Description

This study aims to investigate healthy cortical and subcortical neural processes during induction of general anesthesia with propofol, which is clinically relevant for postoperative delirium, a common cognitive disorder, after surgical intervention in the elderly.

Intrinsic neural oscillations within the alpha frequency band (~8-13Hz) can be measured with the EEG, showing the highest power (i.e., amplitude) in occipital electrodes during eyes-closed wakefulness resting state. Under general anesthesia, especially with propofol, the power of these oscillations decreases in the occipital cortex but increases in the frontal cortex. Although neither the exact mechanisms underlying the generation of alpha oscillations nor their dynamics under anesthesia are entirely understood, it has been suggested that the thalamus might be a key player modulating the shift of alpha-band power throughout the brain.

Post-operative delirium (POD) is a complication after a surgical intervention characterized by an acute impairment of consciousness, attention, and arousal with a fluctuating evolution. This is prevalent mainly in elderly patients, especially in those with pre-existing neurocognitive disorders, neurodegenerative disease, and those undergoing complex or emergency procedures. Despite the functional and economic burden this disorder places on the patient and the health system, e.g., it increases hospital stay and risk of mortality, treatment options and risk management strategies are still limited. Several of our previous studies and those from other groups have highlighted the link between alpha oscillations and clinical outcomes related to POD. For instance, low frontal alpha power - both during maintenance and emergence from general anesthesia - is associated with a higher risk of POD. Low frontal alpha power is also associated with pre-operative neurocognitive impairment, a well-described risk factor for POD. The biochemical nature of this association is still unknown; the role of the cholinergic system as a mediator has been suggested.

Therefore, a better understanding of the mechanisms underlying the generation of alpha oscillations and their dynamics under general anesthesia could open new doors to diagnostic and treatment options for POD.

Based on our past EEG-fMRI experiments in healthy subjects applying visual stimulation at the alpha frequency, the investigators have shown that (i) visual stimulation using a rhythmic flickering light at a specific frequency evokes a reliable response in the occipital brain, which can be measured with EEG and functional resonance magnetic imaging (fMRI), (ii) the response to this stimulation can be evaluated via evoked potential/power/coherence analyses (EEG) or functional connectivity analyses (fMRI), and (iii) visual flicker stimulation at/near to a subject's intrinsic alpha frequency, known as the 'individual alpha frequency' (IAF), generates a response within brain areas beyond the occipital cortex, such as frontal and parietal regions and most importantly, the thalamus, suggesting an interaction with - and a method to assess - intrinsic alpha oscillations.

The investigators propose a simultaneous EEG-fMRI study in which young, healthy participants, anesthetized with propofol, are presented with a visual flicker stimulation paradigm at/around the participant's IAF. Our experimental design includes recordings before the participant is anesthetized (wakefulness pre-anesthesia) and during three different anesthesia concentrations (low, mid, and deep). Functional magnetic resonance spectroscopy will be acquired during resting state throughout all states. A wakefulness post-anesthesia recording in resting state without stimulation is also planned. This approach has several advantages. For instance, the simultaneous acquisition makes it possible to correlate the dynamics of alpha oscillations measured by EEG while having access to a whole-brain resolution via fMRI, including subcortical areas like the thalamus. This is relevant to understanding the interaction between cortical and subcortical neural processes generating alpha oscillations.

Furthermore, it exploits the fact that our modality of stimulation at the IAF enhances intrinsic alpha processes, which can potentially become a treatment to reduce the risk of POD under anesthesia. Furthermore, by acquiring functional spectroscopy data, the investigators can detect biochemical changes in the brain during each state. Finally, our experimental design enables, first, a chronologic follow-up of alpha dynamics during the induction of propofol anesthesia, and second, by acquiring data after the intervention, investigators will have an immediate control to contrast before and after anesthesia.

For our participant's safety, propofol anesthesia will be titrated until deep concentrations without eliciting a burst suppression state, avoiding intubation and artificial respiration support.

This study represents an essential step towards understanding alpha oscillatory processes in the awake and anesthetized brain relevant to the future development of potential preventative/treatment options for POD.