A Physiology- and Measurement-Informed Framework for Evaluating PPG Features in Cuffless Blood Pressure Estimation

Continuous and cuffless blood pressure (BP) estimation using photoplethysmography (PPG) has considerable potential for wearable health monitoring. However, PPG records peripheral optical blood-volume-related changes rather than arterial pressure directly, and its waveform is jointly shaped by systemic hemodynamics, local vascular regulation, sensor--tissue interaction, and measurement conditions. Consequently, high predictive performance does not necessarily establish that a model has learned physiologically valid or mechanism-specific BP information. To address this gap, we propose a physiology- and measurement-informed framework for evaluating PPG-derived descriptors and representations. Rather than treating features as direct measurements of isolated cardiovascular mechanisms, the framework organizes them into four non-exclusive physiological sensitivity domains-cardiac pump-related dynamics, arterial compliance-related dynamics, wave-reflection-related morphology, and peripheral vascular regulation-along with a fifth distinct category for integrative composite representations. Drawing on a qualitative synthesis of 87 studies guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), representative descriptors are examined in terms of their directly observed signal properties, candidate physiological sensitivities, mechanism specificity, susceptibility to confounding, calibration dependence, wearable observability, and the evidential basis and limitations of their proposed interpretations. Importantly, the supporting literature provides uneven and often indirect validation of the assumed relationships between peripheral blood-volume-related signals, arterial pressure or flow, and specific hemodynamic mechanisms. Particular attention is given to practical limitations such as heart-rate dependence, temperature, contact pressure, measurement site, and the loss of morphological landmarks in reflective wearable PPG. A brief illustrative audit of a published feature-guided hybrid model is retained as an example of how the framework can distinguish model use of physiologically motivated descriptors from physiological validation. The proposed framework does not establish or validate PPG as a direct or mechanism-specific measure of BP; rather, it organizes the heterogeneous and incompletely validated physiological and measurement assumptions reported in the literature. It provides a structured basis for identifying conditions under which BP-related information may be observable and for defining stronger validation requirements for cuffless BP estimation systems.