Antepartum, transabdominal near infrared spectroscopy: feasibility of measuring photon migration through the fetal head in utero

Review of optical methods for fetal monitoring in utero

The current technology for monitoring fetal wellbeing during child birth is cardiotocography. However, CTG has high false positive rates that lead to unnecessary emergency Cesarean deliveries and false negatives that result in birth injuries. To curtail these issues, fetal pulse oximetery has been a topic of interest for many decades. Fetal pulse oximetry would yield the oxygen saturation of the fetus in utero and provide a more robust marker for clinicians to make decisions about performing emergency Cesarean deliveries. Here, we present a review of biomedical optical developments related to transabdominal fetal pulse oximetery in the biophotonics field and the challenges that must be overcome to make transabdominal pulse oximetry a clinical reality.

Effect of the presence of amniotic fluid for optical transabdominal fetal monitoring using Monte Carlo simulations

About a third of babies are delivered by Cesarean section. There has been an increase in maternal deaths during labor due to complications with subsequent births after a C-section. Therefore, there is a clinical motivation to reduce the C-section rate. Current techniques are, however, inefficient at determining fetal distress leading to a high false positive rate for complications and ultimately a C-section. For the current study, Monte Carlo simulations were used to calculate the amount of signal received on a model of a pregnant mother, as well as, the percent of the signal that comes from the fetal layer. Models with and without a 1 mm amniotic fluid were compared and showed differing trends.

Design and In Vivo Evaluation of a Non-Invasive Transabdominal Fetal Pulse Oximeter

OBJECTIVE

Current intrapartum fetal monitoring technology is unable to provide physicians with an objective metric of fetal well-being, leading to degraded patient outcomes and increased litigation costs. Fetal oxygen saturation (SpO2) is a more suitable measure of fetal distress, but the inaccessibility of the fetus prior to birth makes this impossible to capture through current means. In this paper, we present a fully non-invasive, transabdominal fetal oximetry (TFO) system that provides in utero measures of fetal SpO2.

METHODS

TFO is performed by placing a reflectance-mode optode on the maternal abdomen and sending photons into the body to investigate the underlying fetal tissue. The proposed TFO system design consists of a multi-detector optode, an embedded optode control system, and custom user-interface software. To evaluate the developed TFO system, we utilized an in utero hypoxic fetal lamb model and performed controlled desaturation experiments while capturing gold standard arterial blood gases (SaO2).

RESULTS

Various degrees of fetal hypoxia were induced with true SaO2 values ranging between 10.5% and 66%. The non-invasive TFO system was able to accurately measure these fetal SpO2 values, supported by a root mean-squared error of 6.37% and strong measures of agreement with the gold standard.

CONCLUSION

The results support the efficacy of the presented TFO system to non-invasively measure a wide-range of fetal SpO2 values and identify critical levels of fetal hypoxia.

SIGNIFICANCE

TFO has the potential to improve fetal outcomes by providing obstetricians with a non-invasive measure of fetal oxygen saturation prior to delivery.

Estimating the Shape of the Fetal Pulse Curve for Transabdominal Pulse Oximetry using Synchronous Averaging. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2020;2020:1-4. [PMID: 33017916 DOI: 10.1109/embc44109.2020.9176692]

A sufficient oxygen supply of the fetus is necessary for a proper development of the organs. Transabdominal fetal pulse oximetry is a method that allows to measure the oxygenation of the fetal blood non-invasively by placing the light sources and photodetectors on the belly of the pregnant woman. The shape of the measured fetal pulse wave is needed to extract parameters for the estimation of the oxygen saturation. This work presents an extension of our previously presented signal processing strategy that allows to extract an average shape of the fetal pulse wave from noisy mixed photoplethysmograms (PPG) with dominating maternal and very weak fetal signal components. An adaptive noise canceller and a comb filter are used to suppress the maternal component. The quality of the resulting fetal signal is sufficient to identify single pulse waves in time domain. Further processing demonstrates the extraction of the mean shape of a single fetal pulse wave by synchronous averaging of several detected pulses. The method is evaluated with different datasets of several simulated and synthetic signals measured with a tissue mimicking phantom. The feasibility of the approach is demonstrated by preparing the mixed PPGs to perform fetal pulse oximetry in future studies. However, clinical measurements are needed to finally evaluate the proposed system beyond synthetic datasets.

Extraction of the Fetal Pulse Curve for Transabdominal Pulse Oximetry using Adaptive and Comb Filters

The fetal pulse curve can be captured by placing light sources and detectors on the belly of a pregnant woman. Following the principle of reflection pulse oximetry, the light emitted into the abdomen is modulated by pulsing maternal and fetal arteries. The acquired signal is a mixture of a weak fetal and a dominating maternal photoplethysmogram (PPG). A first step towards estimation of the fetal oxygen level is the reconstruction of the purely fetal signal in time domain. As already shown in a former work, comb filters are well suited for the task, in case the fetal heart rate is known. In this work we extend the method by utilizing an adaptive noise canceller (ANC) to estimate the fetal pulse rate for comb filter design. Synthetic test signals with constant and time variable pulse rates are generated in order to achieve reproducible conditions. The ANC is fed by the mixed PPG and the maternal reference signal to reduce the dominant maternal components. The fetal pulse rate is computed by evaluating peaks in the resulting signal in time and frequency domain. The findings are used for comb filter design. It is shown that the extraction of the fetal pulse curve from the synthetic mixed PPGs by using the proposed strategy is promising. Clinical test measurements are the next step for evaluation.

Signal Separation for Transabdominal Non-invasive Fetal Pulse Oximetry using Comb Filters

Non-invasive fetal pulse oximetry is the application of reflection pulse oximetry to the abdomen of a pregnant woman. Light sources and detectors areplaced on the belly. Emitted photons travel through maternal and fetal tissue and back to the detectors. The captured photoplethysmogram (PPG) is a complex mixture of the maternal and fetal pulse curve. A purely fetal PPG in time domain is needed to estimate the oxygen level of the unborn child. In this work we describe the application of comb filters to separate the fetalfrom the maternal signal. Finite element simulations and phantom measurements are utilized to generate and measure synthetic signals at different heart rates and noise levels. Comb filters with peak frequencies matched to the fetal heart rate are applied to the mixed PPGs. The filtered signals prove that the extraction of the fetal signal is sufficient even at a distance between the maternal and the fetal signal magnitudes of around 80 dB. The resulting signal quality is sufficient for beat to beat analysis and feature extraction in the time domain. We conclude that comb filtering is a suitable signal separation method for non-invasive fetal pulse oximetry.

Optode Design Space Exploration for Clinically-robust Non-invasive Fetal Oximetry

Non-invasive transabdominal fetal oximetry (TFO) has the potential to improve delivery outcomes by providing physicians with an objective metric of fetal well-being during labor. Fundamentally, the technology is based on sending light through the maternal abdomen to investigate deep fetal tissue, followed by detection and processing of the light that returns (via scattering) to the outside of the maternal abdomen. The placement of the photodetector in relation to the light source critically impacts TFO system performance, including its operational robustness in the face of fetal depth variation. However, anatomical differences between pregnant women cause the fetal depths to vary drastically, which further complicates the optical probe (optode) design optimization. In this paper, we present a methodology to solve this problem. We frame optode design space exploration as a multi-objective optimization problem, where hardware complexity (cost) and performance across a wider patient population (robustness) form competing objectives. We propose a model-based approach to characterize the Pareto-optimal points in the optode design space, through which a specific design is selected. Experimental evaluation via simulation and in vivo measurement on pregnant sheep support the efficacy of our approach.

A near infrared instrument to monitor relative hemoglobin concentrations of human bone tissue in vitro and in vivo

A continuous wave near infrared instrument has been developed to monitor in vivo changes in the hemoglobin concentration of the trabecular compartment of human bone. The transmitter uses only two laser diodes of wavelengths 685 and 830 nm, and the receiver uses a single silicon photodiode operating in the photovoltaic mode. The functioning of the instrument and the depth of penetration of the near infrared signals was determined in vitro using tissue-equivalent phantoms. The instrument achieves a depth of penetration of approximately 2 cm for an optode separation of 4 cm and, therefore, has the capacity to interrogate the trabecular compartment of human bone. The functioning of the instrument was tested in vivo to evaluate the relative oxy-hemoglobin (HbO(2)) and deoxy-hemoglobin (Hb) concentrations of the proximal tibial bone of apparently healthy, normal weight, adult subjects in response to a 3 min on, 5 min off, vascular occlusion protocol. The traces of the relative Hb and HbO(2) concentrations obtained were reproducible in controlled conditions. The instrument is relatively simple and flexible, and offers an inexpensive platform for further studies to obtain normative data for healthy cohorts, and to evaluate disease-specific performance characteristics for cohorts with vasculopathies of bone.

Effect of errors in baseline optical properties on accuracy of transabdominal near-infrared spectroscopy in fetal sheep brain during hypoxic stress

A continuous-wave (cw) near-infrared spectroscopy (NIRS) instrument has been developed to noninvasively quantify fetal cerebral blood oxygen saturation (StO2). A linear Green's function formulism was used to analytically solve the photon diffusion equation and extract the time-varying fetal tissue oxy- and deoxy-hemoglobin concentrations from the NIR measurements. Here we explored the accuracy with which this instrument can be expected to perform over a range of fetal hypoxic states. We investigated the dependence of this accuracy on the accuracy of the reference optical properties chosen based on the literature. The fetal oxygenation of a pregnant ewe model was altered via maternal aortic occlusion. The NIR cw instrument was placed on the maternal abdomen directly above the fetal head, continuously acquiring diffuse optical measurements. Blood was sampled periodically from the fetus to obtain fetal arterial saturation (SaO2) measurements from blood gas analysis. The NIR StO2 values were compared with the fetal SaO2 measurements. Variations in the NIR results due to uncertainty in the reference optical properties were relatively small within the fetal SaO2 range of 30 to 80%. Under hypoxic conditions, however, the variability of the NIR StO2 calculations with changes in the assumed reference properties became more significant.

Transabdominal near infrared oximetry of hypoxic stress in fetal sheep brain in utero

The feasibility of transabdominal near-infrared (NIR) spectroscopy for detecting and quantifying fetal hypoxia in utero is demonstrated in a pregnant ewe model. A frequency domain NIR spectroscopy probe, consisting of two detectors and six sources operating at three wavelengths (675, 786, and 830 nm), was placed on the maternal abdomen directly above the fetal head. Fetal hypoxia was indirectly induced through occlusion of uterine blood flow for approximately 3 min. NIR photon diffusion measurements were made during a baseline period, during hypoxia of the fetus, and during recovery. Fetal blood samples were drawn from the fetal brachial artery and jugular veins at several time points during the cycle. Seven hypoxic cycles were induced in a total of five pregnant ewes. The NIR measurements were analyzed by using a two-layer diffusion model to deconvolve the fetal blood saturation from that of the pregnant ewe. Fetal hypoxia was detected. Good agreement was found between fetal blood saturation determined by the transabdominal NIR method and arterial and venous fetal blood saturation quantified from fetal blood samples by using a hemoximeter.

Cerebral hemoglobin concentration and oxygen saturation measured by intensity modulated optical spectroscopy in the human fetus during labor

AIMS

To quantify hemoglobin concentration and oxygen saturation in the human fetal brain using intensity modulated optical spectroscopy during labor.

METHODS

A specially designed probe was applied to the fetal scalp in 20 women during uncomplicated labor at term. Optical fibres transmitted near infra-red light to and from an intensity modulated optical spectrometer (IMOS), which detected changes in the optical parameters of the infra-red light source. Using novel off-line analytic techniques, these changes were converted into absolute measurements of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb). Summing Hb and HbO2 gives total hemoglobin concentration (HbT) and HbO2/HbT x 100% provided measurements of cerebral saturation (SmcO2).

RESULTS

Of 20 fetuses studied, data from 10 fetuses were suitable for analysis. Over three consecutive uterine contractions, the mean (+/- SD) absolute cerebral concentrations of Hb and HbO2 were 30 +/- 18 and 46 +/- 21 mumol/l, respectively. This gave a mean cerebral HbT of 77 +/- 29 mumol/l and a mean SmcO2 of 59 +/- 12%. In the other ten fetuses insufficient light was detected to allow chromophore quantification.

CONCLUSION

These are the first absolute measurements of cerebral Hb and HbO2 in human fetuses during labor. The values of total hemoglobin are similar to those obtained in neonates with hypoxia-ischemia and the measurements of fetal cerebral oxygen saturation are similar to previously published values.

Intrapartum fetal monitoring: past, present, and future

The concept of intrapartum "monitoring" of the fetal heart rate by auscultation has been extant for almost 200 years and by electronic means for more than 30 years. This article explores historical aspects of fetal monitoring, the advent of electronic fetal monitoring and its controversies, and present and future research opportunities to enhance the reliability, validity, and efficacy of fetal monitoring.

Modeling photon transport in transabdominal fetal oximetry

The possibility of optical oximetry of the blood in the fetal brain measured across the maternal abdomen just prior to birth is under investigation. Such measurements could detect fetal distress prior to birth and aid in the clinical decision regarding Cesarean section. This paper uses a perturbation method to model photon transport through an 8-cm-diam fetal brain located at a constant 2.5 cm below a curved maternal abdominal surface with an air/tissue boundary. In the simulation, a near-infrared light source delivers light to the abdomen and a detector is positioned up to 10 cm from the source along the arc of the abdominal surface. The light transport [W/cm2 fluence rate per W incident power] collected at the 10 cm position is Tm = 2.2 x 10(-6) cm(-2) if the fetal brain has the same optical properties as the mother and Tf = 1.0 x 10(-6) cm(-2) for an optically perturbing fetal brain with typical brain optical properties. The perturbation P=(Tf - Tm)/Tm is -53% due to the fetal brain. The model illustrates the challenge and feasibility of transabdominal oximetry of the fetal brain.