Tod B. Sloan, M.D., Ph.D.

I am sure that many of you realize that monitoring of the brain and spinal cord is in a very dynamic state of growth. For anesthesia, there is a growing interest and availability of processed electroencephalographic (EEG) monitoring. I liken these techniques to monitoring the “pulse” of the brain. For our surgical team, there is a growing availability of methods to allow better decision-making during surgical procedures that place the nervous system at risk. In essence, these techniques have become the “eyes and ears” of the surgical team for assessing the nervous system in new ways, much as the EEG techniques allow us better insight into the pharmacology we use.

For our management of anesthesia, we have recently seen the introduction of a third monitor based on the frontal EEG that gives a numeric value of drug effects based on complex signal processing. This is accompanied by a method using the brain’s response to auditory stimulation, making four commercial devices designed to assist us in our anesthetic management.

As you are aware, these introductions have not come without controversy, and I am reminded of the introduction of the pulse oximeter when it was asked, “Why did we need this new thing?” We all knew when our patients were oxygenated, or we thought we did. Perhaps part of the problem is one of change, where this new form of monitoring challenges our understanding of sleep and what constitutes anesthesia. Admittedly, we all know that movement, blood pressure and heart rate are imperfect measures of anesthesia.

Frankly, some of our resistance may be due to an inability to change our concept of what anesthesia is and what information these monitors provide. On the former issue, one quickly realizes when we depart from the familiar inhalational agents and use total intravenous anesthesia (TIVA) that anesthesia consists of separate components, including analgesia and sedation. On the latter issue, it is helpful to realize that brain monitors may give a better index of the sedation component as opposed to the analgesia component. Hence, their use is perhaps better visualized for adjustment of the sedation component and other measures of pain (such as heart rate and blood pressure) used to adjust the analgesia. To make it challenging, there is clearly an interaction of these two components, so neither gives a complete picture.

For these reasons, these brain monitors cannot be viewed as a single end-point for anesthesia, but rather they offer a novel piece of physiologic information based on a different view of the patient. In many respects, they are like the pulse rate: when they go up or down, they provide information to be integrated into all of the other monitoring to help guide our management. Once viewed this way, I have found them useful when titrating sedation independent from analgesia (such as TIVA or when sedation is added to regional blockade). They also are useful in circumstances where the traditional measures of anesthesia (e.g., heart rate, blood pressure and movement) may not be adequate guides to patient care (e.g., substantial beta blockade, myocardial dysfunction, spinal cord injury, etc.).

Perhaps their greatest value is to challenge our understanding of how we measure anesthesia and to draw us closer to the patient. I suspect that once we accept them as useful but not perfect monitors (and when the costs become manageable), we will realize some benefits of the technology. I wonder if we stand at the verge of a new way of guiding our anesthesia much like when Harvey Cushing, M.D., in the early 20th century, suggested using the heart rate and blood pressure for the anesthesia record and was told they were too variable to be of value?

The second major area of advancement in monitoring has come from the recent Food and Drug Administration approval of an electrical transcranial stimulator for the production of motor-evoked potentials. This technique now becomes the companion of the familiar somatosensory-evoked potentials such that spine surgery can be monitored in both the ascending sensory tracts and the descending motor tracks. Now growing in usage, these techniques demand a more challenging anesthesia management where inhalational agents and muscle relaxants must often be nearly or completely eliminated. Fortunately, the growing body of knowledge and experience with TIVA has helped to nurture these techniques. Further, related techniques such as monitoring of spontaneous electromyographic activity of muscles as indications of neural irritation and monitoring of spinal reflex pathways (such as the equivalent of the knee jerk) give deep insights into neural function in ways where surgeons have previously been blind. Like brain monitoring, these techniques also are not endpoints, but rather, they allow a better understanding of the dynamic nature of the interaction of the nervous system being monitored and the surgery and physiology of the procedure. This is clearly a maturing field, and the American Society of Neurophysiologic Monitoring, an interdisciplinary society devoted solely to intraoperative monitoring, is promulgating guidelines for methods. Also, interdisciplinary credentialing bodies have emerged to recognize expertise in the relevant areas of knowledge and practice (American Board of Neurophysiologic Monitoring and the American Board of Registered Electrodiagnostic Technicians). Like brain monitoring, spine monitoring stands on the verge of innovation as the application of knowledge about neurophysiology from many different disciplines spawns methods to allow the surgical team to better see and hear the functional integrity of the tissues they endeavor to help in the procedure.

It is truly an exciting time for monitoring the brain and spinal cord. For our surgeons, it must be like the introduction of the operating microscope that allowed them to see in ways they could not previously and allowed bolder and more complex procedures matched by advances in surgical technology. For us in anesthesiology, we stand with an opportunity to embrace these techniques and gain deeper insight into how our drugs work and how to create a more effective patient care team for our patients.

Dr. Sloan has received honoraria or research support from Viasys Healthcare/ Nicolet Biomedical, Physiometrix, Aspect Medical Systems and Digitimer, Ltd.

Tod B. Sloan, M.D., Ph.D., is Professor, Deputy Chair for Research and Director of Neuroanesthesiology, University of Texas Health Science Center, San Antonio, Texas.