Electrophysiological Brain Monitoring after Cardiac Arrest with Temperature Management

Abstract

Cardiac arrest (CA) is the leading cause of disability and death annually in the United States. Therapeutic hypothermia (TH) has been recommended as one of the standard practices for improving neurological outcome and survival to treat out-of-hospital CA patients after resuscitation. However, many clinical prognostic markers after resuscitation for predicting outcome have been less reliable under hypothermia. Therefore, there is a strong need to evaluate the prognostic value of current prognostic markers for hypoxic-ischemic brain injury after CA. The first part of this work was to review current literature and assess the prognostic value of current significant breakthroughs in neurophysiologic and electrophysiological methods for CA patients treated with TH in order to provide a comprehensive frame for future work. Due to the restrictions of standard clinical examinations and neuroimaging techniques in detecting brain injury, electroencephalography (EEG) has emerged as one of the commonly used bed-side real-time monitoring tools for prognostication. Instead of the subjective and impractical analysis of waveform-based raw EEG signals, we applied two quantitative methods – information quantity (IQ) and sub-band IQ (SIQ) – to quantify and examine the accuracy of prognostic value of EEG markers on predicting recovery under TH in the second part of this work. Our study discovered that both IQ and SIQ accurately predict neurologic outcome at the early stage of cerebral recovery. Moreover, high-frequency oscillations (HFO) were particularly noticeable during the recovery from severe brain injury indicated by IQ, and SIQ was able to provide additional standard clinical EEG bands of interests. The ischemic brain after CA is sensitive to trivial fluctuations of temperature. Previously, we only observed temperature management strongly affects the recovery of global EEG. However, EEG signals can be decomposed into different frequency sub-bands in clinical practices, which are related to different brain functions, and the association has not been elucidated between the recovery of each sub-band EEG and temperature management. In the third part of this work, we employed SIQ, of which indicative ability has been proven in the last part, to determine the most relevant sub-bands of EEG during brain recovery with temperature manipulation. It was found for the first time that gamma-band EEG activity, linked with high cognitive processes, was primarily affected by temperature and strongly associated with neurologic outcome, while delta-band played a role as constant component of EEG without stable relationship with temperature or outcome. Somatosensory evoked potentials (SSEPs), especially N20 responses in human, are able to evaluate the somatosensory system functioning, which are also regarded as a reliable early prognostic marker for post-CA neurologic outcome. Transcranial direct current stimulation (tDCS) is a non-invasive technique to modulate the cerebral excitability and activity which has been confirmed by motor evoked potentials (MEPs), but it is still unclear whether it can affect the somatosensory cortex. The final part of this work preliminarily studied the alternations of excitability of somatosensory cortex by tDCS and investigated the potential of SSEPs on measuring the after-effect of tDCS