Fetal Heart Rate (FHR) Detection System Based on LabVIEW

Fetal heart rate (FHR) detection is a primary method used to determine the health of the fetus before birth and to help identify potential dangers such as fetal hypoxia or compression. The purpose of early detection is to reduce fetal morbidity and mortality.

Currently, the most common way to detect fetal heart rate is Doppler ultrasound, and the standard antenatal fetal health test is the fetal nonstress test (NST). These tests are usually performed in hospitals with continuous wave equipment.

Although ultrasound fetal heart rate monitors have improved significantly, become cheaper and smaller, they still require precise sensor calibration and expertise to operate correctly. In addition, such devices are sensitive to movement, and the safety of long-term fetal exposure to ultrasound has not yet been determined. Therefore, the use of the monitors is currently limited to short-term testing.

Another method of measuring fetal heart rate is fetal electrocardiogram (FECG), but its steps are more complicated and less practical. In addition, there is no commercial non-invasive FECG device on the market.

Recently, an optical method has been proposed, which is still in the research stage and uses halogen or tungsten lamps as light sources and photomultiplication to achieve detection. However, these techniques are costly, require high light intensities, and are difficult to implement due to instrument size and power consumption limitations.

Optical fetal heart rate monitoring system

Our research team proposed a low-power optical technique based on the photoplethysmogram (PPG) signal to detect the fetal heart rate non-invasively. The PPG signal is generated by light modulated by blood pulsation. The doctor or technician illuminates the abdomen of the pregnant woman with an LED light (less than 68 mW), and the light beam is modulated by the blood circulation of the mother and the fetus. The maximum wavelength of light that can penetrate is 890 nm. This mixed signal can be analyzed by adaptive filtering using digital signal processing, and the PPG of the pregnant woman's index finger is used as a reference input.

An optical fetal heart rate (OFHR) detection system was developed using LabVIEW graphical system design software and NI hardware. In an OFHR system, the SNR decreases as the incident power decreases; the excitation signal is a modulated light beam. The system can implement synchronous detection, and the software subroutine in LabVIEW uses the NI 9474 digital output module to generate the modulation frequency at the counter end.

At the receiver, low-noise amplification and synchronous detection ensure that useful information is preserved with minimal noise power. The 24-bit NI USB-9239 analog-to-digital converter (ADC) reduces the effects of quantization noise. Once digitized, the signal is processed by adaptive noise canceller (ANC) technology to extract the fetal PPG from the mixed signal.

The fetal probe (main signal) was connected to the pregnant woman's abdomen with a belt, so that the IR-LED was kept 4 cm away from the photodetector. The reference probe was connected to the mother's index finger. Since the selected IR-LED can only emit a maximum power of 68 mW, the operating optical power of the OFHR system was set to be less than 87 mW specified by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). In order to modulate the IR-LED, a software subroutine was used to generate a 725 Hz modulation signal, which was connected to the LED driver via the NI 9474 counter terminal (Figure 1). In Figure 1, the diffuse reflected light from the pregnant woman's abdomen is measured by a low-noise photodetector and expressed in the form of I (M1, F), where M1 and F represent the influence of the mother's abdomen and the fetus on the signal, respectively.

Figure 1: The hardware modules in the OFHR system block diagram are implemented by LabVIEW program

A low-noise (6 nV/Hz1/2) transimpedance amplifier converts the current into a voltage. The reference probe (attached to the mother's index finger) consists of an IR-LED and a solid-state photodiode with a built-in preamplifier. The signal from this probe is denoted I (M2); M2 represents the mother's influence on the signal. Synchronous detection is not required for this channel because the photoplethysmogram of the index finger has a high signal-to-noise ratio (SNR).

The NI USB-9239 24-bit resolution data acquisition module synchronously acquires signals from the two probes at a rate of 5.5 kHz. Demodulation, signal filtering, and signal estimation are performed in the digital domain. The software implementation includes modulation signal generation, synchronous detection algorithm, downsampling, high-pass filtering, and adaptive noise cancellation (ANC) algorithm.

The design team used LabVIEW to implement the entire algorithm and some of the instruments. After completing the preprocessing and application of the ANC algorithm, LabVIEW will display the results of the fetal signal and fetal heart rate.

Figure 2a shows the laboratory prototype and graphical user interface of the OFHR system, and shows the maternal index finger PPG (top), abdominal PPG (middle), and estimated fetal PPG (bottom).

Figure 2a: OFHR prototype

Figure 2b shows three selectable displays, including a digital synchronous or lock-in amplifier (LIA), an adaptive noise canceller (ANC), and a heart rate trace. The first two displays can be used to aid development, and the third display is used to show the value of the fetal heart rate relative to time. The user can view the data online or save it for further analysis.

Figure 2b: Graphical user interface of the OFHR system

After development, we tested the functionality of the system on a total of 24 data sets from 6 clinical subjects at 35 to 39 weeks of gestation, provided by the National University of Malaysia Medical Center. All fetuses participating in this study were healthy as examined by obstetricians and were born without complications.

In the study, we obtained a correlation coefficient of 0.97 (p value less than 0.001) between optical and ultrasonic fetal heart rate, with a maximum error of 4%. Clinical results show that the closer the probe is to the fetal tissue (not limited to the brain or buttocks), the better the signal quality and detection accuracy.

in conclusion

The research team developed a novel OFHR detection system using low-cost, low-power IR lamps and commercially available silicon detectors. By using LabVIEW, we were able to quickly and easily implement digital synchronous detection and adaptive filtering techniques. Compared with the standard measurement method (Doppler ultrasound), the fetal heart rate results we measured are more accurate. Based on the novelty of the solution, we are currently applying for a patent for its commercial use.