Introduction: Chronic “purple foot”, a condition known as acrocyanosis, is characterized by compromised peripheral microcirculation. Though purple foot sign is associated with the classic dependent rubor of arterial insufficiency, a purplish foot can exist outside of macro-arterial dysfunction. Purple foot can also be a sign of severe venous congestion potentially confusing clinical assessment and conflating standard assessments, which, while valuable, may lack the precision needed to meaningfully map localized tissue oxygenation. This study aimed to evaluate the use of the MIMOSA Imager in assessing the microvascular response in patients presenting with purple foot.
Methods: A prospective, observational study was conducted involving clinically observed “purple foot”. The MIMOSA Imager, a non-contact, cordless, battery-powered device, was used to capture spatially-resolved StO2 images. The device utilizes both visible and near-infrared LED-illuminated wavelengths, tracking spectral signatures of dominant chromophores in the superficial tissue to calculate and map StO2 values to a heatmap displayed on an Android interface. Baseline measurements were taken on the affected foot in a dependent position to assess the microvascular response to gravitational position.
Results: Preliminary data analysis of a single convenience sample cohort demonstrates distinct and heterogeneous StO2 patterns in affected limbs. In dependent positions, areas of "purple foot" show patchy StO2 values, varied from adjacent tissue. The StO2 heatmaps visually and quantitatively differentiated areas of severe microvascular compromise (i.e., very low StO2) from relatively better-perfused areas. The device provided a clear, objective, and non-invasive measure of the tissue's inability to maintain oxygenation stability under positional stress, which directly correlated with the clinical presentation of this condition representing a combination of rubor and cyanosis.
Discussion: The MIMOSA Imager effectively provided a non-invasive, objective estimation of the spatial distribution of StO2, offering a deeper insight into the microvascular dysfunction underlying the "purple foot" presentation of acrocyanosis. The ability of the NIRS device to map heterogeneous tissue oxygenation is critical here, as the pathophysiology leading to low StO2 differs (supply failure vs. stagnant outflow) and substantially influences both treatment and intervention courses. The spatially-resolved StO2 maps provide information beyond global perfusion markers. This technology demonstrated the compromised microcirculation characterizing this condition, potentially aiding in earlier identification of severe tissue hypoxia and guiding decisions regarding revascularization or wound care strategies.
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