PAD Screening from PPG Waveform Analysis
PPG enables non-invasive peripheral artery disease screening through waveform morphology analysis at ankle and toe sites, where arterial stenosis produces characteristic monophasic waveforms, reduced pulse amplitude, absent dicrotic notch, and prolonged systolic upstroke time correlating with hemodynamic significance.
Normal arterial PPG waveforms are triphasic (systolic forward flow, early diastolic reversal, late diastolic forward flow), reflecting the pulsatile pressure wave and its reflections. Progressive arterial stenosis dampens and delays the waveform: moderate PAD produces biphasic waveforms (lost diastolic reversal), and severe PAD produces monophasic rounded waveforms with prolonged upstroke time >200 ms and absent dicrotic notch.
The Waveform Amplitude Ratio (WAR = ankle PPG amplitude / finger PPG amplitude) correlates with ABI (r = 0.82–0.91) and provides a quantitative PAD severity index. WAR < 0.5 identifies hemodynamically significant PAD with 89% sensitivity and 93% specificity against catheter angiography. This is particularly valuable in diabetic patients with calcified, non-compressible vessels where ABI is falsely elevated (>1.4), affecting 20–30% of diabetic PAD patients.
Machine learning models trained on simultaneous PPG morphology and Doppler ultrasound reference data classify PAD severity (normal, mild, moderate, severe) with 82–88% accuracy. Features include upstroke time, downstroke time, pulse area ratios, dicrotic notch depth, and spectral harmonic content. Smartphone-based PPG using the camera flash as light source shows promise for accessible PAD screening in resource-limited settings, though validation data remains limited.
Frequently Asked Questions
Can PPG replace ankle-brachial index for PAD screening?
PPG waveform analysis complements but does not replace ABI. PPG is superior when ABI is unreliable (calcified vessels in diabetes). Combined ABI + PPG assessment improves screening sensitivity by 15–20% over either alone.
What PPG measurement sites are used for PAD assessment?
Standard sites include great toe, dorsum of foot, posterior tibial (ankle), and finger (reference). Comparing proximal to distal waveforms identifies the level of arterial obstruction. Toe PPG is most sensitive for distal disease.
Can wearable wrist PPG detect PAD?
Wrist PPG cannot directly assess lower extremity arterial disease. However, wrist PPG-derived arterial stiffness indices (augmentation index, stiffness index) correlate with systemic atherosclerotic burden and serve as PAD risk markers.