Date of Award
Master of Science
Harry Whelan, Yongjin Sung
Biophotonics, Cellular Metabolism, Cytochrome C Oxidase, Near-Infrared Spectroscopy, Oximetry
Aims: Although NIRS oximetry has been widely used in clinical and research settings to monitor oxygen consumption in muscles (1), it is not necessarily able to obtain cellular metabolic levels directly. Oxygen saturation will only change with aerobic metabolism (2); however, anaerobic metabolism cannot be assessed by monitoring oxygen saturation. This work aims to measure cell oxygenation and blood oxygenation, simultaneously, by applying oximetry to hemoglobin/deoxyhemoglobin and to cytochrome c oxidase (CCOX). A custom build NIRS device called the cytoximeter was constructed to achieve this goal. This work could have important impacts on monitoring neurological diseases, Postural Tachycardia Syndrome (POTS), and epilepsy.
Introduction: CCOX is the terminal enzyme in the electron transport chain (ETC) and as such will be responsive in changes in either aerobic or anaerobic metabolism (3). CCOX transports a number of electrons over a single cycle of the ETC. As the enzyme accepts electrons, it enters a reduced state. Monitoring the redox state of CCOX in a given tissue will reflect the amount of metabolic activity in that tissue (9). We developed a novel device that monitors the changes in optical densities of a tissue. Using Beer’s law and the difference in absorption spectra of reduced and oxidized CCOX, it is possible to measure the relative changes in concentration of these chemicals (4).
Methods: In order to observe and separate the CCOX signals from absorption changes due to whole blood changes, a six wavelength absorption spectroscopy device is constructed. This device uses the optimal source detector separations, obtained by numerical photon migration simulations, for monitoring superficial muscles and cortical brain tissue. In order to validate the device performance, several phantom studies are conducted followed by clinical investigations. In the clinical setting, the device was applied to the gastrocnemius muscles of patients undergoing a tilt table test during a standard of care neurological examination.
Results: The phantom studies showed that the device was able to obtain changes in the concentration and the redox state of CCOX in a medium with optical properties similar to the tissues found in the calf and skull. When applied in a clinical setting, the device produced reproducible and predictable results. The clinical results are partially verified by the use of a commercially available oximeter, which validates the changes in hemoglobin and oxy hemoglobin obtained by our custom-made cytoximeter.
Conclusion: This work is a novel approach to the non-invasive monitoring of CCOX simultaneously with blood oxygen saturation by use of NIR spectroscopy. While there is currently no gold standard with which to compare these results to, the ability to separate cellular metabolism in the presence of large changes in blood volume during a clinical procedure is a promising first step toward clinically monitoring the energy expenditure of tissues. Further work is underway to correlate changes in CCOX redox state to the level muscular exertion. This will allow the researchers to quantitatively monitor CCOX redox changes during different levels of exercise.
Sugar, Jeffrey Jay, "Monitoring Cellular Metabolism with Near-infrared Spectroscopy" (2016). Theses and Dissertations. 1209.