Technical requirements for evoked potential monitoring in the intensive care unit

A New Quantitative Measure for Monitoring Somatosensory Evoked Potentials

This paper describes the development and testing of a computer algorithm to automate the process of peak identification and somatosensory evoked potential (SSEP) grading. We tested the accuracy of computerized peak detection and evaluated grading schemes using a test set of 60 SSEPs ranked from worst to best by the programmer (RJM) and a blinded grader (PO). The computer algorithm recognized 95% of peaks identified by visual inspection. Twelve percent of peaks identified by the computer were noise. Summed peak to peak amplitude gave the most accurate ranking of SSEPs. Rank correlation between computer and blinded and unblinded expert grading was r = .82 for PO, r = .92 for RJM, p < .0001 for both. Computer and manually summed amplitudes were highly correlated (Pearson r = .98, p < .0001). Correlation between the 2 expert graders was .86, p < .0001. Computer graded SSEPs were significantly related to clinical outcome at 3 months, p < .0001. Automatic grading of SSEPs using summed peak to peak amplitude is highly correlated with expert grading. The measure is objective, continuous, and well suited to statistical analysis.

Neuromonitoring of cochlea and auditory nerve with multiple extracted parameters during induced hypoxia and nerve manipulation

A system capable of comprehensive and detailed monitoring of the cochlea and the auditory nerve during intraoperative surgery was developed. The cochlear blood flow (CBF) and the electrocochleogram (ECochGm) were recorded at the round window (RW) niche using a specially designed otic probe. The ECochGm was further processed to obtain cochlear microphonics (CM) and compound action potentials (CAP). The amplitude and phase of the CM were used to quantify the activity of outer hair cells (OHC); CAP amplitude and latency were used to describe the auditory nerve and the synaptic activity of the inner hair cells (IHC). In addition, concurrent monitoring with a second electrophysiological channel was achieved by recording compound nerve action potential (CNAP) obtained directly from the auditory nerve. Stimulation paradigms, instrumentation and signal processing methods were developed to extract and differentiate the activity of the OHC and the IHC in response to three different frequencies. Narrow band acoustical stimuli elicited CM signals indicating mainly nonlinear operation of the mechano-electrical transduction of the OHCs. Special envelope detectors were developed and applied to the ECochGm to extract the CM fundamental component and its harmonics in real time. The system was extensively validated in experimental animal surgeries by performing nerve compressions and manipulations.

Neuromonitoring in the operating room: why, when, and how to monitor?

This review considers the main principles and indications of EEG and evoked potential (EP) neuromonitoring in the operating room. Neuromonitoring has a threefold purpose: to warn the surgeon that he has to adjust his strategy, to confirm his decision, and to help him improve subsequent procedures. The pathophysiology of intraoperative events liable to alter the EEG or the EPs is first considered. The usefulness of neuromonitoring in preventing neurological complication relies on its ability to detect neurological dysfunction at a reversible stage. This applies especially to ischemia and compressive damage. The anesthetic influences on EEG and EPs are then considered. Knowledge of them is essential to disentangle these neurophysiological alterations due to intraoperative events from those merely due to anesthesia and to use neurophysiological parameters to evaluate the depth of anesthesia. Third, the main indications and limitations of neuromonitoring are considered: prevention of ischemic brain or spinal cord damage, prevention of mechanical injuries of the brain, spinal cord or peripheral nerve, and localization of the motor cortex in cortical neurosurgery or of cranial nerves in posterior fossa surgery. Finally, the 3 levels of neuromonitoring (neurophysiological feature extraction, neurophysiological pattern recognition, clinical integration of the neurophysiological patterns) are discussed together with the rules that should guide the dialogue between the surgeon, the anesthesiologist, and the neurophysiologist.

Dynamics of MLAEP changes in midazolam-induced sedation

This study aimed at assessing the effects of midazolam (MDZ) sedation on auditory brainstem (BAEP) and middle latency (MLAEP) evoked potentials in intensive care conditions. Ten ventilated comatose patients were receiving an intravenous MDZ bolus dose (0.2 mg/kg) followed by a 2 h continuous infusion (0.1 mg/kg/h). MLAEPs and BAEPs elicited by clicks (90 dB HL + masking) were simultaneously and continuously monitored during the first 6 h and for 30 min the next morning. We found no effect of MDZ sedation on BAEPs. Only MLAEP components were modified. However, none of the patients presented any total abolition of the MLAEPs. Bolus injection led to very early alteration of cortical responses, beginning after 5 min and lasting almost 1 h (maximum Pa latency increase, 3.1 ms; maximum Pa-Nb amplitude decrease, 46%). During continuous infusion, MLAEPs remained slightly, although significantly, altered (Pa latency, +1.3 ms; Pa-Nb amplitude, 27%). The Nb wave seemed to be modified earlier and to return to normality later than the Pa wave. These findings incite a careful interpretation of MLAEP tracings acquired during the first hour following MDZ bolus injection. If possible, MDZ should be administered as continuous infusion for reliable interpretation of evoked potential changes in intensive care unit, or during surgery.

Somatosensory evoked potentials and intracranial pressure in severe head injury

The purpose of this study was to explore the relationship between neurologic function, using a quantitative measurement of continuous somatosensory evoked potentials (SSEPs), and intracranial pressure (ICP) following traumatic brain injury. During a 6 year period, severely head-injured patients with a Glascow Coma Scale < or = 8 who were not moribund were monitored with SSEPs and ICP measurements. SSEPs from each hemisphere and ICP were recorded hourly for each patient. Neurologic outcomes were scored using the Glasgow Outcome Scale at three months post injury. Although initial SSEP amplitude did not correlate well with outcome, final SSEP summed peak to peak amplitude from both hemispheres (p = .0001), the best hemisphere (p = .0004), and the worst hemisphere (p = .0001) correlated well with the Glasgow Outcome Scale groups. Of a total of 72 patients, 40 had deteriorating SSEPs and 32 had stable or improving SSEPs. Peak ICP values were not statistically different in these groups (p = .6). Among patients with deteriorating SSEPs, 52.5% lost the greatest proportion of hemispheric electrical activity prior to ICP elevation. In the remaining patients, the percent reduction of SSEP activity after peak ICP levels was not statistically different from the percent reduction in SSEP activity prior to the peak ICP levels (p = .9). This data suggests that in a select group of patients with severe head injury, ICP does not cause SSEP deterioration, but rather is the consequence of deterioration of brain function.