Physiological Analyses of Broadband Near-Infrared Spectroscopy and Electrophysiological Measurements in vivo for Biomedical Applications

Abstract

My thesis concerns analysis of data from optical and electrophysiological measurement tools for calculate potential biomarkers in three biomedical applications: (1) the gastrointestinal immaturity of infants of different ages, (2) prefrontal connectivity in healthy older adults, and (3) hemodynamic-metabolic coherence during vascular occlusion. The first application relied on electrogastrogram (EGG) measurements taken on 81 newborns in a neonatal intensive care unit. Of the 81 babies, 77 were born prematurely. By separating these babies into different gestational age groups and analyzing their EGG data, I was able to find population differences in common EGG analysis parameters. In particular, we found that the mean power ratio between the during- and pre-feeding periods increased as the gestational age of infants increased. Similarly, the full-term (i.e. born at 37 weeks of pregnancy or later) babies consistently showed higher power ratios when comparing the post- and pre-feeding periods. These results imply that power ratio is a robust parameter to use when measuring gastrointestinal maturity in both infants and adults. The second biomedical application regarded prefrontal connectivity in healthy older adults by analyzing infra-slow oscillation (ISO) waves. This area is a pressing topic, as lower rates of prefrontal connectivity have been associated with Alzheimer’s Disease (AD), which affects nearly 10% of older adults. The current study also followed-up on a previous study from our lab, in which the ISO waves of healthy younger adults were analyzed to assess prefrontal connectivity. That study established a range of values for multiple ISO-derived parameters, including spectral amplitude and connectivity. I found that the results from older adults were largely in the same ranges, except for spectral amplitude in the myogenic band and unilateral coupling across all ISO bands. This suggests that older adults may have lower myogenic responses due to the stiffening of the vascular wall with age, and that unilateral coupling of oxygenated hemodynamics and metabolism may actually increase with age. However, the sample size was limited to six subjects, so future measurements are needed to better establish the ranges for ISO-derived parameters in older adults. Lastly, the third biomedical application was to study the robustness of Wavelet Transform Coherence (WTC) in analyzing Near-Infrared Spectroscopy (NIRS) data taken during a vascular occlusion test. Vascular occlusion tests are usually performed in clinical settings to measure blood pressure. However, these tests are also useful for testing tools like NIRS, with which biological signals like oxygenated hemoglobin (HbO) and cytochrome C oxidase (CCO; an enzyme coupled with metabolic rate) can be measured. My lab-mates recorded NIRS data on seven subjects within our lab while conducting a vascular occlusion test. I then analyzed the data using WTC, which is a time-frequency analysis tool that has yet to be applied to HbO and CCO. This resulted in heat maps for each subject, within which we could analyze different the power of different ISO bands during occlusion. This study illustrates how HbO and CCO mediate the supply-and-demand relationship between oxygen and blood when the oxygen supply is starved.