Indications & Contraindications
The electroencephalogram (EEG) is occasionally used during cerebrovascular surgery to confirm the adequacy of cerebral oxygenation. Monitoring the depth of anesthesia with a full 16-lead, 8-channel EEG is not warranted, considering the availability of simpler techniques. There are no contraindications.
Techniques & Complications
The EEG is a recording of electrical potentials generated by cells in the cerebral cortex. Although standard ECG electrodes can be used, silver disks containing a conductive gel are preferred. Platinum or stainless steel needle electrodes traumatize the scalp and have high impedance (resistance); however, they can be sterilized and placed in a surgical field. Electrode position (montage) is governed by the international 10-20 system (Figure 6-7). Electric potential differences between combinations of electrodes are filtered, amplified, and displayed by an oscilloscope or pen recorder. EEG activity occurs mostly at frequencies between 1-30 cycles/sec (Hz). Alpha waves have a frequency of 8-13 Hz and are found often in a resting adult with eyes closed. Beta waves at 8-13 Hz are found in concentrating individuals, and at times, in individuals under anesthesia. Delta waves have a frequency of 0.5-4 Hz and are found in brain injury, deep sleep, and anesthesia. Theta waves (4-7 Hz) are also found in sleeping individuals and during anesthesia. EEG waves are also characterized by their amplitude, which is related to their potential (high amplitude, >50 microV; medium amplitude, 20-50 microV; and low amplitude, <20 microV). Lastly, the EEG is examined as to symmetry between the left and right hemispheres.
International 10-20 system. Montage letters refer to cranial location. F, frontal; C, coronal; T, temporal; O, occipital; Z, middle.
Examination of a multichannel EEG is at times performed during surgery to detect areas of cerebral ischemia, such as during carotid endarterectomy as well as during epilepsy surgery. Likewise, it can be used to detect EEG isoelectricity and maximal cerebral protection during hypothermic arrest. The strip chart EEG is cumbersome in the operating room, and often the EEG is processed using power spectral analysis. Frequency analysis divides the EEG into a series of sine waves at different frequencies and then plots the power of the signal at each frequency, allowing for a presentation of EEG activity in a more manageable way than reviewing the raw EEG (Figure 6-8).
Patient states, candidate depth of anesthesia devices or approaches, key features of different monitoring approaches, and possible readings at different depths of anesthesia. The readings shown represent examples of possible readings that may be seen in conjunction with each frontal electroencephalography trace. The electroencephalography traces show 3-s epochs (x-axis), and the scale (y-axis) is 50 μV. AAI, A-Line Autoregressive Index (a proprietary method of extracting the mid-latency auditory evoked potential from the electroencephalogram); Amp, amplitude of an EEG wave; BIS bispectral index; blinks, eye blink artifacts; BS, burst suppression; BSR, burst suppression ratio; EEG, electroencephalography; ETAG, end-tidal anesthetic gas concentration; f, frequency; γ, β, α, θ, δ, EEG waves in decreasing frequencies (γ, more than 30 hertz [Hz]; β,12-30 Hz; α, 8-12 Hz; θ, 4-8 Hz; δ, 0-4 Hz); K, K complexes; Lat, latency between an auditory stimulus and an evoked EEG waveform response; MAC, minimum alveolar concentration; NI, Narcotrend index; SEF95, spectral edge frequency below which 95% of the EEG frequencies reside; Spindles, sleep spindles. (Reproduced, with permission, from Mashour GA, Orser BA, Avidan MS: Intraoperative awareness: from neurobiology to clinical practice. Anesthesiology 2011;114:1218.)
During inhalational anesthesia, initial beta activation is followed by slowing, burst suppression, and isoelectricity. Intravenous agents, depending on dose and drug used, can produce a variety of EEG patterns.
To reduce the incidence of anesthesia awareness, devices have been developed in recent years that process two-channel EEG signals and create a dimensionless variable to indicate wakefulness. Bispectral index (BIS) is most commonly used in this regard. BIS monitors examine four components within the EEG that are associated with the anesthetic state: (1) low frequency, as found during deep anesthesia; (2) high-frequency beta activation found during “light” anesthesia; (3) suppressed EEG waves; and (4) burst suppression.
Other devices attempt to include measures of spontaneous muscle activity, as influenced by the activity of subcortical structures not contributing to the EEG to further provide an assessment of anesthetic depth. Various devices, each with its own algorithm to process the EEG and/or incorporate other variables to ascertain patient wakefulness, may become available in the future (Table 6-1).
Table 6-1 Characteristics of the Commercially Available Monitors of Anesthetic Depth. ||Download (.pdf)
Table 6-1 Characteristics of the Commercially Available Monitors of Anesthetic Depth.
|Parameters||Machine/Manufacturer||Consumable||Physiologic Signals||Recommended Range of Values for Anesthesia||Principles of Measurement|
|Bispectral index (BIS)||A-2000/Aspect Medical Systems, Newton, MA||BIS sensor||Single channel EEG||40-60||BIS is derived from the weighted sum of three EEG parameters: relative α/β ratio; bio-coherence of the EEG waves; and burst suppression. The relative contribution of these parameters has been tuned to correlate with the degree of sedation produced by various sedative agents. BIS ranges from 0 (asleep)-100 (awake).|
|Patient state index (PSI)||Patient state analyzer (PSA 400)/Physiometrix, Inc., N. Billerica, MA||PSArray2||4-channel EEG||25-50||PSI is derived from progressive discriminant analysis of several quantitative EEG variables that are sensitive to changes in the level of anesthesia, but insensitive to the specific agents producing such changes. It includes changes in power spectrum in various EEG frequency bands; hemispheric symmetry; and synchronization between brain regions and the inhibition of regions of the frontal cortex. PSI ranges from 0 (asleep)-100 (awake).|
- Narcotrend stage
- Narcotrend index
|Nacrotrend monitor/ Monitor-Technik, Bad Bramstedt, Germany||Ordinary ECG electrode||1-2 channel EEG||Narcotrend stage D0-2 to C1, which corresponds to an index of 40-60||The Narcotrend monitor classifies EEG signals into different stages of anesthesia (A = awake; B0-2 = sedated; C0-2 = light anesthesia; D0-2 = general anesthesia; E0,1 = general anesthesia with deep hypnosis; F0,1 = burst suppression). The classification algorithm is based on a discriminant analysis of entropy measures and EEG spectral variables. More recently the monitor converts the Narcotrend stages into a dimensionless number from 0 (asleep) to 100 (awake) by nonlinear regression.|
|Entropy||S/5 Entropy Module, M-ENTROPY/ Datex-Ohmeda, Instrumentarium Corp., Helsinki, Finland||Special entropy sensor||Single-channel EEG||40-60||Entropy described the ‘irregularity’ of the EEG signal. As the dose of anesthetic is increased, EEG becomes more regular and the entropy value approaches zero. M-ENTROPY calculates the entropy of the EEG spectrum (spectral entropy). In order to shorten the response time, it uses different time windows according to the corresponding EEG frequencies. Two spectral parameters are calculated: state entropy (frequency band 0-32 Hz) and response entropy (0-47 Hz), which also includes muscle activity. Both entropy variables have been re-scaled, so that 0 is asleep and 100 is awake.|
|Aline autoregressive index (AAI)||AEP/2 monitor/Danmeter A/S, Odense, Demark||Ordinary ECG electrode||AEP||10-25||AAI is derived from the middle latency AEP (20-80 ms). AAI is extracted from an autoregressive model with exogenous input (ARX model) so that only 18 sweeps are required to reproduce the AEP waveform in 2-6 s. The resultant waveform is then transformed into a numeric index (0-100) that describes the shape of the AEP. AAI > 60 is awake, AAI of 0 indicates deep anesthesia.|
|Cerebral state index (CSI)||Cerebral state monitor (CSM), Danmeter A/S, Odense, Demark||Ordinary ECG electrode||Single-channel EEG||40-60||CSI is a weighted sum of (1) α ratio, (2) β ratio, (3) difference between the two and (4) burst suppression. It correlates with the degree of sedation by an ‘adaptive neuro-fuzzy inference system’. CSI ranges from 0 (asleep) to 100 (awake).|
Controversy still exists as to the exact role of processed EEG devices in assessing anesthetic depth. Some studies have demonstrated a reduced awareness when these devices were used, whereas other studies have failed to reveal any advantage over the use of inhalational gas measurements to ensure a minimal alveolar concentration of anesthetic agent. Because individual EEG responsiveness to anesthetic agents may be variable, EEG monitors to assess anesthesia depth or to titrate anesthetic delivery might not always ensure an absence of wakefulness. Moreover, many monitors have a delay, which might only indicate a risk for the patient being aware after he or she had already become conscious (Table 6-2).
Table 6-2 Checklist for Preventing Awareness. ||Download (.pdf)
Table 6-2 Checklist for Preventing Awareness.
- ✓ Check all equipment, drugs, and dosages; ensure that drugs are clearly labeled and that infusions are running into veins.
- ✓ Consider administering an amnesic premedication.
- ✓ Avoid or minimize the administration of muscle relaxants. Use a peripheral nerve stimulator to guide minimal required dose.
- ✓ Consider using the isolated forearm technique if intense paralysis is indicated.
- ✓ Choose potent inhalation agents rather than total intravenous anesthesia, if possible.
- ✓ Administer at least 0.5 to 0.7 minimum alveolar concentration (MAC) of the inhalation agent.
- ✓ Set an alarm for a low anesthetic gas concentration.
- ✓ Monitor anesthetic gas concentration during cardiopulmonary bypass from the bypass machine.
- ✓ Consider alternative treatments for hypotension other than decreasing anesthetic concentration.
- ✓ If it is thought that sufficient anesthesia cannot be administered because of concern about hemodynamic compromise, consider the administration of benzodiazepines or scopolamine for amnesia.
- ✓ Supplement hypnotic agents with analgesic agents such as opioids or local anesthetics, which may help decrease the experience of pain in the event of awareness.
- ✓ Consider using a brain monitor, such as a raw or processed electroencephalogram but do not try to minimize the anesthetic dose based on the brain monitor because there currently is insufficient evidence to support this practice.
- ✓ Monitor the brain routinely if using total intravenous anesthesia.
- ✓ Evaluate known risk factors for awareness, and if specific risk factors are identified consider increasing administered anesthetic concentration.
- ✓ Redose intravenous anesthesia when delivery of inhalation anesthesia is difficult, such as during a long intubation attempt or during rigid bronchoscopy.
To perform a bispectral analysis, data measured by EEG are taken through a number of steps (Figure 6-9) to calculate a single number that correlates with depth of anesthesia/hypnosis.
Calculation of the Bispectral Index. EEG, electroencephalogram; BSR, burst suppression ratio; BIS, Bispectral Index Scale. (Reproduced, with permission, from Rampil IJ: A primer for EEG signal processing in anesthesia. Anesthesiology 1998;89:980.)
BIS values of 65-85 have been advocated as a measure of sedation, whereas values of 40-65 have been recommended for general anesthesia (Figure 6-10). Bispectral analysis may reduce patient awareness during anesthesia, an issue that is important to the public.
The Bispectral Index Scale (BIS versions 3.0 and higher) is a dimensionless scale from 0 (complete cortical electroencephalographic suppression) to 100 (awake). BIS values of 65-85 have been recommended for sedation, whereas values of 40-65 have been recommended for general anesthesia. At BIS values lower than 40, cortical suppression becomes discernible in a raw electroencephalogram as a burst suppression pattern. (Reproduced, with permission, from Johansen JW et al: Development and clinical application of electroencephalographic bispectrum monitoring. Anesthesiology 2000;93:1337.)
Many of the initial studies of its use were not prospective, randomized, controlled trials, but were primarily observational in nature. Artifacts can be a problem. The monitor, in and of itself, costs several thousand dollars and the electrodes are approximately $10 to $15 per anesthetic and cannot be reused.
Some cases with awareness have been identified as having a BIS of less than 65. However, in other cases of awareness, either there were problems with the recordings, or awareness could not be related to any specific time or BIS value. Whether this monitoring technique becomes a standard of care in the future remains to be seen, and studies are ongoing.
Detection of awareness often can minimize its consequences. Use of the Brice questions during postoperative visits can alert anesthesia providers of a potential awareness event. Ask patients to recall the following:
- What do you remember before going to sleep?
- What do you remember right when awakening?
- Do you remember anything in between going to sleep and awakening?
- Did you have any dreams while asleep?
Close follow-up and involvement of mental health experts may avoid the traumatic stress that can be associated with awareness events. Increasingly, patients are managed with regional anesthesia and propofol sedation. Patients undergoing such anesthetics should be made aware that they are not having general anesthesia and might recall perioperative events. Clarification of the techniques used may prevent patients so managed from the belief that they “were awake” during anesthesia.
Indications for intraoperative monitoring of evoked potentials (EPs) include surgical procedures associated with possible neurological injury: spinal fusion with instrumentation, spine and spinal cord tumor resection, brachial plexus repair, thoracoabdominal aortic aneurysm repair, epilepsy surgery, and cerebral tumor resection. Ischemia in the spinal cord or cerebral cortex can be detected by EPs. EP monitoring facilitates probe localization during stereotactic neurosurgery. Auditory EPs have also been used to assess the effects of general anesthesia on the brain. The middle latency auditory EP may be a more sensitive indicator than BIS in regard to anesthetic depth. The amplitude and latency of this signal following an auditory stimulus is influenced by anesthetics.
Although there are no specific contraindications for somatosensory-evoked potentials (SEPs), this modality is severely limited by the availability of monitoring sites, equipment, and trained personnel. Sensitivity to anesthetic agents can also be a limiting factor, particularly in children. Motor-evoked potentials (MEPs) are contraindicated in patients with retained intracranial metal, a skull defect, and implantable devices, as well as after seizures and any major cerebral insult. Brain injury secondary to repetitive stimulation of the cortex and inducement of seizures is a concern with MEPs.
Techniques & Complications
EP monitoring noninvasively assesses neural function by measuring electrophysiological responses to sensory or motor pathway stimulation. Commonly monitored EPs are brainstem auditory evoked responses (BAERs), SEPs, and increasingly, MEPs (Figure 6-11).
Neuroanatomic pathways of somatosensory-evoked potential and motor-evoked potential. The somatosensory-evoked potential (SEP) is produced by stimulation of a peripheral nerve wherein a response can be measured. The electrical volley ascends the spinal cord by the posterior columns and can be recorded in the epidural space and over the posterior cervical spine. It crosses the mid-line after synapsing at the cervicomedullary junction and ascends the lemniscal pathways having a second synapse in the thalamus. From there, it travels to the primary sensory cortex where the cortical response is measured. The motor-evoked potential (MEP) is produced by stimulation of the motor cortex leading to an electrical volley that descends to the anterior horn cells of the spinal cord via the corticospinal tract. After synapsing there it travels via a peripheral nerve and crosses the neuromuscular junction (NMJ) to produce a muscle response. The MEP can be measured in the epidural space as D and I waves produced by direct and indirect (via internuncial neurons) stimulation of the motor cortex, respectively. It can also be measured as a compound muscle action potential (CMAP) in the muscle. (Reproduced, with permission, from Sloan TB, Janik D, Jameson L: Multimodality monitoring of the central nervous system using motor-evoked potentials. Curr Opin Anaesthesiol. 2008;21:560.)
For SEPs, a brief electrical current is delivered to a sensory or mixed peripheral nerve by a pair of electrodes. If the intervening pathway is intact, a nerve action potential will be transmitted to the contralateral sensory cortex to produce an EP. This potential can be measured by cortical surface electrodes, but is usually measured by scalp electrodes. To distinguish the cortical response to a specific stimulus, multiple responses are averaged and background noise is eliminated. EPs are represented by a plot of voltage versus time. The resulting waveforms are analyzed for their poststimulus latency (the time between stimulation and potential detection) and peak amplitude. These are compared with baseline tracings. Technical and physiological causes of a change in an EP must be distinguished from changes due to neural damage. Complications of EP monitoring are rare, but include skin irritation and pressure ischemia at the sites of electrode application.
EPs are altered by many variables other than neural damage. The effect of anesthetics is complex and not easily summarized. In general, balanced anesthetic techniques (nitrous oxide, neuromuscular blocking agents, and opioids) cause minimal changes, whereas volatile agents (halothane, sevoflurane, desflurane, and isoflurane) are best avoided or used at a constant low dose. Early-occurring (specific) EPs are less affected by anesthetics than are late-occurring (nonspecific) responses. Changes in BAERs may provide a measure of the depth of anesthesia. Physiological (eg, blood pressure, temperature, and oxygen saturation) and pharmacological factors should be kept as constant as possible.
Persistent obliteration of EPs is predictive of postoperative neurological deficit. Although SEPs usually identify spinal cord damage, because of their different anatomic pathways, sensory (dorsal spinal cord) EP preservation does not guarantee normal motor (ventral spinal cord) function (false negative). Furthermore, SEPs elicited from posterior tibial nerve stimulation cannot distinguish between peripheral and central ischemia (false positive). Techniques that elicit MEPs by using transcranial magnetic or electrical stimulation of the cortex allow the detection of action potentials in the muscles if the neural pathway is intact. The advantage of using MEPs as opposed to SEPs for spinal cord monitoring is that MEPs monitor the ventral spinal cord, and if sensitive and specific enough, can be used to indicate which patients might develop a postoperative motor deficit. MEPs are more sensitive to spinal cord ischemia than are SEPs. The same considerations for SEPs are applicable to MEPs in that they are affected by volatile inhalational agents, high-dose benzodiazepines, and moderate hypothermia (temperatures less than 32°C). MEPs require monitoring of the level of neuromuscular blockade. Close communication with a neurophysiologist is essential prior to the start of any case where these monitors are used to review the optimal anesthetic technique to ensure monitoring integrity. MEPs are sensitive to volatile anesthetics. Consequently, intravenous techniques are often preferred.
Cerebral Oximetry and Other Monitors of the Brain
Cerebral oximetry uses near infrared spectroscopy (NIRS). Using reflectance spectroscopy near infrared light is emitted by a probe on the scalp (Figure 6-12). Receptors are likewise positioned to detect the reflected light from both deep and superficial structures. As with pulse oximetry, oxygenated and deoxygenated hemoglobin absorb light at different frequencies. Likewise, cytochrome absorbs infrared light in the mitochondria. The NIRS saturation largely reflects the absorption of venous hemoglobin, as it does not have the ability to identify the pulsatile arterial component. Regional saturations of less than 40% on NIRS measures, or changes of greater than 25% of baseline measures, may herald neurological events secondary to decreased cerebral oxygenation.
Principle of the INVOS® near-infrared spectroscopy technique. (Reproduced, with permission, from Rubio A, Hakami L, Münch F, Tandler R, Harig F, Weyand M: Noninvasive control of adequate cerebral oxygenation during low-flow antegrade selective cerebral perfusion on adults and infants in the aortic arch surgery. J Card Surg 2008;23:474.)
Measurements of jugular venous bulb saturation can also provide estimates of cerebral tissue oxygen extraction/decreased cerebral oxygen delivery. Reduced saturations may indicate poor outcomes. Direct tissue oxygen monitoring of the brain is accomplished by placement of a probe to determine the oxygen tension in the brain tissue. In addition to maintaining a cerebral perfusion pressure that is greater than 60 mm Hg and an intracranial pressure that is less than 20 mm Hg, neuroanesthesiologists/intensivists attempt to preserve brain tissue oxygenation by intervening when oxygen tissue tension is less than 20 mm Hg. Such interventions center upon improving oxygen delivery by increasing Fio2, augmenting hemoglobin, adjusting cardiac output, or decreasing oxygen demand.