Electronic fetal (heart rate) monitoring (EFM) was developed and introduced into clinical practice during the second half of the 20th century, with the expectation that it would provide information on the well-being of the fetus during labor. The hope was that changes in the fetal heart rate (FHR) pattern resulting from fetal hypoxia would alert the clinician to the impending danger of fetal asphyxia and acidosis and would allow timely intervention before permanent damage occurred. Unfortunately, EFM was widely and rapidly adopted throughout the world, and it has become an integral part of modern obstetrics without ever undergoing the necessary scientific scrutiny to validate its merit.1 Indeed, studies have shown that EFM provides no long-term benefit to infants when compared to intermittent auscultation in labor and actually may cause harm by increasing the rate of operative delivery.2 Nevertheless, given its widespread use and perceived merits, EFM is unlikely to be subjected to randomized clinical trials in the foreseeable future. Also, despite its limitations, it provides valuable information on the physiologic and pathophysiologic changes that some fetuses experience during labor.
EFM consists of continuous recordings of the FHR and the tone of the uterine muscle. Both these recordings are obtained simultaneously and are recorded graphically on a strip of paper that (in the United States) runs at a standard rate of 3 cm/min (Figure 4-1). These signals can be obtained either externally (from the maternal abdominal surface) or internally (from the fetus and the intrauterine cavity). The external transducer that records the FHR is an ultrasound Doppler transmitter-receiver that receives signals reflected off the moving fetal heart (Figure 4-2). The device calculates the FHR based on the time interval between consecutive movements of the moving part being recorded. The FHR is continuously calculated from beat to beat and recorded graphically as a running line on the paper strip or on an electronic screen. Internal monitoring of the FHR can be achieved if the mother’s cervix is dilated and the membranes have been ruptured. A metal spiral electrode (Figure 4-3) is attached transvaginally to the fetal scalp and connected to the monitoring device. The electrode picks up the fetal electrocardiographic signal and calculates the heart rate based on the R-R intervals.
Recording of fetal heart rate and uterine activity. The top panel records the fetal heart rate, and the bottom panel records uterine contractions.
External fetal heart rate transducer and tocodynamometer.
Internal fetal heart rate transducer attached to fetal scalp. From Intrapartum assessment. In: Cunningham F, Leveno L, Bloom S, Hauth J, Rause, D, Spong. Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.