Photoplethysmography imaging to assess facial perfusion under simulated hypovolemia.

Objective: This study evaluates the potential of photoplethysmography imaging (PPGI) with automated facial tracking for detecting hemodynamic and autonomic changes induced by lower-body negative pressure (LBNP). The goal is to assess whether PPGI-derived facial perfusion variations are related with stroke volume (SV), systemic vascular resistance (SVR), heart rate variability (HRV), and autonomic responses to progressive hypovolemia. Approach: Twenty-four healthy adults (8 females, 16 males; aged 28.7 ± 3.5 years) underwent a seven-stage LBNP protocol (-15 to -60 mmHg, recovery). Facial perfusion was recorded using cross-polarized PPGI, along with SV, SVR, heart rate (HR), and mean arterial pressure (MAP). Facial landmark tracking (MediaPipe) was used to extract region-specific PPGI signals. Wavelet synchrosqueezing transform enabled spectral analysis, and HRV was assessed with NeuroKit2. Main Results: At -60 mmHg, the LBNP-intolerant group showed a 25.2% decrease in SV (p < 0.0001) and a 19% increase in SVR (p = 0.041). At -30 mmHg recovery, SV remained reduced by 21% (p < 0.001), with SVR elevated by 30.1% (p = 0.002). In contrast, the tolerant group exhibited SV increases of 12% and 18% at these stages (both p < 0.0001), and a HR reduction of up to 5% (p < 0.05), with a decreasing SVR trend. HRV analysis indicated greater sympathetic activation in the intolerant group, with reduced HF power (p = 0.037) and increased LF/HF ratio (3.5 at -60 mmHg, p = 0.020). First harmonic PPGI amplitudes significantly declined in the intolerant group, most notably in the cheeks (-44.2%, p = 0.005). Significance: These findings suggests that PPGI, combined with AI-based face tracking and wavelet analysis, enables non-invasive, spatially resolved monitoring of vascular and autonomic responses. PPGI differentiates tolerant and intolerant groups, supporting its potential for real-time cardiovascular assessment in critical care and emergency settings.