Wearable Sensor Could Detect Postpartum Blood Loss

Postpartum hemorrhage, the leading and most preventable cause of maternal mortality, can be hard to detect, because physiological compensation mechanisms can mask excessive bleeding. In the early stages of hemorrhage, the sympathetic nervous system becomes activated to ensure adequate perfusion of vital organs and, in turn, vital signs remain stable.

Worldwide, the most common methods of diagnosing postpartum hemorrhage are visual estimation of blood loss or monitoring of vital signs. Blood loss is routinely underestimated beyond the point of early intervention.

To diagnose postpartum hemorrhage in its early stages, a multidisciplinary research team at Washington University in St. Louis developed a wearable optical device that is worn on the wrist. The device uses laser speckle flow index (LSFI) to continuously monitor the body’s compensatory mechanisms triggered by blood loss elsewhere in the body.

A wearable, wrist-worn device developed by Washington University in St. Louis uses laser speckle flow imaging to catch early signs of postpartum hemorrhage. Courtesy of Francesca Bonetta-Misteli/Washington University in St. Louis.

“Because of peripheral vasoconstriction, vital signs can remain unchanged until there is very severe blood loss, meaning that in many instances we cannot use vital signs to detect postpartum hemorrhage before it becomes severe,” said research leader professor Christine O’Brien.

The LSFI device uses a laser and a camera to detect and quantify spatial and temporal changes in speckle patterns that are formed when the light interacts with moving blood cells, in order to extract information about blood flow. The LSFI measurements are linearly proportional to blood flow.

The LSFI device is wireless and battery-powered, with a low signal-to-noise ratio. Software controls the laser module and camera sensor, performs real-time video capture and data processing, and transfers data via Bluetooth to external devices such as a tablet, where results are tracked. The team developed a fast speckle processing protocol for onboard video data processing.

“By monitoring blood flow in the wrist, our device can detect peripheral vasoconstriction as it occurs in response to postpartum hemorrhage before the patient starts to show symptoms,” O’Brien said.

To test the device, the researchers placed it on a tissue-mimicking phantom with a hollow channel, through which they flowed a liquid to simulate blood flow. When the researchers changed the rate at which the liquid flowed, they observed a nearly perfect linear response to the flow rate, indicating that the device was highly sensitive to alterations in blood flow.

In a study in an animal model, the response of the LSFI device was found to be almost perfectly linear, with an average correlation of 0.94 between detected blood flow measured and volume of blood removed. Additionally, in most animals, the device measured an increase in blood flow during intravenous injection with saline, indicating that the added volume helped increase blood flow to the periphery. These experiments showed that the wearable LSFI sensor could be used to monitor patients for early signs of postpartum hemorrhage and track their response to resuscitation efforts.

Tests quantifying the effects of skin pigmentation in both phantom measurements and patient studies found no significant difference based on the level of pigmentation. “We are obtaining feedback and conducting many of these tests with our research prototypes, including the consideration of artifacts caused by motion, and testing performance across a range of skin pigmentation levels to ensure the results are not biased by skin color,” O’Brien said.

In the early stages of hemorrhage, low-cost, accessible treatment options such as pharmacologic agents and intrauterine devices can be used. Later stages of hemorrhage require blood transfusion and surgical interventions, which are not always accessible, depending on where a patient lives.

“Postpartum hemorrhage most severely impacts people in low- and middle-income areas, who have limited access to high-quality medical diagnostics and treatments,” O’Brien said. “We were inspired to develop an accessible tool that can be used in both low- and high-resource settings to detect this condition earlier than current methods. This work is the first step in the development and proof-of-concept testing of our initial prototype.”

The team is developing new prototypes that use a smaller, more stable laser with lower power, have a longer battery life, and could provide faster results. In addition to testing the device with healthy volunteers, the team plans to begin a clinical study with pregnant women in the coming months. The researchers also hope to partner with investigators from countries with high mortality rates due to postpartum hemorrhage to test the device’s performance and utility in resource-limited settings.

For the LSFI device to move to clinical use, O’Brien said that it would need to be manufactured by a commercial partner that could create a device that meets regulatory criteria and would require testing under various conditions and with a diverse population. “It will also be critical to obtain feedback from patients, nurses, physicians, and other members of the health care team throughout the development process,” she said.

The wearable LSFI device provides a framework for a low-cost, noninvasive technology to identify ongoing blood loss in time to use accessible interventions effectively.

“Our hope is that this work will lead to a device that will enable prevention of morbidity and mortality caused by postpartum hemorrhage, ensuring new moms make it home safely with their babies,” researcher Francesca Bonetta-Misteli said. “In a medical setting, it could be useful for monitoring how patients are responding to treatment for a hemorrhage. It could also offer a stand-alone tool for use at home or in other settings that alerts mothers giving birth when they may be in danger and should seek medical attention.”

The research was published in Biomedical Optics Express (www.doi.org/10.1364/BOE.494720).