The electroencephalogram and evoked responses
In anesthesia, we expend much more effort in monitoring the cardiovascular and respiratory system than the nervous system, even though anesthesia is all about putting nervous function out of commission long enough to abolish awareness or at least the perception of pain. The reason for our bias against monitoring nervous activities is that we can afford to overdose the nervous functions and put them completely to rest as long as we continue to satisfy the basal needs for substrate and oxygen to brain and nerves. Hence, we worry more about the circulation than about the brain. However, when the systemic circulation is doing well but blood supply to all or parts of the brain or spinal cord is threatened, we need to monitor their function. Two methods are available: the electroencephalogram and evoked responses.
The EEG as recorded by experts requires a montage with many electrodes. In anesthesia, that is not practical and in the operating room, you will rarely see more than a couple of leads plus a ground. The typical EEG of an awake individ-ual shows rapid fire wiggles of low amplitude. With increasing depression of the central nervous system, the frequency of the wiggles decreases, and the ampli-tude increases. Before the EEG becomes flat, showing no electrical activity, it goes through a stage of burst suppression in which brief electrical activity alternates with longer periods of electrical silence. Figure 7.6shows these typical patterns which can be described by the frequency of their waves and the amplitude of their excursions.
The EEG can be processed to make interpretation more convenient. Several methods have been published. A commercial success has been the BIS (Bispec-tral index) monitor, which translates an automatic analysis of the EEG wave-forms (obtained from forehead leads) into a unit-less number between 0 and 100 – the higher the number, the more awake the patient. In general, a BIS of 60 or less is associated – most of the time – with general anesthesia. However, even in physiologic (not pharmacologic) sleep, the BIS can dip well below 60. Thus, we still need to consider the context (drugs, surgical stimulation) in which we observe BIS values. It will have served us well if it helps us to avoid exces-sively deep anesthesia – which might be harmful – and all too light anesthesia – which carries the risk of intraoperative awareness. Anesthesia that is neither too deep nor too light can speed postoperative recovery (wake-up and PACU time).
When we need to monitor the integrity of specific neuronal pathways, we use the evoked potential. Here, we apply a volley of either somatic, auditory, or visual stimuli. The system then automatically scans the EEG, looking for responses to the stimuli and filtering out all other activity in the EEG. It then presents an evoked potential response with characteristic latencies and amplitude of positive and negative deflections. Categorically, we can say that a central response to a peripheral stimulus signifies that the sensory pathways between periphery and brain are conducting impulses and that the brain is capable of responding. If the response is delayed or muted, it is either because the pathways have been affected or the brain is depressed, for example by anesthetics. You can easily imagine that the monitoring of evoked responses can be helpful when the integrity of the pathways are jeopardized by trauma or the surgical intervention, e.g., scoliosis correction.