CHAPTER 32 Intraoperative Neurophysiologic Monitoring

Ross P. Martini

Introduction

Surgery involving or adjacent to the brain, spinal cord, peripheral nerves, or their blood supply may risk damaging those structures. Unintentional damage may occur 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, and changes in neuromonitoring signals can alert the surgeon and anesthesiologist that something is wrong before irreversible injury occurs. These monitoring systems can be an essential part of anesthetic management in patients whose nervous system is altered and can substantially alter the plan of the anesthesia provider. As an anesthesia technician, your ability to provide support will depend on your understanding of this wide variety of technologies. This chapter will focus both on the care of neurosurgical patients and on the use of neurologic monitoring to understand anesthetic depth.

Many varieties of neuromonitoring exist that can be utilized in different circumstances, and it is common for more than one modality to be used simultaneously. Evoked potentials rely on stimulating a peripheral or central site and measuring the response to the stimulus—that is, the “potential” that is “evoked” when a nerve is stimulated (see Chapters 11 and 12, Central and Peripheral Nervous System). The integrity of the system is determined from characteristics of the electrical response signal. Measurement of brain electrical activity, without application of a stimulus, is called electroencephalography (EEG). EEG may require an expert neurophysiologist for interpretation or rely on complex and proprietary signal processing algorithms that the anesthesia provider interprets as a numerical value, called processed EEG. Processed EEG monitors can be used to approximate depth of anesthesia and may assist in assessing the risk of intraoperative awareness. Nonelectrical neuromonitoring may also be utilized in the operating room and most commonly takes the form of intracranial pressure (ICP) monitoring during neurosurgical procedures in patients with severe brain injury. This chapter provides an introduction to the major neuromonitoring techniques and their implications for anesthesia.

Evoked Potentials: Stimulus and Response

Electrodes placed along peripheral nerves or over the skull can be used to generate an electrical signal that travels across the entire length of the sensory or motor neural pathways, to and from the peripheral nerves and brain. Precise measurement of the signal, called an “evoked potential,” can provide information about the integrity and health of those neural pathways. Both needle and surface electrodes may be used to initiate the stimulus and measure the response. Many electrical stimuli and evoked potentials are transmitted and recorded repeatedly, and hundreds of signals are continuously averaged over time. Both the amplitude (height or strength) and the latency (the time it takes to travel) of the evoked potential provide important information. Changes in amplitude or latency can indicate damage to the neural pathway.

Evoked potential neuromonitoring is commonly used in surgery involving the brain and spinal cord. Carotid artery operations and cerebral aneurysm repair, where blood flow to the brain may be inadequate or interrupted, may also involve evoked potential monitoring. Direct damage to nerves and blood supply can change the waveforms, but so can change in the patient’s physiology: hypoxia, hypotension, and profound anemia all can alter signals, because they alter oxygen delivery to the nerves, as we have seen in previous chapters (Chapter 4, Cardiovascular Anatomy and Physiology, and Chapter 5, Cardiovascular Monitoring, and others) on cellular metabolism. Hypothermia, anesthetic drugs, and other factors can also alter signals. Therefore, it is important for the anesthesiologist to control these variables and communicate changes to neuromonitoring personnel during surgery to avoid preventable changes. Any changes in signals are reported by the neuromonitoring team to the surgeon and anesthesia provider. Neuromonitoring technicians may be present in the operating room and practice under the guidance of an expert neurophysiologist; they are ultimately overseen by a physician. Factors about the patient’s anatomy and the planned procedure dictate which specific varieties of evoked potential monitoring will be utilized—arrangements are usually made between the surgeon, the anesthesia provider, and the neurophysiologist prior to the start of surgery. The following sections will briefly describe the major neurophysiologic monitoring techniques and the anesthetic implications.

Types of Evoked Potential Monitoring

Somatosensory Evoked Potentials

Somatosensory evoked potentials (SSEPs) are obtained by repeated electrical stimuli delivered to a peripheral nerve. The signal is recorded by electrodes on the surface of the scalp to measure the responses in the cerebral cortex. Common peripheral nerves stimulated are the posterior tibial nerve near the ankle and the median nerve in the wrist. SSEPs can monitor the integrity of the sensory pathways in the dorsal (posterior) portion of the spinal cord and monitor the cerebral cortex directly. Changes in the amplitude or latency of the waves signal a problem in the spinal cord or in the cerebral cortex. SSEPs are utilized in most major spine surgeries where there is a concern for damage to the spinal cord and in vascular operations of the head, neck, and great vessels where cerebral blood flow may be interrupted.

Motor Evoked Potentials

Electrodes placed on the scalp can generate an electrical current over the motor cortex and cause an electrical impulse to travel downward from the brain through the spinal cord to the peripheral nerve and muscles. Motor evoked potentials (MEPs) are measured at the level of the peripheral nerve and are commonly used in spine surgery. The electrical current generated in the brain is not very precise; a single electrical impulse will stimulate peripheral nerves of the arm, legs, and face. Caution must be taken to prevent jaw clenching, which may damage the patient’s tongue and lips. Muscle relaxants are not used with MEP monitoring. The muscles of the hands, feet, and face will also move slightly during surgery when the brain is stimulated; this small movement can momentarily startle you if you don’t recognize its cause! This motion can also cause artifact on the pulse oximeter.

Brainstem Auditory Evoked Potentials

Earpieces placed in the auditory canal generate clicking sounds, which stimulates the sensory neurons of the ear. This impulse is carried through the cochlear nerve, brainstem, and to the auditory cortex. Brainstem auditory evoked potentials (BAERs) are used commonly for the removal of tumors at the base of the brain near the cochlear nerve, brainstem, and cerebellum. This space is difficult to access by craniotomy, and there is a unique risk for compression and retraction injury of the nerve and brainstem in this area.

Visual Evoked Potentials

Lights repeatedly flashed in front of the eyes stimulate neurons on the retina, and an impulse travels via the optic nerve to the visual cortex in the brain, where scalp electrodes record the impulse over the occiput. Visual evoked potentials (VEPs) may be useful in patients who have tumors involving the optic pathway (the optic nerve and the pituitary gland).

Electromyography

Electromyography (EMG) involves only the peripheral nervous system and musculature. Sensing electrodes placed in particular muscles are able to detect irritation or damage in the nerves that innervate them. In spine surgery, irritation of a nerve root will produce muscle twitches that are detected by EMG, which alerts the surgeon that the nerve may be nearby or at risk. Surgeons may also use a stimulator to attempt to find the nerve during placement of screws during spinal fusion.

EMG is also used during operations of the ear (acoustic neuroma, cochlear implant), neck (thyroid), and brainstem. In these cases, electrodes placed in the muscles of the face (innervated by the facial nerve), or a specialized breathing tube with electrodes at the level of the vocal cords (vagus or recurrent laryngeal nerve), are used for monitoring. The neural integrity monitor (NIM) EMG endotracheal tube is a specialized device with two sensing electrodes just proximal to the inflatable cuff (Fig. 32.1). When the tube is placed into the trachea with the cuff just past the vocal cords, the sensors will detect signals from the vocal cords. If the surgeon irritates the recurrent laryngeal nerve, the electrodes will sense vocal cord signals and communicate the motion to the surgeon as a series of auditory alarms. EMG is a test involving motor nerves; therefore, muscle relaxants should not be used during the anesthetic.

Guimaraes2e-ch032-image001 — FIGURE 32.1. The NIM EMG Tube. The *blue* sensor segment is placed between the vocal cords under direct vision with the laryngoscope. It is electrically connected to an amplifier by the *red* and *blue* sensing wires. If the surgeon nears a nerve that stimulates the vocal cords to contract, the sensor produces an amplified sound. This helps identify and prevent injury to the nerve.

Evoked Potential Monitoring and Anesthesia

Anesthetic agents affect the evoked potential signals in varying degrees (Table 32.1). 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 possibly etomidate) may improve 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 reduced amount of inhaled anesthetic gas, supplemented by intravenous anesthesia. MEPs are very sensitive to even small amounts of volatile anesthetic, and sometimes, total IV anesthesia is required. Opioids are frequently used to supplement the anesthetic without affecting evoked potentials. Muscle relaxants should be avoided in all cases where the motor response of a nerve will be monitored visually or by EMG or MEP.

Table 32.1. The Effect of Anesthetic Agents on Evoked Potential Neuromonitoring

Supporting an anesthesia provider who must administer an anesthetic with little or no volatile anesthetic and without muscle relaxant can be challenging. The anesthetic depends on intravenous infusions, which lack some of the advantageous qualities of gases and muscle relaxants. Because propofol infusions often cause hypotension, it is important to maintain perfusion of the brain and the spinal cord. Vasopressors may be required. Cases involving neurophysiologic monitoring are often complex and carried out for many hours; therefore, large amounts of intravenous anesthesia 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, because propofol has a long context-sensitive half-life (i.e., the longer you run it, the longer it takes to clear; see Chapter 3, Pharmacology). 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 be available. Anesthesia technicians should prepare for these cases with the following:

A multichannel infusion pump with appropriate tubing
100-mL propofol vials for infusion
Possible vasopressor infusion and need for invasive blood pressure monitoring
Soft bite block
Transport equipment for an intubated patient

Other Monitors

Electroencephalography

EEG measures brain activity through an array of 20 electrodes placed at specific locations on the scalp. It may also be measured directly during a craniotomy by electrodes placed on the brain (electrocorticography). Hypoxia, hypotension, temperature changes, carbon dioxide tension, and all anesthetic drugs may affect the EEG. EEGs may be used during neurosurgical procedures for monitoring of reduced blood flow, for mapping of areas of the brain that have important functions, and for titrating anesthetic depth to suppress brain activity. Common operations requiring EEG are those for treatment of seizures, awake craniotomy for resection of a tumor, treatment of a vascular malformation, or carotid surgery.

Processed EEG

Proprietary EEG monitors with fewer electrodes that can be easily placed using surface landmarks have been developed to assist in the monitoring of the depth of consciousness. These monitors collect EEG information and display a processed waveform and/or numerical value. Two common monitors are the bispectral index (BIS) (Covidien) and SedLine (Masimo). Each monitor consists of a strip of sensors, placed on one side or both sides of the forehead, which detects a frontal EEG (Fig. 32.2). Note that the sensors are not numbered sequentially. After cleaning the skin well with alcohol, the edges of the sensors should be pressed down for 5 seconds to ensure adhesion of the sensors to the skin and sealing in of the electrode gel. Appropriate contact and proper positioning of the electrodes are critical to produce accurate measurements. The electrode strip is then connected to the monitor.

Guimaraes2e-ch032-image002left — FIGURE 32.2. Electrode interface, SedLine. (Courtesy of Masimo.)

Technical details of the application of the monitoring pads are slightly different, but both devices provide similar information. BIS monitoring displays a BIS number from 0 to 100, whereas the SedLine displays a patient state index (PSI). Both the BIS index and the PSI range from 0 to 100, but with different thresholds for deep hypnosis, unconsciousness, and awareness (Table 32.2). Both monitors provide additional information about the quality of the waveform, the underlying artifact, and a graphical representation of the EEG activity over time.

Table 32.2. Comparative Depth Thresholds of BIS and SedLine Monitoring

Processed EEG monitoring may be used in the following situations:

To assist anesthesia providers in optimizing anesthetic doses for individual patients, resulting in faster wake-up times and cost savings from decreased drug dosages.
To guide the management of sedation in critically ill patients in ICUs, especially during mechanical ventilation both with and without neuromuscular blockade and during management of drug-induced coma.
During anesthesia for neurosurgical procedures and in which it is necessary to induce pharmacologic EEG silence or burst suppression on the EEG (electrical silence with intermittent short bursts of EEG activity). Patients with increased ICP or sustained seizures fall into this category. This may be achieved by using BIS monitoring instead of the more complex full EEG monitoring.
As a component of monitoring for intraoperative awareness, in addition to other physiologic variables.

It is important to note that there is no specific monitor for determining whether a patient is actually unconscious. Adequacy of anesthesia is based on a combination of knowledge of drug doses, monitoring of inhaled concentrations of anesthetics, and monitoring of physiologic variables such as spontaneous respirations, heart rate, and blood pressure. Multiple studies involving thousands of patients have estimated an incidence of awareness under anesthesia between 1 in 1,000 and 1 in 10,000 patients anesthetized, but may be even less common when limited to patients receiving general anesthesia. The most serious type of intraoperative awareness involves the inability to move or breathe while experiencing pain (this can happen if the patient is paralyzed with neuromuscular blocking agents). Subsequent long-term psychological sequelae including posttraumatic stress disorder may ensue in up to a third of these patients. Awareness under anesthesia may be more common in the following situations:

Volatile anesthetic at doses that are high enough to ensure unconsciousness in all patients may cause the patient to become critically unstable (e.g., very sick patients, severely injured trauma patients, emergency obstetric surgery); the anesthesia provider may be forced to limit anesthetic gas or face potentially fatal overdose.
Muscle relaxants; patients who are aware and not paralyzed may move.
Anesthesia machine malfunction (e.g., the vaporizer is not delivering the set amount of agent, problems with gas flows diluting a volatile agent).
Anesthetic has run out (e.g., an empty vaporizer or infusion pump that goes unrecognized) End-tidal anesthetic gas monitoring is conventional per ASA guidelines for prevention of intraoperative awareness, but is not an ASA standard monitor (see Chapters 30 and 31, Gas Analyzers and ASA Standard Monitors).
Total IV anesthesia.
Sedated patients in whom awareness is normal, but not consistent with patient expectations. (See Chapter 15, Sedation.) Patients often are not fully educated, or fully accepting, that recall of intraprocedure events may be normal during any procedure under sedation: only a general anesthetic ensures unconsciousness and amnesia.
Partial awareness during emergence that is interpreted by the patient as intraoperative awareness.
Patients using chronic opioids, alcohol, or other substances of abuse (these patients may be tolerant to the usual doses of anesthetic medications).

In 2004, the Joint Commission described intraoperative awareness as a sentinel event and promoted a heightened attentiveness to this issue, but did not mandate the use of brain monitoring devices. The American Society of Anesthesiologists issued a practice advisory regarding processed EEG monitoring stating that it should be used at the discretion of the anesthesia provider. Maintaining low–brain function monitor values in an attempt to prevent intraoperative awareness may conflict with other anesthesia goals, for example, preserving vital functions. Many studies have now been carried out with conflicting results on the value of processed EEG monitoring in preventing awareness under general anesthesia. Low processed EEG values may also be associated with postoperative cognitive dysfunction in elderly patients. Processed EEG usage has now become very dependent on individual practitioner’s preferences.

A strategy for troubleshooting high or low values is important for proper interpretation of the data (Table 32.3).

Table 32.3. Troubleshooting Unexpectedly High or Low Processed EEG values

Intracranial Pressure Monitoring

The adult skull is a fixed volume and contains the brain tissue (80%), the cerebrospinal fluid (CSF) (10%), and the blood vessels (10%). These volumes create a pressure inside the skull known as ICP. Any condition that increases the volume inside the skull will increase the ICP (e.g., brain tumors, bleeding into the brain, or brain swelling after a head injury). Normal values for ICP are 8-12 mm Hg. Elevated ICP (>15-20 mm Hg) can compromise blood flow and lead to cell death or cause physical herniation of brain tissue across rigid skull and connective tissue structures. Measurement of the cerebral perfusion pressure (CPP) can be obtained by subtracting the ICP from the mean arterial pressure (MAP) (CPP = MAP − ICP). Maintenance of a CPP greater than 60 is associated with better outcome. Either hypotension (low MAP) or increased ICP can compromise the CPP; thus, the treatment for low CPP is to raise the blood pressure and/or decrease ICP.

Multiple devices are capable of measuring the ICP (Fig. 32.3). Catheters may be placed directly into the ventricle, inside or outside the dura, or directly into the brain tissue. Pressure monitoring bolts may be placed beneath the skull and either on top of or beneath the dura mater. The external ventricular drain (EVD) is a commonly used monitor because it allows for drainage of the CSF and therefore treatment of elevated ICP. All methods require connection of the device to a transducer to convert the pressure signal to a waveform that can then be displayed on a screen. Most patients who have an ICP monitor will also have invasive blood pressure monitoring to allow for determination of CPP.

Guimaraes2e-ch032-image003 — FIGURE 32.3. Several possible locations for intracranial pressure monitors.

External Ventricular Drains

You will commonly see EVDs and should understand their management. The EVD (Fig. 32.4) is placed through a small hole in the skull and passed directly into the ventricle of the brain. Its drainage system appears complicated, but understanding its basic principles is important for you as an anesthesia technician, as you may be involved in the transportation of critically ill neurosurgical patients. The EVD’s stopcocks and precise hanger height are essential to patient safety at all times, but particularly during patient transport. The catheter is attached to a transducer and also to a drainage chamber, which allows for removal of CSF. CSF drainage is gravity dependent, so the rate of drainage will depend on the height of the drainage chamber relative to the height of the patient’s head. The drainage device must be secured at the proper height at all times. This is particularly true during patient transport. If the drain is placed too low, too much CSF can drain out, with severe consequences for the patient, including intracranial bleeding. It is safer to close the EVD drainage valve during transport, but this should only be done in consultation with all physicians taking care of the patient, and usually only for the brief period of transport. If the drain becomes obstructed or is inadvertently closed for a long period of time when CSF drainage is necessary, ICP may rise to very high levels and compromise brain perfusion. After transport, the transducer should be rezeroed, the drainage device adjusted to the appropriate height, the stopcock opened to drainage, and proper drainage (dripping CSF) confirmed. If it is found not to be functioning properly, the neurosurgical personnel should be contacted. Its stopcock and transducer resemble familiar vascular transducers from the operating room, but the EVD’s sterile pathway should never be interrupted except under strict sterile procedure. It should never be flushed. The EVD connects directly into the brain, which is not well defended against infection.

Guimaraes2e-ch032-image004 — FIGURE 32.4. Functioning external ventricular drain. The external ventricular drain runs in a sterile path from the patient to a drainage system. The surgically placed drain is attached at stopcock A to a separately packaged drainage system. Tubing runs down, briefly out of the frame, and then up to stopcock B, where it is zeroed level with the patient’s ear. Note the zero marking on the white hanging ruler and the blue bubble level used to ensure precise leveling. At this stopcock, the system can be “clamped” (stopcock turned off toward the patient) for transportation. The stopcock can also be turned off toward the drainage system, permitting a continuous column between patient and pressure transducer so that ICP can be measured. This stopcock is currently open to the drainage system. CSF does not drain until the pressure is high enough to push CSF up 15 cm to point C, where it can drip into the collecting chamber. The pressure at which CSF drains is determined by the height at which that chamber hangs; it is critical that the collecting chamber hangs at the correct height (note the string hanging from the IV pole). The stopcock at point D allows the collecting chamber and its measured amount of CSF (in this case, CSF mixed with blood) to be emptied periodically into the lower bag for collection. The EVD’s stopcocks and hanger height are essential to patient safety at all times, but particularly during patient transport. (The pressure bag behind point C is NOT part of the EVD but the patient’s arterial monitoring—a good example of how complex these patients are, and how easy it can be to mix up the different stopcocks and connectors of the many systems, with potentially harmful results.)

Intraparenchymal Fiberoptic Catheters and Bolts

Fiberoptic catheters can also measure ICP when directly inserted into brain matter. The Camino (Integra) monitor is a common variety that allows for additional channels to monitor brain temperature and other values. The catheter is placed through a bolt drilled into the skull and inserted into the brain tissue of the nondominant (usually right) hemisphere of the brain. The device is zeroed relative to the atmosphere while being held at the level of the external auditory meatus and then cannot be recalibrated. If the display on the monitor does not read zero, there is a zero adjustment screw that can be turned with a special tool provided with the insertion kit. The catheter is then placed inside the bolt and secured. A strain relief sheath, which prevents kinking and bending of the catheter, is then slid down over the catheter to prevent damage to the fiberoptics. ICP monitoring bolts utilizing strain gauges are also commonly placed in patients with traumatic brain injury.

Other Monitors

Several additional, highly specialized neuromonitoring modalities exist for use in research and special patient populations. Brain tissue oxygen monitoring is gaining popularity in patients with traumatic brain injury, but its use is not widespread or evidence-based. The catheter can be placed through the same bolt as a fiberoptic ICP monitor. Cerebral microdialysis, subcortical grid EEG electrodes, cerebral oximetry, and near-infrared spectroscopy (NIRS) may be used in highly specialized academic medical centers.

Summary

Neurophysiologic monitoring can be used as an early warning system to monitor the status of both the peripheral and the central nervous systems. Anesthesia technicians should be familiar with the basic physiology of these techniques and the anesthetic implications if they are used. In addition, anesthesia technicians should be familiar with the setup, operation, and maintenance of ICP monitors and processed EEG monitors.

Review Questions

1. In which of the following situations is processed EEG monitoring most likely to be requested by an anesthesia provider?

A) Monitoring a patient for carotid surgery

B) Managing moderate sedation in the GI suite

C) Monitoring a patient under total intravenous anesthesia for rigid bronchoscopy with muscle relaxation

D) Monitoring a patient with elevated ICP for aneurysm clipping in the neuro IR suite

Answer: C

Processed EEG monitors are useful as one of many components of monitoring for intraoperative awareness. Patients undergoing total intravenous anesthesia in whom monitoring of anesthesia gas is not possible, particularly if they must be paralyzed, are at particular risk; those patients may benefit most from a processed EEG monitor. Moderate sedation patients are not usually monitored with EEG. Carotid surgery patients often have standard EEG monitoring. Patients with elevated ICP require ICP and MAP monitoring to assist with CPP but do not usually require EEG monitoring unless requiring monitoring of sedation regimens to reduce CMRO₂.

2. A patient has a blood pressure of 128/60, a mean arterial pressure of 75 mm Hg, and an intracranial pressure of 22 mm Hg. What is the cerebral perfusion pressure, and how do you think the anesthesia provider will address it?

A) CPP is low—provider should try to raise blood pressure and ICP.

B) CPP is high—provider should try to lower blood pressure and ICP.

C) CPP is low—provider should try to raise blood pressure and lower ICP.

D) CPP is high—provider should try to raise blood pressure and lower ICP.

E) None of the above options are correct.

Answer: C

To calculate CPP, subtract ICP from MAP (75 − 22 = 53). This is low—cerebral perfusion pressures greater than 60 mm Hg are associated with better outcomes. To raise the CPP, the anesthesia provider should take steps to raise the patient’s blood pressure and lower their ICP. Notably, it is important to note that this patient’s ICP is high (normal values are 5-15), which is also associated with poor outcomes.

3. Which of the following correctly matches the type of monitoring with the procedure it is most likely to be used for?

A) Somatosensory evoked potential monitoring—removal of tumors involving the optic pathway

B) Brainstem auditory evoked potential monitoring—removal of acoustic neuromas, posterior fossa craniotomies, and tumors near the cochlear nerve

C) Motor evoked potential monitoring—carotid surgery

D) Electromyography—in operations where cerebral blood flow may be interrupted

E) Camino catheter monitoring—patients requiring CSF drainage

Answer: B

Brainstem auditory evoked potential monitoring is often used for removal of tumors at the brainstem, such as those near the cochlear nerve or the cerebellum. Somatosensory evoked potential monitoring is often used in operations where cerebral blood flow may be interrupted or in most major spinal surgeries where there is a concern for damage to the spinal cord. Likewise, motor evoked potential is commonly used for spinal surgery. Electromyography is often used for operations of the peripheral nervous system and musculature, such as operations of the ear, neck, and brainstem; it does not monitor perfusion of the central nervous system. Camino catheters monitor intracranial pressure but cannot drain CSF.

4. Inhaled anesthetics often cannot be used at all with which type of neuromonitoring?

A) Somatosensory evoked potentials

B) Motor evoked potentials

C) Brainstem auditory evoked potentials

D) Processed EEG

E) Electromyography

Answer: B

For cases in which neurophysiologic monitoring is used, anesthesia providers will often choose to reduce the amount of inhaled anesthetic gas used, supplementing this with intravenous anesthesia. This is because inhaled anesthetic gases depress signal amplitude and prolong latency. Motor evoked potentials (MEPs) are particularly susceptible to the effects of inhaled anesthetics and may require total intravenous anesthesia.

5. Which of the following neuromonitors was developed specifically to assess a patient’s level of consciousness and risk for awareness under anesthesia?

A) Somatosensory evoked potential

B) Motor evoked potential

C) Brainstem auditory evoked potential

D) Electromyography

E) Processed EEG monitoring

Answer: E

There is no specific monitor that can determine whether a patient is unconscious. Instead, adequacy of anesthesia is based on a combination of knowledge of drug doses, monitoring of inhaled concentrations of anesthetics, monitoring of physiologic variables such as spontaneous respirations, heart rate, and blood pressure. Processed EEG monitoring was developed to assess the level of consciousness under anesthesia but is not a perfect measure, as it is subject to artifact as well as pharmacologic and physiologic influences. Its information must be integrated with the anesthesia provider’s judgment. Evoked potentials and electromyography all measure disruptions in nerve transmission: each is used depending on which nerve transmission is near the site of surgery (sensory pathways, motor pathways, brainstem and auditory pathways, or peripheral nerves).

Introduction

Evoked Potentials: Stimulus and Response

Types of Evoked Potential Monitoring

Somatosensory Evoked Potentials

Motor Evoked Potentials

Brainstem Auditory Evoked Potentials

Visual Evoked Potentials

Electromyography

Evoked Potential Monitoring and Anesthesia

Other Monitors

Electroencephalography

Processed EEG

Intracranial Pressure Monitoring

External Ventricular Drains

Intraparenchymal Fiberoptic Catheters and Bolts

Other Monitors

Summary

Review Questions

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