Surgery often involves operating in close proximity to peripheral nerves, the spinal cord, and the brain, or their blood supply. These structures can be damaged unintentionally from scalpels, retractors, electrocautery devices, or other surgical instruments. Neurophysiologic monitoring can be used during surgery to assess the status of peripheral nerves, the spinal cord, and the brain. It can serve as an early warning system to alert the surgeon that something is wrong while there is still time for corrections to be made before permanent injury occurs. The basic technique is to apply a stimulus in the central or peripheral nervous system and to measure the response. The health of the system is determined from the nature of the measured response. Neurophysiologic monitoring may also be used to monitor intracranial pressure (ICP) during neurosurgery or when the brain is injured. Finally, neurophysiologic monitoring can be used to assess the depth of general anesthesia to reduce the risk of intraoperative awareness. This chapter provides an introduction to the major neurophysiologic monitoring techniques and their implications for anesthesia.

Neurophysiologic monitoring involves the measurement of electrical signals generated along the entire length of motor or sensory neural pathways from peripheral nerves to the brain. Needle or surface electrodes may be used to both initiate the stimulus and measure the response. There are two basic methods of performing this type of monitoring. First, an electrical stimulus is applied to a peripheral sensory organ or nerve and the response signal is measured as it travels to the brain. In the second method, the stimulus is applied to the scalp over a particular brain region and the response is measured as it travels along the nerve pathways to the periphery. Measurements are averaged with a computer, and the results are displayed on a screen as continuously changing waveforms (older systems recorded the signals on graph paper). Both the amplitude (the strength) and the latency (the time it takes to travel) of the signal yield important information about the health of the pathways (Fig. 41.1). Amplitude and latency are continuously measured during the surgery, and changes in either of these may indicate damage in the neuronal pathway.

This type of monitoring will help to detect impending nerve damage along any part of the pathway produced by surgical manipulation of the brain, the spinal cord, peripheral nerves, or the blood supply to these structures. It provides an early warning system of altered nervous tissue function, thus allowing the surgeon to take steps to avoid permanent postoperative neurologic damage. Some examples of surgeries where neurophysiologic monitoring is utilized include carotid artery surgery (interrupts blood flow to the brain), spine surgery (the operation is in close proximity to nerves), and operations directly involving nerves, the spinal cord, or the brain.

Although nerve damage causes changes in monitored waveforms, they are also affected by changes in the physiologic milieu. Hypoxia, hypotension, hypothermia, and anesthetic drugs (see below) all can alter signal latency and amplitude. These variables must be controlled as much as possible during surgery to avoid affecting neurophysiologic monitoring. Any changes in signal latency or amplitude must be interpreted by taking into account any changes in physiologic parameters or the administration of anesthetic agents.

In most institutions, neurophysiologic monitoring is carried out by certified neurophysiology
technicians under the guidance of an expert neurophysiologist and ultimately overseen by a physician. Arrangements are usually made between the surgeon, the anesthesiologist, and the neurophysiologist regarding the appropriate monitoring for the specific case. The following sections will briefly describe the major neurophysiologic monitoring techniques.

Anesthetic agents affect the evoked potential signals in varying degrees. Inhaled anesthetic gases have the greatest effect by depressing signal amplitude and prolonging latency. Low-dose intravenous (IV) agents have less effect on waveforms, but at higher doses, they can significantly decrease amplitude. Some IV agents (e.g., ketamine and etomidate) will even augment the signals. Not all evoked potential signals are equally susceptible to anesthetic agents.

For most cases in which neurophysiologic monitoring will be utilized, anesthesia will consist of a small amount of inhaled anesthetic gas, supplemented by IV infusions, primarily propofol (some institutions require total IV anesthesia when monitoring MEPs). Opioids are frequently used to supplement the anesthetic. Muscle relaxants should be avoided in all cases where the motor response of a nerve will be monitored visually or by EMG or MEP. Because propofol infusions often cause hypotension, it is important to maintain perfusion of the brain and the spinal cord. A phenylephrine infusion may be required.

Cases involving neurophysiologic monitoring are often complex and carried out for many hours; therefore, large amounts of propofol may be administered. At the end of the procedure, it may not be possible for the patient to rapidly emerge from anesthesia and undergo extubation of the trachea. Transport of an intubated patient to either the postanesthesia care unit or the intensive care unit (ICU) is always a possibility, and a transport monitor and an Ambu bag should always be available.

Anesthesia technicians should prepare for these cases with the following: