Central Nervous System Monitoring

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Conscious State Monitoring

Conscious state monitoring is necessary to detect the impact of drugs, metabolic response, and/or cardiovascular and respiratory changes. Propofol is frequently used to sedate patients, but methohexital,™ barbiturates, and other drugs, especially benzodiazepines, are also used. Sometimes fentanyl or remifentanil™ in variable doses is administered as a "rescue" analgesic during the operation, because often it is not possible to continue with regional anesthesia. Thus, the regional anesthetic technique is changed to other complex techniques that must also be monitored.

Hypotension or hypoxemia can cause loss of consciousness (possibly transient) because of sympathetic blockade, bleeding, vagal stimulation, local anesthetics, drug absorption, or such events as pulmonary or myocardial infarction or stroke, which are secondary to pathologic complications. One frequent cause of loss of consciousness is inadvertent pneumocephalus secondary to regional anesthesia techniques (epidural and spinal).133

To evaluate and record the level of consciousness, the Glasgow scale can be useful in patients with high risk for loss of consciousness. Simple scales can be employed such as the alertness/sedation (OAA/S) score, with a range of responses from 1, which signifies awake/alert to 5, which indicates that the patient is asleep.™

The bispectral index (BIS) of the electroencephalogram and middle latency auditory evoked potentials (AEP) are likely candidates to distinguish between the conscious and unconscious state in all patients during repeated transitions from consciousness to unconsciousness, which can happen in regional anesthesia with additional propofol infusion. However, the AEP index offers better discriminatory power in describing the transition from the conscious to the unconscious state in the individual patient.™

Electroencephalography

Electroencephalography (EEG) measures the neuron electric activity of the cerebral cortex and is useful in detecting ischemia resulting from improper cerebral perfusion. Specific evaluation of cortical, or deep, activity is measured in neurosurgery to determine specific areas or to map seizure activity spots by means of special electrodes.

The EEG waveforms are classified by their frequencies as delta—1 to 3 Hz; theta—4 to 7 Hz; alpha—8 to 13 Hz; beta—14 to 30 Hz. Loss of fast activity (alpha, beta) and slow activity increase (delta, theta, gamma) are the most common changes associated with cerebral ischemia. Special waveforms are specific for specific seizure patterns.

An EEG reading is necessary when there is a risk of cerebral hypoxemia, or when the effect of drugs on cerebral function must be evaluated, especially drugs that are capable of depressing the cerebral function. In many neurosurgical centers, use of EEG with carotid endarterectomy is a good example,™ because during this type of surgery, the patient is kept awake under regional anesthesia (superficial cervical block plus local infiltration) to monitor the conscious state and to indicate mental performance as an indirect measure of the neuronal metabolic state. There are many studies that attempt to show what type of monitoring is best™ We think that a mixed technique is best, but the data obtained from the EEG reading are more sensitive and are the earliest data obtainable, with fewer false-positive or false-negative evaluations of transitory ischemic states.

Reports exist that show that there is no significant difference in the outcome of patients who were sedated by either a general or a regional anesthetic, with low stroke and mortality rates achieved in carotid endarterectomy without EEG monitoring but with neurologic assessment in awake patients or maintenance of cerebral protection criteria in patients under general anesthesia.™

Bispectral Index

The BIS of the EEG measures the level of unconsciousness and thus may improve the early recovery profile and guarantee hypnosis under general anesthesia.™

Psychomotor agitation can have many causes: emotional, pharmacologic, metabolic, and so forth. It can, therefore, be very difficult to make a rapid determination between emotional acute disturbance and procedure complications. We have to take enough time to define the most probable cause, without forgetting the toxicity of local anesthetics and other drugs, which must be recognized primarily.™ We must also be mindful of the effect of the drugs on each patient of respiratory impairment (muscle paralysis or airway obstruction), especially in patients with premorbid states (e.g., elderly, hyponatremic, starved). In these patients, the drug impact is far less predictable.

The flexibility of shifting from an initially simple procedure to another much more complex procedure is the daily challenge of anesthesiologists who practice regional procedures, and they must be prepared to face this ever-changing situation. A previous emotional excitatory response to pain or other elements related to the illness or surgical procedure should lead the anesthesiologist to suspect that superficial sedation might be insufficient to maintain a state of calm in the patient; therefore, the patient must be evaluated to detect early signs or symptoms of emotional discomfort.

It is expected that some emotional reaction will accompany certain surgical procedures that imply a significant loss to the patient, such as abortion or hysterectomy.

Generally, children between 6 and 12 years of age are not able to tolerate regional anesthesia without sedation.

Neurologically, we evaluate not only awakeness but also superior cerebral functions such as speech, memory, and other abilities. This evaluation is more sophisticated when there is risk compromise of the central flow, as in carotid surgery or during Wada's test of cerebral dominance under cerebral arteriography. For such procedures, specific tests are performed, usually by an expert in the field."2 For regional anesthesia procedures, a continuous record of mental status should be kept in the anesthetic record, including the regularly recorded score of anxiety, comfort level, drowsiness, and pain status; this record should be permanent.142

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