Diagnostic performance of photoplethysmography for early vascular compromise in a customisable in vitro flap model

BACKGROUND: Free flap reconstruction is a standard procedure in surgery, yet vascular thrombosis occurs in 3% to 5% of cases. Early detection within the first few hours is essential for successful salvage, as the success rate of surgical re-exploration decreases over time. Since current clinical assessment remains subjective and depends on experience, objective and continuous monitoring is required. Photoplethysmography (PPG), which monitors blood volume changes non-invasively, is a candidate for this application. However, the influence of specific waveform changes during early hemodynamic shifts is not fully understood. Methods: This study evaluated the diagnostic capacity of PPG for detecting early vascular changes using a custom silicone phantom. Synthetic vessels mimicking human arterial and venous mechanics were embedded at depths of 3, 9, 15, and 21 mm. A perfusion system simulated normal, early ischaemic, and early congested states. Signal quality was assessed using the Signal-to-Noise Ratio (SNR), and only signals with SNR > 15 dB were used for morphological analysis. Over 50 parameters, including Time, Area, and Slope, were extracted from the waveforms to identify those that characteristically respond to each hemodynamic state. Results: The custom-made free flap phantom was successfully validated. Signal quality assessments limited morphological evaluation to depths up to 15 mm. Analysis showed that Intensity and Area-based parameters were the most effective indicators at all depths. At shallow positions, Time-related features showed clear changes during ischaemia, while Slope and Second Derivative (SDPPG) features emerged as key indicators at depth. Red light was useful for superficial monitoring at 3 mm, whereas Infrared (IR) was necessary for assessing deeper states. Conclusion: A custom phantom capable of replicating early hemodynamic compromise was developed. Identifying specific feature variations across depths provides a framework for objective, continuous monitoring. These findings suggest that combining multiple morphological features can improve the reliability of flap assessment.