Inter-sensor fractional-order distance derived from in-ear dual photoplethysmography correlates with pulse pressure in supine conditions: a pilot study

Photoplethysmography (PPG) enables wearable, cuffless blood pressure monitoring. However, the relationship between PPG morphology and blood pressure is confounded by factors such as contact state and venous pooling, limiting interpretability and accuracy—particularly for pulse pressure. We developed an in-ear dual-PPG probe with two vertically separated reflective channels (upper and lower) to provide a controlled gravitational contrast within the ear. To quantify inter-site morphological differences while suppressing amplitude and timing variability, we defined a new index—the inter-sensor fractional-order distance (dfo_UL). It is defined as the differential/integral order of a fractional-order filter that best deforms one phase-normalized, ensemble-averaged pulse waveform to match the other. Seventeen healthy adults (25.6 ± 8.3 years) were studied under four conditions—baseline supine rest, right lateral position, passive leg raise, and the cold pressor test—with simultaneous brachial cuff blood pressure measurements used as the reference. Across supine observations (rest, passive leg raise, cold pressor test), dfo_UL showed the strongest association with pulse pressure among examined morphology-based indices (Spearman coefficient ρ = −0.627), whereas the association was weak in the right lateral position. In addition, single-sensor indices exhibited stronger correlations with pulse rate than dfo_UL, suggesting that the differential feature may be less confounded by heart rate. These pilot results indicate that the inter-sensor fractional-order distance captures a posture-dependent component of waveform deformation that relates to pulse pressure under supine conditions, motivating larger studies to clarify physiological mechanisms and to evaluate pulse-pressure-oriented feature engineering for cuffless blood pressure estimation.