Assessment of Skeletal Muscle Oxygenation during Exercise in Patients with Peripheral Artery Disease Using Near Infrared Spectroscopy: Impact of Skin Color and Adipose Tissue Thickness

Abstract

IntroductionNear infrared spectroscopy (NIRS) is a potentially valuable tool to examine disease severity and the effects of therapeutic interventions on microvascular leg oxygenation in patients with peripheral artery disease (PAD). However, multiple factors can confound the interpretation of NIRS signals and therefore limit its clinical applicability, including adipose tissue thickness (ATT) and skin melanin content. We tested the hypothesis that the changes in the concentration of oxygenated [oxy‐Hb+Mb] and deoxygenated [deoxy‐Hb+Mb] hemoglobin + myoglobin would be reduced in the calf of black PAD patients during exercise when compared to white patients. Similarly, we hypothesized that patients with a thicker calf ATT would exhibit a lower change in oxy[Hb+Mb] and deoxy[Hb+Mb] during exercise relative to patients with low ATT.MethodsTwenty‐four participants with a history of exertional calf pain (18 males, 6 females, 67±7 years; ankle‐brachial index: 0.6±0.1) were asked to complete two graded tests on the treadmill. The protocol consisted of a constant walking speed of 2 mph and progressive increases in grade of 2% every 2 min. The NIRS device was secured to the skin over the medial gastrocnemius muscle of the most symptomatic leg. Oxy[Hb+Mb] and deoxy[Hb+Mb] were measured continuously at rest and during exercise. The final 20‐s averages of stages 1, 2 and peak exercise were utilized for analysis. Magnetic resonance imaging of the calf was used to measure ATT. The average ATT among all individuals (7±3 mm) was used to divide participants into ‘low’ (n=12) or ‘high’ (n=12) ATT groups. A three‐way repeated measures analysis of variance was used to evaluate the effects of skin color, ATT, and stage of the treadmill test (i.e. stage 1, stage 2, peak walking time) on the changes from baseline in calf oxy[Hb+Mb] and deoxy[Hb+Mb].ResultsSeven participants were black and the remaining 17 were white. The average ATT at the medial gastrocnemius site was 4±1 mm in the low ATT group and 9±2 mm in the high ATT group. A significant interaction was observed between skin color, ATT, and exercise stage for the change from baseline in oxy[Hb+Mb] (p=0.049). Post‐hoc testing revealed that among white participants, the change in oxy[Hb+Mb] during exercise was significantly lower in the high ATT group when compared to the low ATT group (p=0.039). Among black participants, the change in calf oxy[Hb+Mb] was similar between high and low ATT (p=0.57). There was a trend for a main effect of ATT on the changes in deoxy[Hb+Mb] during exercise (p=0.1), but the main effects of skin color (p=0.2) and the interaction between skin color, ATT, and exercise stage (p=0.5) were not significant.ConclusionsIn conclusion, the changes in NIRS‐derived calf oxy[Hb+Mb] during exercise are impacted by both skin color and ATT in patients with symptomatic PAD.