(M-482) Wearable short-wave infrared photoplethysmography sensor for real-time detection of hemodilution

Introduction:: Postpartum hemorrhage (PPH), defined as excessive blood loss after delivery of the fetus, is the global leading cause of maternal death, particularly effecting low resource settings and racial minorities in the United States². Current diagnosis methods include visual estimation of blood loss and vital sign monitoring. Visual estimation is prone to underestimation and fails to detect internal bleeding. Compensatory mechanisms keep vital signs like heart rate and blood pressure stable during early hemorrhage, and thus the vital sign monitoring can only reliably identify late stages of PPG when surgical intervention or blood transfusions are required³. A timely diagnosis of PPH is key in providing optimal patient care, as an estimated 90% of PPH-related deaths are preventable with early diagnosis and early treatment. This is especially critical for low-resource settings with limited access to blood transfusion and surgical care. One compensatory response to PPH is hemodilution, in which water from the intravascular space is drawn into the circulation to increase blood volume, resulting in reduced hemoglobin concentration. If hemodilution is severe, it may compromise a patient’s ability to deliver oxygen throughout their body, creating a medical emergency. To combat this issue, a wearable short-wave infrared (SWIR) photoplethysmography (PPG) sensor was developed for real-time detection of hemodilution⁴.


Materials and Methods:: The optical absorption of biological chromophores in the SWIR were first spectrally characterized to determine ideal wavelengths to monitor hemoglobin and water contributions from blood while minimizing absorption of melanin⁵. Based on these spectra, wavelengths for hemoglobin and water absorption were chosen for illumination in the wearable device. A SWIR-sensitive InGaAs photodiode was chosen as the photodetector. The SWIR LEDs fire sequentially to provide two-channel illumination. The photodiode is sampled at frequencies > 60Hz to obtain PPG signals across the cardiac cycle. The signal undergoes analog filtering via third order active high and low pass filters, followed by channel-specific amplification with a programmable gain amplifier, and finally undergoes digital filtering with a Butterworth filter.


Results, Conclusions, and Discussions:: SWIR spectra from biological chromophores showed strong absorption from hemoglobin at 900 nm and strong, but not saturated absorption from water at 1300-1350 nm. These spectra informed the choice of a 900 nm LED for hemoglobin and 1300 nm LED for water measurements. A wearable SWIR PPG sensor has been constructed in both reflectance and transmission modes and PPG signals have been successfully acquired in reflectance and transmission modes. The device has been tested in human volunteers across a range of skin tones, demonstrating the ability to collect in vivo PPG signals in a diverse patient cohort. Next steps include validation of the hemodilution sensing capabilities by flowing sheep blood with hemoglobin concentrations spanning 4-14g/dL through a pulsatile flow phantom with tissue-mimicking optical properties that represent a range of skin tones. The ratio of the hemoglobin and water PPG signals collected from these studies will be used to estimate the hemoglobin concentration via linear regression. After validation, the sensor will be tested in a swine controlled-volume hemorrhage model.

Our wearable hemodilution sensor aims to help prevent complications caused by PPH by providing a low-cost continuous monitoring tool for early diagnosis of PPH and preventing dangerous blood loss. Importantly, the use of SWIR LEDs will minimize effects caused by melanin absorption, preventing racial disparities seen by other light-based diagnostics. Continued development of the device will yield an inexpensive, fully wireless, wearable watch or finger clip that is accessible to patients across the world and particularly in low resource settings where PPH burden is the highest.


Acknowledgements (Optional): :

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