Lasers: The Next Generation of Fetal Monitoring

Researchers have developed a novel monitoring system using optoacoustic technology to provide accurate, real-time measurement of cerebral venous blood oxygen saturation in fetuses during late-stage labor. This pre-clinical patient monitoring system from Noninvasix, Inc., Galveston, TX, enables obstetricians to promptly recognize if a fetus is in distress and take immediate, corrective action. With its enhanced technology, the company aims to decrease the rate of cesarean sections and neurodevelopment issues associated with brain hypoxia.

Fig. 1 – Compact for portability, the third-generation prototype features a graphical user interface for real-time display of present numerical oxygen concentration.

Designed to directly access a fetus’ anterior or posterior fontanel (incomplete closure of skull bone structure) during labor, the monitoring system uses advanced light and sound technology as an adjunct to inefficient electric fetal heart rate sensors currently used in all US hospitals. Although fetal heart rate monitoring, an ineffective and antiquated technology, provides an indirect indicator of fetal oxygenation, to date, there are no commercially available technologies that directly monitor the adequacy of fetal cerebral blood flow.

A tiny probe, about the size of a stethoscope ear tip, is inserted transvaginally and placed atop one of the baby’s easily accessible soft spots. Near infrared light pulses are sent at 1,000 times per second into the brain’s superior sagittal sinus, the vein that drains the brain. Hemoglobin in the blood absorbs the light at different frequencies depending on whether or not it is carrying oxygen. Absorption causes rapid thermal expansion of the hemoglobin resulting in a measureable acoustic wave. The software analyzes the acoustic signal and returns an absolute measurement of oxygen saturation. Direct measurement is taken without any influencing variables, unlike problems encountered with pulse oximetry or near-infrared spectroscopy, such as degradation of accuracy with low blood pressure.

Demonstrated in in vitro, animal, and clinical tests, the optoacoustic prototype measures cerebral oxygenation (SO2) in individual brain blood vessels accurately and precisely (correlation: r2 = 0.99; bias = 2.47%; SD = ±2.3%) in comparison to invasive hemoximetry, the gold standard for those measurements.

Integrated Technologies

Based on generation of acoustic waves by pulsed laser light and detection of these waves by sensitive acoustic transducers, the optoacoustic system requires several integrated technologies including: a multi-wavelength, nanosecond pulsed laser to generate optoacoustic signals; a transvaginal probe consisting of both an optical fiber for laser light supply and a piezoelectric detector for optoacoustic signal detection; and a user interface running custom software for real-time control and processing of the signals.

The primary engineering challenge to developing a multiwavelength laser light to generate an optoacoustic signal is developing a nanosecond laser that has outputs with peak power of several thousand watts over a significantly short nanosecond pulse duration while, at the same time, controlling the light wavelength to within a few nanometers. The earliest prototype utilized a large optical parametric oscillator (OPO) laser that required a water-cooling system mounted in a custom cart about the size of a washing machine. The third-generation prototype has and electronic console with a 13-inch by 13-inch footprint and is fitted with laser diode arrays that have thermoelectric cooling elements. The current iteration is lightweight and compact for portability, and moreover, provides improved signal measurement performance. (See Figure 1)

Extending from the electronic console is a two-meter-long cable bundle with an electrical cable and an optical fiber. A onemillimeter diameter fiber is needed to transmit high-energy pulses while reducing background noise. The current prototype features a glass optical fiber optic connected to a flexible plastic fiber approximately 45 millimeters from the fetal probe. The flexibility of the plastic fiber allows the physician to insert the fetal probe transvaginally and position it over the fontanel to locate the sagittal sinus vein for sensing the venous oxygenation.

Fig. 2 – The probe houses the optical fiber, which passes through the acoustic sensor to make solid contact with the skin.

The probe, designed with molded medical-grade plastic, houses the optical fiber, which passes through the acoustic sensor to make solid contact with the skin. To account for fetuses with an abundance of hair, the optical fiber protrudes slightly past the bottom of the probe to get through the hair and deliver light pulses to the superior sagittal sinus. (See Figure 2)

Also enclosed in the plastic housing is the acoustic sensor, which is spaced a few millimeters from the skin surface. This piezoelectric sensor is connected to an electronic preamplifier in the sensor housing to improve signal-to-noise. The sensor and amplifier require careful electronic shielding to protect the circuit from electromagnetic interference.

Fig. 3 – UTMB obstetricians collaborated on the probe design for use with by feel alone and with just two fingers.

Obtaining feedback from obstetricians at University of Texas Medical Branch (UTMB) was paramount in designing a probe for accurate placement and use by feel alone and with just two fingers. A small indention on the top of the probe gives the obstetrician freedom to manipulate the probe with one finger, while locating the fetus’ most accessible fontanel with the other. (See Figure 3)

Development of customized signal-processing hardware and software was the primary focus of the innovation. The signalprocessors measure the low-level acoustic signal and average it from hundreds of repetitive cycles, extract the waveform out of the background noise, and analyze the waveform to compute the oxygen concentration. Engineers successfully configured the system to make accurate measurements while disregarding motion artifacts.

Coordination of Custom Design

For six years, Noninvasix collaborated with Cooper Consulting Service, Houston, TX, to integrate the laser module with the acoustic signal processing system. A LabVIEW™ software program running under Microsoft Windows on a single board computer and with an LCD touchscreen display was used to bring these technologies together. The graphical user interface was designed to meet clinician needs for real-time display of present numerical oxygen concentration and for strip charts to provide historical reference to track patient condition.

The graphical user interface features were considered early in the development process and were reviewed with clinicians for feedback during the development cycle. Additionally, a number of safety features were incorporated to ensure compliance with FDA regulations and with laser standards. These include custom-built software to control the laser module, monitoring the stability of the measured acoustic signal, separate circuitry to monitor the laser’s temperature for ensuring wavelength stability, a separate light pulse energy monitor, and lock-out features to prevent laser output until all connections are secure and the sensor is properly mounted on the patient.

Presently, the prototype optoacoustic system has been safely tested in adult humans, fetuses, and neonates. Clinical tests show the optoacoustic system provides accurate measurement of oxygen saturation specifically in the superior sagittal sinus. The high (optical) contrast and high (acoustic) resolution of the optoacoustic system permits direct probing of blood vessels.

While the initial commercial application for optoacoustic technology is monitoring fetal well-being, Noninvasix has also demonstrated that the system can noninvasively detect cerebral ischemia in premature newborn infants and monitor brain ischemia in traumatic brain injury. In these applications, clinicians would fit patients with a head strap to hold the sensors.

This article was written by Tommy Cooper, owner and founder, Cooper Consulting Service, Houston, TX. For more information on Cooper Consulting Service, visit http://info.hotims.com/61057-163 . For more information on Noninvasix, Galveston, TX, visit http://info.hotims.com/61057-164 .