The combination of SFDI with a mathematical model links perturbation in microcirculation to early stages of sepsis

Abstract: The microcirculation system is crucial for the function of delivering biological markers such as oxygen and removing carbon dioxide from all the cells forming the complex ma- trix of tissue in the body. To keep up with the demands of each and every cell, there is a response from the microvasculature - resulting from for instance changes in blood flow to the tissue area. Infections cause disturbances in this important system, which increases the risk of development into one of the world’s most common syndrome - sepsis. This con- dition can be explained as a biological response affecting each and every vital organ, and can as a result of the dysfunction be life threatening. Studies have shown when monitoring pulse and respiratory rate the response is not visually quick enough to be able to determine the gravity in the state of the patient. The primarily chosen biological markers were oxy- genated hemoglobin and deoxygenated hemoglobin present in blood, respectively melanin in the skin. This was performed using the optical instrument Spatial Frequency Domain Imaging in combination with a Tissue Viability Imager respectively an Enhanced Perfusion and Oxygen Saturation-equipment. The formulated aim for this thesis was separated into an optical part and a mathematical modeling part. Regarding the optical section the aim was to understand if there were any optical methods more preferable to detect changes in the microcirculation, whilst the modeling section aimed to understand how to construct the best adjusted model for the changes in the biological markers and how these could be related to sepsis. Spatial Frequency Domain Imaging is an optical technique able to generate two- dimensional maps of the absorption coefficient and the reduced scattering coefficient of a biological tissue surface. The skin of healthy subjects were illuminated with RGB-LEDs to detect the chromophores of interest. The data obtained from the experimental sessions was then collected to work as a base for building a mathematical model. The experimental session was performed with a total of six healthy subjects and the data was collected dur- ing a control-measurement and a simulated sepsis-measurement using a pressure chamber and applying negative pressure to the lower part of the body. The mathematical model was based on theory regarding the biological events of sepsis in the microcirculation and was described by ordinary differential equations. The results were presented in graphs and the resulting model likewise, with an addi- tional figure to describe the source of associated equations written to describe the events. An observation of a distinct difference in the deoxygenated, respectively oxygenated hemoglobin could be observed and did show in general more changes during the measure- ments using a lower body negative pressure chamber. The chosen optical approach was the Spatial Frequency Domain Imaging equipment along with the mathematical model named as the Macro-Micro model due to its more realistic design. Future improvements were dis- cussed and summarized as a repetition of the experimental sessions and including more parameters and relationships between the biological markers and the model. This would contribute to more robust results.