck cord, or clip it to clothing when mobilising. The monitor can be used in the antenatal period and during the first stage of labour. Its use has not at present been validated for the second stage of labour. The FHR is displayed on the screen or printed along with the maternal heart rate and uterine contractions (see Figure 4).

The addition of the FHR means that confusion cannot be made with the maternal heart rate, especially when the mother is tachycardic. This is highlighted in a report by Neilson et al (2008): ‘We report a confusing situation, which may occur during external fetal monitoring in which the fetal signal is replaced by an alternative from the mother or second fetus without the usual recognisable transition associated with such signal source shifting. This masks the condition of the fetus without the attending staff being alerted to the loss of fetal signal. In approximately 10,000 deliveries, we have encountered five examples of unexpected adverse fetal outcome attributable to this signal ambiguity.’ The abdominal fetal ECG monitor is unable to be used in twin pregnancies, as it would not be able to distinguish between the two heart rates, but will dismiss the confusion with maternal pulse in singleton pregnancies.

Once the fetal ECG electrodes are placed on the maternal abdomen, it is able to continue to detect the FHR as the fetus moves, requiring no readjustment as the unique positioning of the electrodes produces three channels of data (see Figure 5). Therefore as the fetus moves, regardless of fetal lie, presentation or position it is able to pick up the fetal ECG signal.

Calculation of BMI

BMI is a simple index of weight-for-height that is commonly used to classify underweight, overweight and obese adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 70kg and whose height is 1.75m will have a BMI of 22.9. It is not a standalone tool in obstetrics, as other factors such as maternal ethnicity, parity, and smoking habits need to be considered to provide good care and assessment, but it will alert midwives to possible increased risk factors associated with either a low or high BMI.


Clinical trial

As part of an independently assessed clinical trial, undertaken in the Department of Perinatology, Fertility and Gynaecology at Holland’s University of Utrecht, 120 mothers were recruited with singleton pregnancies – the gestation of each varied between 20 to 40 weeks. The BMI was recorded for every woman participating in the clinical trial.

The abdominal fetal ECG monitor was used to record an average of 15 hours of fetal and maternal heart rate data. Five high-quality ECG electrodes were placed in the same position on each woman’s abdomen – the only differentiating factor was the distance between electrodes in relation to the gestation of the pregnancy. The recording was either in a home or hospital setting. A total of 92% of the overnight recordings as judged by the lead clinical investigator were of an acceptable clinical standard offering extended periods of high-quality FHR data. The BMI range was between 18 and 44. The highest BMI in the study was 44, the success of data obtained was 95% illustrating that BMI did not affect the ability to obtain a successful recording. The study data suggests that a higher BMI does not interfere with the ability of the monitor to successfully detect the FHR (see Figure 6). Hence monitoring the fetus of an obese mother is no more difficult than a non-obese mother.

In this research, the FHR was monitored in gestations from 20 weeks onwards. However in practice, only the viable fetus would be monitored.

Conclusion

The increased incidence of obesity may be a reflection in the choices made, be it at societal or an individual level. Many health initiatives are in place to tackle the growing problem of obesity in its prevention and management.

Maternity units should ensure that a protocol is in place for obese women and also have a multidisciplinary approach, always ensuring that respect and dignity is maintained, as well as a non-judgmental approach.

Units also need to ensure that equipment is suitable for obese women, such as larger blood pressure cuffs and reliable monitors for recording the FHR. The abdominal fetal ECG monitor has bridged one gap for healthcare professionals in that it monitors the FHR over an extended period of time, regardless of maternal BMI. This is very useful in clinical practice, when women need multiple recordings during a 24-hour period. This may also prove a useful form of monitoring in early labour.

As clinicians, it is imperative to ensure that the provision of care to all pregnant women should be of a high quality regardless of BMI.


References and bibliography

Chu S, Bachman D, Callaghan W, et al. (2008) Association between obesity during pregnancy and increased use of health care. The New England Journal of Medicine 358: 1444-53.

Cicinelli E, Bartone A, Carbonara I. (1994) Improved equipment for abdominal fetal electrocardiogram recording: description and clinical evaluation. Int J Bio-Med Comp 34: 193-205.

Confidential Enquiry into Maternal and Child Health. (2008) Perinatal mortality 2006: England, Wales and Northern Ireland. Confidential Enquiry into Maternal and Child Health: London.

Fraser R, Chan K. (2003) Problems of obesity in obstetric care. Current Obstetrics and Gynaecology 13(4): 239-43.

Neilson D, Freeman R, Mangan S. (2008) Signal ambiguity resulting in unexpected outcome with external fetal heart rate monitoring. American Journal of Obstetrics and Gynecology 198(6): 717-24.

Ramsey J, Greer I, Sattar N. (2006) ABC of obesity: obesity and reproduction. BMJ 333: 1159-62.

World Health Organization. (2008) Global data on body mass index: BMI classification. See: http://www.who.int/bmi/index.jsp?introPage=intro_3.html (accessed 14 November 2008).