MONITORING MODALITIES

Multiple types of neurologic monitoring are used in the operating room ( Table 38-1 ). To better understand the uses and limitations of each monitoring technique, neurologic monitors can be divided into monitors of electrical function and monitors of blood flow. Monitors of electrical function also indirectly monitor blood flow, because electrical function fails after a critical reduction in blood flow but before the onset of permanent cellular damage.

1512

The most commonly used monitoring modalities are the EEG, sensory evoked responses (SERs), and the electromyogram (EMG). The EEG is a surface recording of the summation of excitatory and inhibitory postsynaptic potentials spontaneously generated by the pyramidal cells in the cerebral cortex. The signals are very small, and each electrode records only information generated directly beneath the electrode. Monitoring with the EEG is usually directed toward three perioperative uses. First, the EEG is used to help identify inadequate blood flow to the cerebral cortex caused by a surgical or anesthetic-induced reduction in blood flow or by retraction on cerebral tissue. Second, the EEG may be used to guide reduction of cerebral metabolism in anticipation of a loss of cerebral blood flow (CBF) or when a reduction in CBF and blood volume is desired in the patient with high intracranial pressure. Third, the EEG may be used to predict neurologic outcome after a brain insult.

More than one-half century of experience in monitoring with the EEG has led to many known correlations of electroencephalographic patterns with normal and pathologic clinical states of the cerebral cortex. The electroencephalographer can accurately identify consciousness, unconsciousness, seizure activity, stages of sleep, and coma. In the absence of changes in drug level, the electroencephalographer can also accurately identify inadequate oxygen delivery to the brain (from hypoxemia or ischemia). In the past decade, using high-speed, computerized analysis and statistical methods, the electroencephalographic patterns in the continuum from awake to deeply anesthetized are becoming, with few exceptions, much better understood. Although still far from perfect, correlations between electroencephalographic or evoked potential patterns and depth of hypnotic state have significantly improved,^{[1] [2] [3] [4] [5] [6]} and the likelihood of consciousness or amnesia can be assessed with most anesthetic techniques. These computer advances have made possible high-speed mathematic manipulation of the electroencephalographic signal to present the data in a manner more suitable to continuous, trended monitoring for surgical purposes.

Monitoring of the EEG in the operating room for surgical purposes requires significant training and experience. In contrast, electroencephalographic monitoring directed at assessing the depth of hypnosis can be readily mastered by most practicing clinicians in a short period.

Evoked potentials are electrical activity generated in response to a sensory or motor stimulus. Measurements of evoked responses may be made at multiple points along an involved nervous system pathway. The evoked responses are generally smaller than other electrical activity generated in nearby tissue (i.e., muscle or brain) and are readily obscured by these other biologic signals. Repeated sampling and sophisticated electronic summation and averaging techniques are needed to extract the desired evoked potential signal from background biologic signals.

SERs are by far the most common type of evoked potentials monitored intraoperatively. During the past 2 decades, much research has been carried out regarding the use of intraoperative motor evoked potentials (MEPs); however, routine use of MEP monitoring is not widespread. There are three basic types of SERs: somatosensory (SSEP), auditory (BAEP), and visual (VEP) potentials. The SSEP is produced by electrically stimulating a peripheral or cranial nerve. In the case of peripheral nerve stimulation, responses may be recorded proximally over the stimulated peripheral nerve, the spinal cord, and the cerebral cortex with assessment of the function of the peripheral nerve, the posterior and lateral aspects of the spinal cord, a small portion of the brainstem, the ventral posterolateral nucleus of the thalamus, the thalamocortical radiation, and a portion of the sensory cortex. The BAEP is usually produced by a series of rapid, loud clicks applied directly to the external auditory canal. Responses are most commonly recorded from scalp-applied electrodes, although more invasive direct recordings from auditory structures and nerves may be made. BAEPs assess function of the auditory apparatus itself, cranial nerve VIII, the cochlear nucleus, and a relatively small area of the rostral brainstem, the inferior colliculus, and the auditory cortex. After significant clinical research, cortically generated auditory responses (i.e., middle latency auditory responses) are being used to determine the depth of anesthetic induced hypnosis, similar to the bispectral index (BIS) monitor. The VEP is produced by

1513