Electrode-Based Sensor for Non-Invasive Fetal Heart Rate Monitoring With Improved Reliability

The majority of obstetric deliveries in the US undergo electronic monitoring and continuous uterine activity and fetal heart rate (FHR) monitoring is the standard of care. Typically, external transducers are employed, the reliability of which depends on their proper positioning, which may be disturbed by patient or fetal movement. The tocodynamometer (strain gauge, toco for short) provides frequency and timing of contractions, but requires transmission of tension from the uterus to the sensor. Fetal heart rate is acquired with an external Doppler ultrasound transducer. The reliability of this monitor depends on the ability to obtain a window to the fetal heart.

In some patients, particularly the obese, the toco and ultrasound may fail to monitor consistently. In others both transducers require frequent repositioning by the nursing staff, and the Doppler may erroneously report maternal heart rate instead of fetal.

The alternative uterine activity monitor is an intrauterine pressure catheter (IUPC), which is placed through the cervical os in the adequately dilated patient with ruptured membranes. While this monitor usually provides a more reliable signal than the toco, as well as quantitative information regarding intrauterine pressure, it is invasive and there is an increased risk of infection. The alternative FHR monitor is via fetal scalp electrode (FSE), which is applied transvaginally to the fetal presenting part, also requiring adequate cervical dilation and ruptured membranes. While the FSE usually provides a more reliable signal, it is similarly invasive and increases risk of infection.

Maternal obesity (defined as BMI at presentation ≥ 30) is an ever-increasing problem in the US, and is even more prominent in the obstetric suite. Meanwhile, obese women have an increased risk for labor problems, infections and other complications. Noninvasive labor monitoring, while preferable to reduce the infection risk, is particularly unreliable in this population.