Our pilot studies suggest that time-varying changes in the fundamental fraction, comprising the lower harmonics, of the somatosensory evoked potential (EO) may be a sensitive early indicator of cerebral hypoxia. Recently we have also acquired experimental evidence that near-infrared (NIR) spectroscopy appears to estimate noninvasively the cerebral oxygen delivery. We now have the unique ability to acquire and analyze, simultaneously and continuously, the time-varying changes in the NIR spectra and the neuroelectric signals (EP and EEG) that should allow us to assess oxygen deprivation and its effect on electrical function under the conditions of acute hypoxic or ischemic injury to the brain.
The aim of the current project is to answer the questions: Is time-frequency mapping (time-varying changes in the harmonics or the spectrum) of neuroelectric signals (EP and EEG) a sensitive approach to detecting acute oxygen deprivation of the brain? Is the fundamental fraction of the neuroelectric signals a specific indicator of cerebral ischemic injury? Do the spectral changes in the neuroelectric signals correlate with oxygen delivery to the brain estimated by NIR spectroscopy? What components (oxy- and deoxy-hemoglobin, cytochrome a,a3) of the NIR spectra are sensitive to hypoxic and ischemic injuries and the associated neuroelectric response? To answer these questions, we propose the following research plan: Develop signal processing methods to obtain time-frequency distributions of EP and EEG signals by adaptive Fourier series modeling and coherence estimation techniques. Conduct experiments to determine: (i) confounding effects on the neuroelectric signals, if any, of various anesthetics, and (ii) the transient response to acute cerebral ischemia generated by temporary occlusion of cerebral arteries. Analyze, and correlate with time-frequency distributions of neuroelectric signals, the time-varying changes in the NIR spectra under conditions of altered oxygen delivery to the brain. Conduct experiments: (i) to monitor cerebral hemoglobin saturation under the conditions of global cerebral hypoxia, and (ii) to selectively evaluate the NIR response in the cytochrome band. In high risk surgeries, as well as in neurological intensive care situations, the brain may experience acute injury due to hypoxia or ischemia. Our research should help establish the noninvasive, continuous, and rapid techniques of neuroelectric signal processing and NIR spectroscopy as tools for monitoring injury to the brain at times of initial dysfunction when the insult may be reversible.
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