A toe-flexion NIRS-assisted test for rapid assessment of foot perfusion in peripheral arterial disease: feasibility, validity and diagnostic accuracy

Objectives: We assessed feasibility, validity and diagnostic accuracy of a non-invasive dynamic ambulatory test with Near-Infrared Spectroscopy (NIRS) evaluating foot perfusion in Peripheral Arterial Disease (PAD). Design: Prospective observational study. Materials and Methods: Eighty PAD patients (63 males, 71±9 y) including 41 patients with coexisting diabetes), participated. Thirteen healthy subjects (8 males, 26±8 y) were also studied by Echo-Color-Doppler providing 160 diseased and 26 non-diseased limbs. Under clinostatic conditions, participants performed a 10-repetition toe-flexion test with NIRS probes on the dorsum of each foot; the area-under-curve of the oxygenated hemoglobin trace (‘toflex-area’) was calculated and the ankle brachial index (ABI) was measured. Time of execution, rate of wrong tests and adverse reactions were recorded. Within-session reliability was assessed by administering the test twice after five-minute interval. Validity was assessed determining whether the toflex-area was: i) dependent on the oxygen delivery from the lower limb arteries simulating PAD conditions by a progressive blood flow restriction (40‒120% of systolic pressure) in healthy subjects; ii) consistent with the degree of PAD ranked by ABI and correlated with ABI and ankle pressures values in PAD patients. The diagnostic accuracy in detecting PAD was compared to Echo-Color-Doppler examination. Results: All tests were rapidly, satisfactorily (<1% mistakes) and safely performed. Toflex-area values, superimposable in the two sessions (ICC: 0.92), were: comparable to PAD values following blood flow restriction, consistent with PAD severity, correlated to dorsal pedis artery pressure (r=0.21; p=0.007) and ABI (r=0.65; p<0.001) in PAD, but not in the presence of diabetes. Toflex-area was comparable to Echo-Color-Doppler for detection of PAD following receiver-operating characteristic curve analysis (area=0.987, p