Published in Microvascular Research 162, 104864 (2025) — doi:10.1016/j.mvr.2025.104864 — Open Access (CC BY 4.0)
At a Glance
differentiation (95% CI 0.88–0.94)
102 lower limbs evaluated
for ABI prediction
(RMSECV = 0.186)
Background
Peripheral artery disease (PAD) affects over 230 million adults worldwide, yet remains frequently underdiagnosed. The ankle-brachial index (ABI) is the standard first-line test, but has known limitations — particularly in patients with arterial calcification, diabetes, or chronic renal disease, where falsely elevated values can mask compromised flow.
Near-infrared spectroscopy (NIRS) offers a non-invasive, portable alternative that penetrates superficial tissues to assess hemoglobin oxygenation and tissue composition. Despite its established role in cerebral oximetry, its application to PAD remains underexplored. This pilot study investigated whether NIRS spectral signatures can differentiate ischemic from non-ischemic limbs and whether they carry information correlated with ABI — all using a portable spectrometer at rest in a routine outpatient setting.
Key Findings
🔴 Strong Limb Discrimination
Random Forest classification achieved an AUC of 0.91, with the most discriminative wavelengths below 1060 nm and in the 1250–1375 nm range — consistent with oxygenated hemoglobin absorption peaks.
🟢 ABI Spectral Correlation
iPLS identified a narrow band at 1429–1463 nm as the most informative for ABI prediction. This region captures O–H stretching (water), N–H stretching (proteins), and CH₂ combination bands (lipids) — pointing to vascular tissue composition signatures.
📡 Portable & At-Rest Protocol
Unlike previous NIRS–PAD studies requiring exercise testing, we demonstrated feasibility at rest in an outpatient clinic, using a compact InGaAs-based spectrometer (900–1700 nm) with no cooling requirements.
🔗 Complementary to ABI
NIRS may address ABI limitations in patients with non-compressible vessels. The modest R² was expected — the goal was not to replicate ABI but to identify whether NIRS carries independent hemodynamic information.
Spectral Windows
The most discriminative wavelengths for limb classification and ABI prediction fall in distinct regions of the NIR spectrum, reflecting different physiological information:
The 900–1060 nm window aligns with the absorption characteristics of oxygenated hemoglobin (typically peaking between 800–1000 nm), while the 1429–1463 nm region captures molecular signatures from water (O–H), proteins (N–H), and lipids (CH₂) — potentially reflecting vascular tissue composition and perfusion status rather than oxygenation alone.
Methods
NIRS Acquisition
20 scans per site (right thumb, both halluces), 900–1700 nm, 225 data points. Supine, at rest.
Pre-processing
Standard Normal Variate (SNV) transformation for scatter correction and baseline normalisation.
Classification
Random Forest (upper vs. lower limb), 2/3–1/3 split preserving within-patient measurements.
Regression
Interval PLS (iPLS) for wavelength selection and ABI prediction across 21 spectral intervals.
Study Design
This was an observational, cross-sectional study employing a convenience sample of 51 outpatients from the Angiology and Vascular Surgery service at Unidade de Saúde Local de São João, Porto. Patients with PAD (Rutherford 0–3, ABI ≤ 0.90 or abnormal Doppler) were recruited between November 2023 and May 2024. The study received approval from the Ethics Committee (ref. CE-382-24).
Cohort Summary
| Characteristic | Value |
|---|---|
| N | 51 patients (102 lower limbs) |
| Age | 65.5 ± 8.3 years |
| Sex | 86.3% male |
| Rutherford 1–3 | 94.1% |
| Median ABI | 0.65 (IQR 0.4) |
| Smoking | 94.1% (37.2% current) |
| Hypertension | 72.5% |
| Diabetes | 27.5% |
| Coronary artery disease | 23.5% |
| Claudication (limb-level) | 66% |
| No palpable distal pulse | 84.6% |
| Monophasic / occlusion (Doppler) | 64.1% |
NIRS Device
Spectroscopic measurements used a portable NIR reflectance system (NIR-S-G1) with an InGaAs detector covering 900–1700 nm. The device requires no cooling, holds FCC and CE certifications, and allows rapid spectral acquisition — making it suitable for bedside and outpatient use.
Limitations & Future Directions
This pilot study has important limitations: a relatively small convenience sample, a predominantly male cohort limited to Rutherford stages 1–3, and the absence of patients with non-compressible vessels (ABI > 1.3) — a subgroup where NIRS could have particular clinical value. The assumption that upper limbs were non-ischemic was based on clinical evaluation without formal hemodynamic confirmation.
Future work should prioritise larger and more diverse cohorts, matched healthy controls, integration of exercise or provocative protocols (e.g., transient leg elevation), and comparison with complementary modalities such as TcPO₂. Standardised guidelines for NIRS sensor placement and acquisition parameters are also essential for cross-study comparability.
Team
* Corresponding author † Equal contribution
Citation
BibTeX
@article{BragaAmorim2025,
title = {Pilot study on near-infrared spectroscopy in peripheral
artery disease: {Differentiating} upper and lower limbs
and its correlation with the ankle-brachial index},
author = {Braga Amorim, J{\'e}ssica and Dias Neto, Marina and
Magalh{\~a}es, Sandra and Barros, Ant{\'o}nio S.},
journal = {Microvascular Research},
volume = {162},
pages = {104864},
year = {2025},
doi = {10.1016/j.mvr.2025.104864}
}Funding
This research was supported by Fundação para a Ciência e Tecnologia (FCT) under the CardioNIR project — CARDIOvascular Near-InfraRed spectroscopy probing — 2021 (PTDC/EMD-EMD/3822/2021).