Transabdominal oximetry offers non-invasive monitor of foetal health – Physics World


Daniel Fong (left) and Soheil Ghiasi (right) with their novel foetal oximetry system. (Courtesy: University of California, Davis)

Researchers in the US have designed and tested a novel oximetry system that offers a non-invasive approach for monitoring foetal blood-oxygen levels, an indicator of foetal wellbeing, during active labour. A more objective and accurate measure of foetal blood oxygenation could improve both mother and infant birth outcomes.

If left unaddressed, foetal asphyxia, or oxygen deprivation, can cause long-term disabilities, developmental delays or even death. Therefore, any indication of foetal asphyxia during active labour commonly prompts an emergency Caesarean delivery, or C-section.

The current method of evaluating foetal wellbeing, known as cardiotocography, attempts to indirectly assess foetal oxygen saturation by monitoring the relationship between uterine contractions and foetal heart rate over time. Despite its widespread use for over 50 years, cardiotocography has failed to decrease the rates of complications associated with foetal asphyxia. Instead, it has contributed to increasing rates of C-section, which itself poses dangers to both mother and baby.

A multidisciplinary team of researchers at the University of California, Davis has developed a new transabdominal foetal pulse oximetry system that directly assesses foetal oxygen saturation. The team recently published a detailed description and assessment of the system in IEEE Transactions on Biomedical Engineering.

Pulse oximetry

Pulse oximetry is a non-invasive method used to measure blood-oxygen saturation, similar to the technology commonly employed in smart watches to monitor heart rate.

Haemoglobin, the protein in blood that carries oxygen, absorbs distinct wavelengths of light differently depending on whether it is loaded with or lacking oxygen. Pulse oximetry leverages this characteristic by using a pair of light-emitting diodes, or LEDs, to send a known light signal into the body, and a detector to collect the light that is reflected back to the skin surface.

Standard oximetry systems use LEDs of specific wavelengths such that one is selectively absorbed by oxygen-lacking haemoglobin and the other by oxygen-loaded haemoglobin – commonly 660 nm and 940 nm, respectively. The ratio of the light signals of each wavelength that reach the detector indicates the relative blood-oxygen saturation.

Transabdominal foetal pulse oximetry

Adapting this well-established method to measure foetal blood-oxygen saturation in utero posed interesting obstacles for Daniel Fong and colleagues to overcome.

First, the light must travel deeper into the body to reach the foetus than is possible with conventional oximetry systems. The researchers cleared this hurdle by optimizing the wavelength selection to reduce scattering in the tissue and allow the light to travel further while maintaining a detectable intensity. The team identified the optimal LED wavelengths as 740 nm and 850 nm.

Second, because the light must pass through the mother to reach the foetus, the resulting signal is a combination of maternal and foetal interactions. To address this, the investigators came up with an innovative design involving an additional detector. This extra detector is placed nearer to the LEDs than the primary detector, resulting in a shallow depth of measurement to supply the maternal signal alone. With this information, the software can filter out the maternal portion of the mixed signal and extract the foetal signal for assessment.

The researchers tested their new system on a pregnant ewe. Results from their transabdominal foetal pulse oximetry system agreed with gold-standard, but invasive, measurements of foetal arterial blood gases. Next, they plan to further characterize the system’s performance under various conditions, like active labour, with the ultimate goal of improving the “health and safety of mothers and babies during labour and delivery”, via the start-up spinoff Storx Technologies.

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