Mitigating Hypovolemia-Induced Miscalibration of Photoplethysmogram-Derived Blood Pressure

Pulse transit time (PTT) is a hemodynamic indicator that may be obtained non-invasively using photoplethysmogram (PPG) signals for continuous blood pressure (BP) monitoring. Among the most promising applications of this technology are military and civilian trauma cases, where reduced blood volume due to hemorrhage, or absolute hypovolemia, is the leading preventable cause of death. However, the drawback of this method is that it requires calibration for each patient; additionally, changes in physiological state may affect PTT calibration. In this work, a porcine model (n = 6) was used to demonstrate that changes in blood volume lead to miscalibration of PTT for BP estimation. To mitigate hypovolemia-induced miscalibration, this work first defines a template-based signal quality index (SQI) for characterizing the morphology of PPG signals; it is then shown that the subject-specific calibration of SQI to BP is more robust to changes in blood volume than PTT. Though changes in PPG signal quality are not necessarily specific to changes in BP, these results suggest that PPG-based monitoring systems may benefit from incorporating morphological information for cuffless BP estimation in trauma settings.