■ INTRODUCTION
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 STIMULUS AND RESPONSE
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. 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 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 neuro- physiologist regarding the appropriate monitoring for the specific case. The following sections will briefly describe the major neurophysiologic monitoring techniques.
■ TYPE OF STIMULUS RESPONSE NEUROPHYSIOLOGIC MONITORING
Somatosensory Evoked Potentials. Repeated electrical stimuli are delivered to a peripheral nerve and the signal is recorded as it passes up the spinal cord into the cerebral cortex. For example, the stimulus could be applied to the posterior tibial nerve near the ankle because the surgeon is working on the spine near the spinal nerve roots. The roots may be difficult to see, and they contribute to the makeup of the tibial nerve. Somatosensory evoked potential (SSEP) waveform changes would alert the surgeon to potential injury to the nerve roots. In another example, the blood flow to the carotid artery must be interrupted during surgery. SSEP waveform changes could indicate that the brain is not receiving enough blood flow. Motor Evoked Potentials. Electrical stimulation of the scalp overlying the motor cortex of the brain produces a response passing through the spinal cord to the peripheral nerve and finally to the muscle. The response can be detected at any point in this pathway. Of particular importance to anesthesia providers is that stimulation of the motor cortex in the brain intending to stimulate nerves to the legs often stimulates the facial nerve as well. This may lead to jaw clenching during stimulation. A soft bite block should be placed, so that the tongue does not get bitten. Brainstem Auditory Evoked. Potentials or Responses. Repeated clicking sounds are delivered via an earpiece placed in the auditory canal, and the responses are picked up by electrodes on the scalp. This technique monitors the condition of the ear, the cochlear nerve, and the pathway to the auditory cortex through the brainstem. It is most useful during resection of acoustic neuromas (tumor on a nerve leading from the ear to the brain), brain surgery close to the junction of the brainstem and the cerebellum, and during decompression of certain cranial nerves. Visual Evoked Potentials. Lights are repeatedly flashed in front of the eyes, and the pathway to the visual cortex in the brain is recorded. Visual evoked potentials (VEPs) may be useful information in patients who have tumors involving the optic pathway (the optic nerve and the pituitary gland). Electromyography. The sensing electrodes are placed in the muscles innervated by the nerve at risk for damage. When the surgeon touches the nerve, an electrical signal will be generated in the muscle. This method is used in spine surgery, surgery where the facial nerve is at risk of damage, and more recently in thyroid surgery with the introduction of the NIM electromyography (EMG) endotracheal tube. The NIM endotracheal tube has two sensing electrodes just proximal to the cuff. 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. EMG is a test involving motor nerves; therefore, muscle relaxants should not be used during the anesthetic.
■ IMPLICATION FOR ANESTHESIA PERSONNEL
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., keta mine 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 post anesthesia 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:
• A multichannel infusion pump with appropriate tubing
• 100-mL propofol vials for infusion
• Possible phenylephrine infusion
• Soft bite block
• Transport equipment for an intubate patient
■ OTHER BRAIN MONITORS
Electroencephalography. 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). The standard recording has 16 channels and requires special training and experience to interpret. The signal may be processed to produce a single number, which may be more easily interpreted and indicates in which general direction the EEG is going. The bispectral index (BIS) monitor is a form of processed EEG. Hypoxia, hypotension, temperature changes, carbon dioxide tension, and all anesthetic drugs may affect the EEG. EEGs may be used during neurosurgical ablation of a seizure focus, awake craniotomy for resection of a tumor or vascular malformation, or carotid surgery. Bispectral Index (BIS Monitor) Although anesthesia has been delivered safely for many years, 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 and monitoring of changes in heart rate and blood pressure. Many have argued that these are not reliable indicators of the depth of anesthesia. Multiple studies involving thousands of patients have estimated an incidence of awareness under anesthesia of between 1 in 1,000 and 1 in 10,000 patients anesthetized. Awareness mainly consists of remembering conversations and an inability to move or breathe while experiencing pain (this can happen if the patient is paralyzed with neuromuscular blocking agents). Subsequent significant long-term psychological sequelae including posttraumatic stress disorder may ensue in about 33% of these patients. The causes for awareness under anesthesia have been attributed to the following situations:
• It was unsafe to administer deep anesthesia to the patient (e.g., very sick patients, severely injured trauma patients, emergency obstetric surgery where it is important to minimize drugs to the fetus).
• 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).
• Total IV anesthesia
• Sedated patients where the patient experiences awareness. They sometimes do not understand that awareness is normal and common when a patient is sedated and not under general anesthesia (e.g., sedation only or sedation with regional anesthesia).
• 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 maybe tolerant to the usual doses of anesthetic medications)
The BIS was developed and introduced in 1994 as a more objective tool to monitor patients’ levels of consciousness and to decrease the level of awareness under anesthesia. In addition, the BIS monitor has been reported to assist anesthesia providers in optimizing anesthetic doses for individual patients, resulting in faster wake-up times and cost savings from decreased drug dosages. BIS has also been used to guide the management of sedation in critically ill patients in ICUs, especially during mechanical ventilation both with and without neuromuscular blockade and management of drug-induced coma. Another use of BIS monitoring is 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. In recent years, awareness under anesthesia has received a great deal of media attention. In 2004, the Joint Commission deemed awareness under anesthesia a sentinel event and described 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 BIS monitoring stating that it should be used at the discretion of the anesthesiologist. They also added a caveat that 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 BIS monitoring in preventing awareness under general anesthesia and its usefulness in both decreasing levels of awareness and cost and time saving. In addition, an association has been found between low BIS values and postoperative cognitive dysfunction in elderly patients, although the reasons and true significance of this are yet to be determined. Thus, BIS monitor usage has now become very dependent on individual practitioner’s preferences. BIS Monitor Operation. The monitor consists of a sensor, placed on one side of the forehead, which detects a frontal electroencephalograph. The signal is then converted mathematically into a numbered continuous measure, scaled from 1 to 100 (BIS number). This conversion algorithm was derived from EEG data from about 5,000 volunteers. It is the propriety property of Aspect Medical and has been modified several times since the introduction of the monitor. Applying Sensors. After cleaning the skin well with alcohol, the sensor, which consists of a strip of four gelled electrodes, should be applied to one side of the forehead. Sensor number 1: center of the forehead, 2 in (5 cm) above the nose. Sensor number 4: directly above and adjacent to the eyebrow. Sensor number 3: temple area between the corner of the eye and the hairline. Sensor number 2: between sensor number 1 and sensor number 4. Note that the sensors are not numbered sequentially. 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 BIS measurements. The electrode strip is then connected to the monitor. Once powered up, the monitor will display a message indicating the status of the electrodes:
• PASS indicates that there is good contact between the electrodes and the patient.
• HIGH indicates a need to reprep the skin under the electrode and reapply at it. The “high” indicates that the impedance (resistance to flow of electrical currents between the patient and the monitor) is high.
• NOISE may appear in the status window if the electrode is pressed upon during the check or in the presence of a large external stimulus.
• LDOFF indicates an electrode has become detached from the patient. The electrode status may be accessed at any time during use from the setup menu. The BIS monitor may be started at any time as there is neither calibration nor baseline required for its use. The BIS monitor shows the BIS number in the upper left hand corner of the screen. The monitor displays an indication of which power supply is in use (AC electrical supply or battery). If possible, the BIS monitor should be connected to AC power as typical battery life is only about 1 hour. The monitor will also display a warning if the sensor gets disconnected from the machine. The signal quality index (SQI) bar indicates the quality of the EEG signal over 60-second time increments. It is optimal when the bar extends all the way to the right. As the SQI decreases, the BIS numeric display changes form a solid to an outlined number. This should prompt a check of both sensors and connectors. Electrocautery may also interfere with BIS values. The electromyographic bar reflects muscle tone in the underlying frontalis muscle. Increased tone in the frontalis from light anesthesia or recovery from muscle relaxants may be interpreted by the machine as an EEG signal. This can artificially raise the BIS number. Troubleshooting BIS Values BIS is higher than expected:
• Increased surgical stimulus
• Inadequate anesthesia: Vaporizers and IV lines should be checked to ensure that accurate doses are being administered.
• Frontalis muscle twitching or shivering BIS: Check for EMG activity on the monitor. Possible neuromuscular blockage wearing off.
• Interference from pacemakers and other electrical devices BIS is lower than expected:
• Decreased patient requirements (e.g., decreased surgical stimulus)
• Decrease in frontalis muscle tension: Check EMG tracing.
• Hypothermia
• Cerebral ischemia
• Improper lead placement: Reposition or replace sensor.
■ INTRACRANIAL PRESSURE MONITORING
The adult skull is a rigid box whose contents include 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. If the ICP becomes elevated, it can compromise blood flow to the brain and lead to cell damage, impaired neurologic function, or even death. The cerebral perfusion pressure (CPP) maybe calculated according to the following formula: Cerebral perfusion pressure (CPP) = mean arterial pressure (MAP) − intracranial pressure (ICP). Most clinicians strive to keep the CPP greater than 50-60 mm Hg. Either hypotension or increased ICP can compromise the CPP; thus, the treatment for low CPP is to raise the blood pressure and/or decrease ICP. Devices to measure ICP include the following:
• Catheter placed directly into the ventricle of the brain (an external ventricular drain [EVD])
• Small fiberoptic catheter placed inside the brain tissue (Camino catheter)
• Pressure monitoring catheters placed either subdurally or epidurally The most commonly used devices today are EVD and Camino catheters. 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. EVD. The EVD is placed through a small hole in the skull and passed directly into the ventricle of the brain. The catheter is attached to a transducer and also to a drainage chamber, which allows for removal of the CSF. CSF drainage may be necessary to decrease ICP. 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. Great care must be taken that the drainage device is 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. It may be 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. If the drain becomes kinked off or is inadvertently closed for a long period of time when CSF drainage was necessary, ICP may rise to very high levels and compromise blood flow to the brain. It may be advantageous to monitor ICP during transport as sudden changes in ICP can occur. After transport, the transducer should be rezeroed, the drainage device adjusted to the appropriate height, reopened if it has been closed, and checked to ensure that it is dripping and working again.
Camino Catheter. In the 1980s, researchers at Camino Laboratories developed a small fiberoptic device that could be directly inserted into brain matter. A bolt, which acts as a conduit for the catheter, is placed in a sterile fashion in a small hole in the skull. The catheter is then passed through the hole in the skull directly into the frontal lobe of the nondominant hemisphere of the brain. Before placement through the bolt, the cable connector end of the fiberoptic catheter is handed off to a nonsterile
assistant. The assistant attaches the connector to the monitor. The sterility of the catheter must be maintained throughout the procedure. The device is zeroed relative to the atmosphere while being held at the level of the external auditory meatus. 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 transducer is built into the tip of the catheter and requires calibration before insertion. It does not have to be leveled or recalibrated and cannot be calibrated in vivo. 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. It is very important not to bend the fiberoptic catheter as the delicate
transmission fibers can be easily damaged. The monitor can either be a free-standing screen or can be integrated with the bedside patient monitor using a standard interface cable. The monitor can display an ICP waveform, digital ICP, CPP, and brain temperature. For transport, the external monitor can be unplugged from the electrical outlet, the catheter can be disconnected from the cable, and the monitor can be transported to the new location and reconnected. No additional zeroing is required.
■ SUMMARY
Surgery often can unintentionally damage peripheral or central nervous system structures with scalpels, retractors, electrocautery devices, or other surgical instruments. In addition, surgical procedures may interrupt the blood supply to the central nervous system. Neurophysiologic monitoring can be used as an early warning system to monitor the status of both the peripheral and the central nervous system. Anesthesia technicians should be familiar with the basic physiology of these techniques and the anesthetic implications if they are to be used. In addition, anesthesia technicians should be familiar with the setup, operation, and maintenance of ICP monitors and processed ECG (e.g., BIS) monitors.
REVIEW QUESTIONS
- The following are common uses of neurophysiologic monitoring EXCEPT
A) Assess the status of peripheral nerves, the spinal cord, and the brain during the surgical case.
B) Monitor ICP during neurosurgery or when the brain is injured.
C) Navigate through the brain by using computer images.
D) Assess the depth of general anesthesia.
E) None of the above. - The use of a NIM endotrachael tube utilizes which of the following type of neurophysiologic monitoring?
A) Somoatosensory evoked potentials (SSEPs)
B) Motor evoked potentials (MEPs)
C) Brainstem auditory evoked potentials or responses (BAEPs or BAERs)
D) Visual evoked potentials (VEPs)
E) Electromyography (EMG) - When preparing for a case that will require neurophysiologic monitoring, the anesthesia technician should have all the following items available EXCEPT
A) A multichannel infusion pump with appropriate tubing
B) Extra propofol
C) Transport equipment
D) Long-acting muscle relaxants
E) Phenylephrine infusion - BIS is a monitor that measures true patient awareness.
A) True
B) False - Normal values for ICP are
A) 8-12 mm Hg
B) 22-30 mm Hg
C) 15-18 mm Hg
D) 2-6 mm Hg
E) None of the above - When transporting with an EVD catheter, it is important that the anesthesia technician
A) Always place the drain very low in order to drain off the CSF
B) Always close the drain
C) Always make sure the catheter and the drain are not kinked
D) B and C
E) All of the above