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, HR, and mean arterial pressure. 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.