Influence of click polarity on the brain-stem auditory evoked response (BAER) revisited.

Brain-stem auditory evoked responses (BAERs) were recorded both to rarefaction and condensation click stimuli in 92 normal hearers and 78 patients with varying degrees of cochlear hearing loss (N = 340 ears). Frequency distributions of rarefaction minus condensation (R - C) latency and amplitude differences revealed clinically significant polarity effects in a substantial percentage of the patients studied. Bivariate plots of R - C latency and amplitude differences versus average high frequency hearing loss (PTA 3) demonstrated that the magnitude of the R - C latency and amplitude differences also seemed to be influenced by degree of high frequency hearing loss. Results are discussed relative to the phase-locking properties of the afferent auditory nerve fibers and the possible electrodiagnostic consequences of recording the BAER either to alternating or condensation clicks.

A comparison of EEG determinants of near-awakening from isoflurane and fentanyl anesthesia. Spectral edge, median power frequency, and delta ratio.

To evaluate the efficacy of the computer-processed electro-encephalogram (EEG) for determining near-awakening from anesthesia, 14 patients were monitored during emergence from either isoflurane or fentanyl anesthesia at the termination of major surgical procedures. The raw EEG was obtained using bilateral frontomastoid electrodes. The compressed spectral array was digitized and recorded on disk in 4-s epochs using a Tractor Northern "Nomad" processor. The EEG information was displayed in four formats: 1) the frequency-power spectrum from 1-20 Hz, 2) the 95% power frequency, 3) the 50% power frequency, and 4) the ratio of power in the 8-20 Hz frequency range to the power in the 1-4 Hz frequency range (delta ratio). During emergence from isoflurane, there were obvious changes in the EEG frequency-power spectrum that occurred several minutes before patients opened their eyes in response to verbal stimuli. Although no one descriptor of EEG activity could be shown to be superior in anticipating when patients would respond by opening their eyes, awakening was always presaged by an abrupt decrease in power in the 1-4 Hz frequency range; this resulted in a marked increase in the delta ratio value. During emergence from fentanyl anesthesia, however, there was no obvious change in the overall EEG frequency-power spectrum. However, the same numeric EEG descriptors that were predictive of awakening from isoflurane also occurred during emergence from fentanyl, even though they usually occurred within 1 min of awakening. It is concluded that EEG criteria for identifying when patients will awaken from anesthesia are more reliable with isoflurane than with fentanyl.

The effect of nicardipine on neuronal function following ischemia.

In cerebral ischemia, it has been proposed that calcium influx into neurons results in irreversible cellular injury during reperfusion. We administered nicardipine, a dihydropyridine calcium entry blocker, by continuous subcutaneous infusion to twenty five rats beginning before (PR) or following (PO) ischemia, and compared somatosensory evoked potentials (SEPs) to twenty eight ischemic control animals. Comparable ischemic cellular changes were seen in the hippocampi of all animals. SEP amplitude was higher in both the PR (p less than .005) and PO (p less than .0005) groups compared to controls. This effect was found in all three components (P1, N1, P2) of the evoked response. Plasma nicardipine levels of 6-10 ng/ml were associated with mild hypotension. We conclude that nicardipine improved neuronal function as measured by SEPs when administered before or after ischemia, most likely by interrupting the cytotoxic events occurring in cortical neurons during reperfusion.

Baseline hemodynamic state and response to hemodilution in patients with acute cerebral ischemia.

Hemodynamic data were obtained in 9 patients (mean age 65 yrs) with carotid territory cerebral infarct within the preceding 24 hours (mean 14 +/- 8) as part of a pilot study testing the feasibility and safety of hypervolemic hemodilution. Pulmonary arterial catheters (PACs) were placed without complication in all patients, and after baseline measurements were obtained, up to 1500 cc of 6% hetastarch in 0.9% sodium chloride was administered the first day and up to 1000 cc per day the second and third days. Pulmonary wedge pressure (PWP) rose from 6.3 +/- 3.5 to 14.4 +/- 3.4 mm Hg (p less than 0.001) without development of congestive heart failure in any patient. This was accompanied by a drop in hematocrit (Hct) from 40.3 +/- 3.4 to 32.9 +/- 2.0 (p less than 0.001) and rise in cardiac output (CO) from 4.3 +/- 1.0 to 5.3 +/- 0.6 (p less than 0.05). Phlebotomy of 250 cc was performed in 2 patients and 500 cc in one in order to reduce Hct to desired levels. The volume of fluid needed to raise PWP to 15 was unpredictable (2361 +/- 1106 cc) and therefore PACs were necessary to monitor the rate and volume of fluid administration. The data show that PWP is sufficiently low and Hct sufficiently high following stroke in most patients that hemodilution by volume expansion with phlebotomy added if necessary can be undertaken safely with appropriate monitoring of hemodynamic function, and that this therapy results in optimal reduction of Hct and increased CO without risk of hypotension.

The 40 Hertz auditory event-related potential: normal values and effects of lesions.

A model of the 40 Hz auditory event-related potential (40 Hz AERP) was developed and evaluated in a group of normal subjects and two patient groups. The model views the 40 Hz AERP as a combination of the brain-stem auditory evoked potential (BAEP) and a sinusoidal component presumed to arise from structures rostral to the brain-stem. Fourier analysis techniques were used to quantify changes in the sinusoidal component. The results obtained demonstrated that the phase of the sinusoidal component was quite stable in normal subjects, but was predictably altered as a consequence of thalamic or midbrain lesions. Lesions of the temporal lobe did not alter the phase of the sinusoidal component. These results were interpreted as being consistent with the model developed and suggesting a midbrain or thalamic origin for the 40 Hz sinusoid.

Visual field perimetry analysis using evoked potentials in normal and corpus callosum sectioned cats.

The use of the visual evoked potential (VEP) was investigated in terms of its ability to accurately reflect the behaviorally determined extent of the visual field in non-verbal subjects. Two major adult groups of cats were studied: normals, and those with neonatal section of the corpus callosum at 15-19 postnatal days of age. A light-emitting diode was used as the visual stimulus. For both groups, the VEP determined monocular visual field extent closely matched the behaviorally determined field extent (Elberger 1979). This indicates that the VEP method is accurate, and that it is sensitive enough to detect differences between different populations of subjects.

Speech discrimination in children: auditory and auditory/visual processing with binaural and monaural presentation.

The Word Intelligibility by Picture Identification test was recorded with electret microphones inserted at the ear canals of a child listener. This recording paradigm has been shown to preserve binaural cues. Thirty normal-hearing children (ages 6 to 14) responded to binaural and monaural stimulation under auditory and auditory/visual presentation at six signal-to-noise (S/N) ratios (varying from +3 to -12 dB). At the higher and lower S/N ratios, the binaural advantage was minimized. For the auditory alone mode, the largest mean difference of 21% intelligibility improvement from monaural to binaural presentations occurred at -6 dB S/N, whereas in the auditory/visual mode, an intelligibility improvement of 20.8% occurred at -9 dB S/N. Implications for binaural amplification for the hearing impaired who operate on minimal residual hearing follow from the results of this study.