Exploring the Effect of LED-to-Photodetector Spacing on Subcutaneous Photoplethysmography for Continuous Blood Pressure Measurement

Photoplethysmography (PPG) is widely used to measure heart rate, blood oxygenation, and more recently, blood pressure. Implanted PPG systems offer the possibility to measure similar real-time measures of cardiovascular health, however, the detection method may vary due to a lack of capillary vessels for PPG sensors to observe in muscle tissue. To improve volumetric blood detection in large muscles, without relying on the capillary density of skin, a flexible PPG sensor was developed. The sensor included multiple spacing of illuminating infrared (IR) light emitting diodes (LEDs) and a single IR photodetector. This arrangement was expected to enable detection of blood volume changes at variable distances from the sensor face, potentially at much longer depths then feasible with skin-mounted PPG devices. IR bench phantoms simulating a large blood vessel embedded in IR absorbing, tissue-mimicking rubber were developed and used to determine the sensor performance in vitro. A preliminary in vivo test used an adult rabbit to provide additional performance validation. Test results reveal an observed trend of increased SNR for deeper vessel depths for the farthest LED to detector spacing which is aligned with our initial prediction. However, ANOVA and post-hoc tests reveal that these trends did not reach statistical significance. The in vivo test showed a relationship consistent with relevant literature. Future experiments are required to improve the phantoms representation of the biological setting and to confirm a reduced SNR variation for the farthest spacing. Clinical RelevanceDevelopment of a subcutaneous, continuous blood pressure sensor may provide benefits towards monitoring autonomic dysreflexia and hypertension for people affected by spinal cord injury.