How is the patient’s neurologic status monitored?

How is the patient’s neurologic status monitored?

The assessment of neurologic status depends on whether regional or general anesthesia is used. With regional anesthesia, intermittent evaluation of motor strength, sensation, and language is performed. This should be timed to coincide with interventions of relatively high risk. These include manipulation of the carotid artery, arterial occlusion, and reperfusion. The choice and timing of seda-tive may markedly interfere with proper evaluation. For example, administration of a benzodiazepine may elicit re-emergence of a focal deficit following recovery from a recent TIA.

With general anesthesia, neurologic status is most often assessed by electroencephalogram (EEG), somatosensory evoked potential (SSEP), or transcranial Doppler (TCD). EEG is the most widely used and considered the “gold standard” for monitoring neurologic function during general anesthesia. It is the most sensitive method for detecting cerebral ischemia in the unconscious patient. The sensitivity of the EEG will depend on the number of elec-trode channels monitored and the experience of the person evaluating the EEG. EEG changes indicative of ischemia are most likely to be observed from electrodes positioned near the anatomic site of brain suffering from ischemia. Furthermore, unilateral changes especially observed in regions dependent on the operative artery are more likely to reflect ischemic insult. EEG changes most indicative of ischemia are decreased amplitude (voltage), slowing (decreased frequency), or burst suppression. It is useful to maintain a steady state of anesthetic agent during monitoring in order to best appreciate EEG changes not confounded by altered anesthetic dose.

SSEP depend on processing of signals from stimulation of a peripheral nerve. Since the response is described by latency and amplitude alone, analysis of SSEP requires less experience or training than does EEG. SSEP is probably less sensitive than EEG for detecting ischemia. Ischemia is indi-cated by an increase in latency or a decrease in amplitude of the SSEP waveform. This is of greatest concern when it is unilateral and on the operative side.

TCD is used to measure the middle cerebral artery blood flow velocity on the ipsilateral operative side. It does so by assessing the Doppler shift of sound waves reflected by moving red blood cells in the artery. It is also extremely sensitive for the detection of embolic material. Embolization is quite common during carotid surgery and depends on surgical technique and the presence of atherosclerotic plaque or entrained air. Positioning of the detector probes may be quite cumbersome and the read-ings will markedly depend on the probe direction (angle of insonation). Therefore, any movement of the Doppler probe may result in diminished or lost waveforms.

Less common, but useful monitors include motor evoked potential (exquisitely sensitive to ischemia) and cerebral blood flow measurement by washout of intra-arterial or intravenous xenon-133.