Relationship of somatosensory evoked potentials and cerebral oxygen consumption during hypoxic hypoxia in dogs

Hemodynamic Consideration in Intraoperative Neurophysiological Monitoring in Neuromuscular Scoliosis Surgery

OBJECTIVE

To prove the hypothesis that the parameters of intraoperative neurophysiological monitoring (IONM) during will be more deteriorated in neuromuscular scoliosis (NMS) than in adolescent idiopathic scoliosis (AIS).

METHODS

This retrospective study reviewed the data of 69 patients (NMS=32, AIS=37) who underwent scoliosis surgery under IONM. The amplitude of motor evoked potentials (MEPs), and the amplitude and the latency of somatosensory evoked potentials (SEPs) were examined. Demographic, preoperative, perioperative and postoperative data were analyzed to determine whether they affected the IONM parameters for each group.

RESULTS

Of the items analyzed, the bleeding amount was the only significant risk factor for SEP latency deterioration in the NMS group only. The amplitude of SEP and MEP did not correlate with the hemodynamic parameters. The NMS/AIS ratios of the bleeding-related parameters were higher in the order of bleeding amount/weight (2.62, p<0.01), bleeding amount/body mass index (2.13, p<0.01), and bleeding amount (1.56, p<0.01). This study suggests that SEP latency is more vulnerable than SEP or MEP amplitude in ischemic conditions during scoliosis surgery.

CONCLUSION

In NMS patients, it should be considered that the bleeding amount can have a critical effect on intraoperative electrophysiological deterioration.

Prone Position Impairs Oxygen Supply-Demand Balance During Systemic Hypoxia in Rabbits

Ventilation in the prone position improves the prognosis of patients with severe acute respiratory distress syndrome (ARDS). Contraindications to ventilation in this position include unstable systemic circulation. Only a few reports exist on the effects of prone ventilation in respiratory failure on systemic circulation. This animal study compared systemic hemodynamic changes between supine and prone positions in anesthetized rabbits under acute systemic hypoxia (breathing 15% O₂). Cardiac output and the systemic O₂ extraction ratio increased under the hypoxia, but only in the supine group. Besides, the rate pressure product was higher in the prone group than in the supine group. This study showed that prone ventilation increases myocardial O₂ consumption and suppresses compensatory mechanisms to maintain aerobic metabolism during systemic hypoxia. First of all, it will be necessary to examine the effect of prone ventilation on the O₂ supply-demand balance in the ARDS model.

Median somatosensory evoked potential as a predictor of clinical outcome after urgent surgical extracranial internal carotid artery recanalization

OBJECTIVE

Changes in the N20/P25 amplitude of somatosensory evoked potentials (SEP) of the median nerve have been found to correlate with those in cortical regional cerebral blood flow (rCBF). Our study presents the use of median nerve SEP amplitude in predicting the clinical outcome of urgent surgical internal carotid artery (ICA) recanalization.

METHODS

A total of 27 patients suffering an acute ischemic stroke (AIS) with extracranial ICA occlusion within 24 h were prospectively recruited. The primary preoperative endpoints included the SEP amplitude absolute value (SEP-amp) and the SEP amplitude side-to-side ratio (SEP-ratio). Clinical outcome at 3 months postoperatively was assessed using the modified Rankin scale (mRS-3M).

RESULTS

The positive predictive values (PPVs) for SEP-amp and SEP-ratio were 95.5% and 100%, respectively, with the negative predictive values (NPVs) being 60.0% and 100%, respectively. The SEP-ratio correlated fully with mRS-3M.

CONCLUSION

The median SEP side-to-side N20/P25 amplitude ratio seems to be a very strong positive and negative predictor of the clinical outcome of urgent recanalization of an extracranial ICA occlusion.

SIGNIFICANCE

The results suggest that cortical evoked activity may help in selection patient for surgical recanalization and predict clinical recovery after an acute ischemic stroke.

Inner retinal oxygen extraction fraction in rat

PURPOSE

Oxygen extraction fraction (OEF), defined by the ratio of oxygen consumption to delivery, may be a useful parameter for assessing the retinal tissue status under impaired circulation. We report a method for measurement of inner retinal OEF in rats under normoxia and hypoxia based on vascular oxygen tension (PO(2)) imaging.

METHODS

Retinal vascular PO(2) measurements were obtained in 10 rats, using our previously developed optical section phosphorescence lifetime imaging system. Inner retinal OEF was derived from retinal vascular PO(2) measurements based on Fick's principle. Measurements of inner retinal OEF obtained under normoxia were compared between nasal and temporal retinal sectors and repeatability was determined. Inner retinal OEF measurements obtained under normoxia and hypoxia were compared.

RESULTS

Retinal vascular PO(2) and inner retinal OEF measurements were repeatable (ICC ≥ 0.83). Inner retinal OEF measurements at nasal and temporal retinal sectors were correlated (R = 0.71; P = 0.02; n = 10). Under hypoxia, both retinal arterial and venous PO(2) decreased significantly as compared with normoxia (P < 0.001; n = 10). Inner retinal OEF was 0.46 ± 0.13 under normoxia and increased significantly to 0.67 ± 0.16 under hypoxia (mean ± SD; P < 0.001; n = 10).

CONCLUSIONS

Inner retinal OEF is a promising quantitative biomarker for the adequacy of oxygen supply for metabolism under physiologically and pathologically altered conditions.

Does very premature birth affect the functioning of the somatosensory cortex?--A magnetoencephalography study

Increased survival of extremely low birth weight infants has led to a need for new prognostic methods to predict possible future neurological impairment. We investigated the early development of the somatosensory system by recording the somatosensory evoked magnetic fields (SEFs) during natural sleep at fullterm age in 16 very prematurely born infants and 16 healthy newborns born at term. The purpose was to determine possible changes in the function of the somatosensory cortex in the prematurely born infants by comparing the latency, strength, location and morphology of the SEFs with those of healthy fullterm newborns. We recorded reliable SEFs in all patients and controls. The equivalent current dipole (ECD) strength of the first cortical response, M60, was significantly lower in the patients. Otherwise, the general morphology and latency of the SEFs were similar in the two groups of babies. The similar response latencies in the two groups indicate normally developed conduction in the somatosensory system of the prematurely born infants. The attenuated ECD strength may reflect weaker synchrony in firing or a smaller number of the cortical neurons activated by the somatosensory stimulation. At the individual level, in four of the preterm infants, a later M200 response was not present or could not be modeled: all of them had lesions of the underlying hemisphere depicted by ultrasound and magnetic resonance imaging.

The effect of infusing hypoxanthine or xanthine on hypoxic–ischemic brain injury in rabbits

Xanthine oxidase (XO), an enzyme that converts hypoxanthine to xanthine and xanthine to uric acid, is thought to contribute to hypoxic-ischemic brain injury by generating oxygen-free radicals during reperfusion. This is based largely on the observation that inhibition of XO reduces brain damage, but the precise mechanism by which the enzyme contributes to cerebral ischemic injury has not been specifically evaluated. We examined the role of XO in generating oxygen-free radicals that cause brain injury, hypothesizing that if XO generated a significant amount of free radicals during hypoxia-ischemia and reperfusion, providing additional substrate at the time of injury should increase brain damage. Anesthetized rabbits were first subjected to 8 min of cerebral hypoxia by breathing 3% oxygen and then to 8 min of ischemia by raising intracranial pressure equal to mean arterial pressure with an artificial CSF. In order to promote oxygen-free radical generation, hypoxanthine (n=9) or xanthine (n=9), XO substrates, or the vehicle (n=8) was infused intravenously beginning 30 min before and continuing until 30 min after the insult. Animals were sacrificed after 4 h of reperfusion. Neither hypoxanthine nor xanthine infusion increased brain damage. However, administration of hypoxanthine significantly improved somatosensory evoked potential recovery and preserved neurofilament 68 kDa protein, a neuronal structural protein. This study does not support free radical generation by XO as a major cause of damage in cerebral hypoxia-ischemia. Infusion of hypoxanthine reduced cerebral injury suggesting that another mechanism may explain why inhibition of XO reduces brain damage.

Lamotrigine suppresses neurophysiological responses to somatosensory stimulation in the rodent

Neurotransmitter release and voltage-gated ion channel activity in excitatory neurons are critical for understanding and interpreting neuroimaging signals. Couplings between changes in neural activity and energetic/vascular responses are assumed for interpretation of neuroimaging signals. To investigate involvement of neural events to neuroenergetic/neurovascular responses, we conducted multi-modal magnetic resonance imaging (MRI) measurements (at 7.0 T) and electrophysiological recordings (with high impedance microelectrodes) for local field potential (LFP) and spiking frequency (nu) in alpha-chloralose-anesthetized rats. The rats underwent forepaw stimulation before and after treatment of lamotrigine, a neuronal voltage-gated ion channel blocker and glutamate release inhibitor. Multi-modal MRI measurements of cerebral blood flow (CBF) and blood oxygenation level-dependent (BOLD) signal were combined to estimate changes in cerebral metabolic rate of oxygen consumption (CMRo2). Lamotrigine did not appreciably affect values of nu, CBF, and CMRo2 in the resting state. After lamotrigine treatment, evoked changes in LFP and nu were attenuated, which were consistent with commensurate declines in deltaCBF and deltaCMRo2. While number of evoked BOLD-activated voxels was considerably reduced with lamotrigine, intensities of voxels in middle cortical layers were affected to a lesser degree by lamotrigine. The results suggest that lamotrigine suppresses evoked neurophysiological (i.e., neural/energetic/vascular) responses, both in terms of volume of tissue activated and degree of activation in the foci. Since lamotrigine affects evoked responses but not the basal signals, it can be suggested that glutamate release and activity of voltage-gated ion channels are essential for initiating evoked energetic/vascular responses, and thereby important for interpretation of incremental changes in neuroimaging signal.

Systemic hypoxia and the depression of synaptic transmission in rat hippocampus after carotid artery occlusion

The relationship between step reductions in inspired oxygen and the amplitude of evoked field excitatory postsynaptic potentials (fEPSPs) recorded from hippocampal CA1 neurons was examined in anaesthetized rats with a unilateral common carotid artery occlusion. The amplitudes of fEPSPs recorded from the hippocampus ipsilateral to the occlusion were significantly more depressed with hypoxia than were the fEPSPs recorded from the contralateral hippocampus. The adenosine A1-selective antagonist, 8-cyclopentyl-1,3-dimethylxanthine (8-CPT), blunted the hypoxic depression of the fEPSP. Tissue partial pressure of oxygen (Ptiss,O2) was measured in the ipsilateral and contralateral hippocampus using glass Clark-style microelectrodes. Ptiss,O2 fell to similar levels as a function of inspired oxygen in the ipsilateral and contralateral hippocampus, and in the ipsilateral hippocampus after administration of 8-CPT. Hippocampal blood flow (HBF) was measured using laser Doppler flowmetry. A decline in HBF was associated with systemic hypoxia in both hippocampi. HBF, as a function of inspired oxygen, fell significantly more in the ipsilateral than in the contralateral hippocampus. We conclude that endogenous adenosine acting at the neuronal A1 receptor plays a major role in the depression of synaptic transmission during hypoxic ischaemia. The greater susceptibility of the fEPSP in the ipsilateral hippocampus to systemic hypoxia cannot be explained entirely by differences in Ptiss,O2 or HBF between the two hemispheres.

The effect of acute hypoxemia and hypotension on adenosine-mediated depression of evoked hippocampal synaptic transmission

The present study was designed to investigate the relative contributions of arterial P(O(2)), local cerebral blood flow, and oxygen delivery to the adenosine A(1) receptor-mediated depression of evoked synaptic transmission recorded in the rat hippocampus. Urethane-anesthetized rats were given a unilateral common carotid artery occlusion and then placed in a stereotaxic apparatus for stimulation and recording of bilateral hippocampal field excitatory postsynaptic potentials (fEPSPs). Arterial blood gases, mean arterial blood pressure (MAP), and bilateral hippocampal blood flow (HBF) were also measured. Arterial P(O(2)), HBF, and oxygen delivery were manipulated using normoxic hypotension, hypoxic hypotension, and hypoxic normotension. Both hypoxic hypotension and normoxic hypotension resulted in decreased HBF, decreased oxygen delivery, and a depression of the evoked fEPSP limited to the hippocampus ipsilateral to the occlusion. The enhanced HBF and oxygen delivery associated with increased MAP resulted in a restoration and maintenance of hippocampal fEPSPs despite sustained hypoxemia. The adenosine A(1) receptor-mediated depression of the fEPSP was more strongly correlated with changes in HBF and oxygen delivery than with arterial P(O(2)). We propose that adenosine plays an important role mediating the depression of neuronal activity associated with reduced oxygen delivery characteristically observed in ischemic brain tissue.

Multimodality evoked potential findings in infants with congenital heart defects

Evoked potentials are sensitive prognostic tools in young infants at risk for developmental disability. The objective of this prospective study was to determine whether infants with congenital heart defects demonstrate evoked potential abnormalities prior to or following open heart surgery, and to examine the association between these abnormalities and developmental status 1 year following surgery. A consecutive series of newborns (less than 1 month old) and infants (1 month to 2 years old) were recruited. Somatosensory and brain stem auditory evoked potentials were carried out before or after cardiac surgery, or both. One year later, neurologic examination and standardized measures of motor performance and functional independence were carried out. Twenty-seven newborns and 31 infants underwent perioperative somatosensory evoked potential recordings. Results indicate that perioperative somatosensory evoked potential abnormalities were common in newborns (41%) but not in infants (13%) with congenital heart defects. Brainstem conduction times were within normal limits in all subjects; however, 32% presented with mild elevations in hearing thresholds. All newborns with abnormal somatosensory evoked potentials had abnormal neurologic examinations both perioperatively and again 1 year after open heart surgery. Moreover, standardized developmental assessments 1 year following surgery indicate that all newborns with somatosensory evoked potential abnormalities had developmental deficits in one or more domains. Somatosensory evoked potential abnormalities in the perioperative period are common in newborns with congenital heart defects, and are strongly predictive of persistent developmental delay later.

Simultaneous recording of evoked potentials and T2*-weighted MR images during somatosensory stimulation of rat

Somatosensory evoked potentials (SEP) and T2*-weighted nuclear magnetic resonance (NMR) images were recorded simultaneously during somatosensory stimulation of rat to investigate the relationship between electrical activation of the brain tissue and the signal intensity change in functional NMR imaging. Electrical forepaw stimulation was performed in Wistar rats anesthetized with alpha-chloralose. SEPs were recorded with calomel electrodes at stimulation frequencies of 1.5, 3, 4.5, and 6 Hz. At the same time, T2*-weighted imaging was performed, and the signal intensity increase during stimulation was correlated with the mean amplitude of the SEP. Both the stimulation-evoked signal intensity increase in T2*-weighted images and the amplitude of SEPs were dependent on the stimulation frequency, with the largest signals at a stimulation frequency of 1.5 Hz and decreasing activations with increasing frequencies. The feasibility of simultaneous, artifact-free recordings of T2*-weighted NMR images and of evoked potentials is proved. Furthermore, the study demonstrates-in the intact brain-the validity of functional magnetic resonance imaging for estimating the intensity of electrocortical activation.

Mode-actions of the Na(+)-Ca2+ exchanger: from genes to mechanisms to a new strategy in brain disorders

Mode-actions of the Na(+)-Ca2+ exchanger from genes to mechanisms to a new strategy for brain disorders were comparatively studied in oxidative stress. In transfected Chinese hamster ovary (CHO) cells steadily expressing the Na(+)-Ca2+ exchanger's gene, Ca(2+)-efflux via an active mode of the Na(+)-Ca2+ exchanger was elicited by hydrogen peroxide (H2O2) after preincubation of the cell with a Ca(2+)-free medium, whereas Ca(2+)-influx via a reverse mode of the Na(+)-Ca2+ exchanger was dramatically evoked by H2O2 after preincubation of the cell with a Ca2+ medium, as a prelude to neuronal death. According to [45Ca2+] uptake of transfected CHO cells at given time intervals or extracellular Na+[Na+]o gradients, hyperbola, logarithmic and sigmoid curve equations of the Na(+)-Ca2+ exchanger's mode-actions were respectively defined in the absence and the presence of H2O2. The Na(+)-Ca2+ exchanger's conformational transition in oxidative stress was dominated by adenosine triphosphate (ATP)-dependent cytoskeletal redox modification, cation-pi interactions and secondary Ca2+ activation. These mechanisms were used to generate an intracellulary distributed tetra-cluster (named VISA931) for rescuing G-protein agonist-sensitive signal transduction and cortico-cerebral somatosensory evoke potential (SEP) from oxidation via activating forward operation of the Na(+)-Ca2+ exchanger, the beta-adrenergic and the P2-purinergic receptors, blocking Ca2+ influx and catalyzing the dismutation of superoxide anions (O2-.) to H2O2. In conclusion, knowledge-based drug design is a new strategy for developing promising candidates of neuroprotective agents.

Hypoxemic resuscitation in newborn piglets: recovery of somatosensory evoked potentials, hypoxanthine, and acid-base balance

We tested the hypothesis that hypoxic newborn piglets can be successfully resuscitated with lower O2 concentrations than 21%. Severely hypoxic, 2-4-d-old, anesthetized piglets were randomly divided into five resuscitation groups: 21% O2 (n = 10), 18% O2 (n = 9), 15% O2 (n = 9), 12% O2 (n = 8), all normoventilated, and a hypoventilated 21% O2 group (PaCO2; 7.0-8.0 kPa, n = 9). Base excess (BE) reached -20 +/- 1 mmol/L at the end of hypoxia. After 3 h of resuscitation, BE had risen to -4 +/- 1 mmol/L in the 21% O2, 18% O2, and hypoventilated groups, but was -10 +/- 2 mmol/L in the 15% O2 group (p < 0.05 versus 21% O2 group) and -22 +/- 2 mmol/L in the 12% O2 group (p < 0.05 versus 21% O2 group). Four animals died during resuscitation, all allocated to the 12% O2 group (p < 0.05 versus 21% O2 group). Somatosensory evoked potentials (SEPs) recovered in 39 of 45 piglets, and remained present during resuscitation in all except the 12% O2 group. SEP recovered initially even in six of eight animals in the 12% O2 group, but disappeared again in all later during resuscitation. The SEP amplitude recovered to levels not significantly different from the 21% O2 group in all groups except the 12% O2 group. Plasma hypoxanthine concentrations and extracellular hypoxanthine concentrations in the striatum decreased during resuscitation to levels not significantly different from the 21% O2 group in all but the 12% O2 group (p < 0.05 versus 21% O2 group). In conclusion, severely hypoxic newborn piglets were resuscitated as efficiently with both hypoventilation and 18% O2 as with 21% O2.

Somatosensory evoked potentials during hypoxia and hypocapnia in conscious humans

PURPOSE

The objective of the study was to evaluate the effects of moderate hypoxia and hypocapnia on the latency and amplitude of cortical somatosensory evoked potentials (SSEPs) in conscious human subjects.

METHODS

In ten volunteers the amplitude and latency of the cortical somatosensory evoked potentials were recorded during stimulation of the left posterior tibial nerve. Measurements of SSEPs and respiratory variables were made breathing ambient air, air containing a reduced oxygen percentage (17% O2, 14% O2 (n = 6) or 11% O2 (n = 10)), and again during voluntary hyperventilation breathing ambient air (PETCO2 = 20 mmHg, n = 10).

RESULTS

Hypoxia (11% O2) caused mild stimulation of ventilation (P < 0.05) but had no effects on the latency or amplitude of the SSEP. Lesser degrees of hypoxia had no effects. Hyperventilation caused a small (2-4%) decrease) in the latency of the SSEP and an increase in the amplitude of the SSEP (P < 0.05).

CONCLUSIONS

These findings in conscious subjects were consistent with previous observations in anaesthetized humans and anaesthetized dogs and show that the decrease in latency of the SSEP associated with hypocapnia is not due to changes in the depth of anaesthesia. These effects of hypocapnia may contribute to small variations in the latency of the SSEP when monitoring is performed during surgery, but are unlikely to be large enough to be of clinical concern.

Evoked potentials as predictors of outcome in neonatal intensive care unit survivors: review of the literature

Neonatal intensive care unit survivors are at substantial risk for a range of neurodevelopmental sequelae, and therefore a variety of clinical diagnostic techniques have been evaluated as predictors of outcome. We summarize the prognostic value of evoked potentials in newborns at risk. A review of the literature reveals that brainstem conduction abnormalities in auditory brainstem evoked potentials are associated with neuromotor impairment; however, there are many false negative studies. Visual evoked potentials are highly accurate in predicting neurologic deficits in early childhood in asphyxiated term neonates. Sensitivity and specificity are consistently high for somatosensory evoked potentials in term newborns; however, correlations with outcome in premature infants is controversial. Several studies have compared neonatal findings on neuroimaging studies and evoked potentials, and concordant results between these two tests are highly predictive. However, neurologic sequelae often can most accurately be predicted by visual or somatosensory evoked potentials. Evoked potentials may therefore be a useful adjunct to the clinical investigation and prognostication of outcome in the high risk newborn.

Inter- and intraindividual variability of posterior tibial nerve somatosensory evoked potentials in comatose patients

Thirty-two sequential posterior tibial nerve somatosensory evoked potentials (PTN-SEP) were recorded in 30 neurologically impaired, ventilated, comatose patients. To establish the time invariance of PTN-SEP in this population, Spearman rank correlations of latencies, interpeak latency, amplitudes, and mean absolute amplitude with time were computed. The results revealed no significant time dependency. The mean, standard deviation, and 5th and 95th percentiles for the inter- and intraindividual distribution of PTN-SEP parameters, pairwise PTN-SEP parameter differences, and direct cross-correlation of PTN-SEP were estimated using bootstrap procedures. The standard deviations of the interindividual distribution of PTN-SEP parameters are two to three times higher than the standard deviations of the intraindividual distributions. The coefficients of variation, that is, standard deviations divided by means, for the intraindividual distribution of latencies ranged from 0.012 to 0.042, of amplitudes from 0.146 to 0.230. The mean maximal cross-correlation coefficient of two randomly chosen PTN-SEP across patients equaled 0.65, and within patients 0.91. These data demonstrate the interindividual variability and intraindividual stability of PTN-SEP. The normal limits of intraindividual variability (1.96 x standard deviation) are 3.76, 1.33, 2.92, 6.00, and 3.04 ms for latencies N1, P1, N2, P2, and interpeak latency P1-N2, respectively. The intraindividual differences of amplitudes N1/P1, P1/N2, N2/P2, and the mean absolute amplitude should not exceed 0.67, 0.67, 0.90, and 0.27 microV or, expressed as quotients, 61, 52, 41, and 61%, respectively. The intraindividual maximal cross-correlation coefficient should not be lower than 0.74 with a lag of < 2.00 ms. The results of this study are of use for discrete and continuous PTN-SEP monitoring on intensive care units and during neuroradiological interventions and neurosurgery.

Prediction of outcome at school entry in neonatal intensive care unit survivors, with use of clinical and electrophysiologic techniques

OBJECTIVE

To determine the predictive value of multimodality evoked potentials as well as the neonatal neurobehavioral assessment in neonatal intensive care unit survivors at school entry.

STUDY DESIGN

In this prospective study, healthy (n = 24) and high-risk newborn infants (n = 78) were assessed in the newborn period with auditory brain-stem responses (ABRs) and somatosensory evoked potentials (SEPs), as well as the Einstein Neonatal Neurobehavioral Assessment Scale (ENNAS). Healthy and high-risk newborn infants were assessed in a blind fashion at 1, 3, and 5 years of age by a psychologist and a pediatric neurologist. Of those healthy (all 24) and high-risk newborn infants (72/78) with a neonatal ABR, SEP, or both, 62.5% were assessed at 5 years of age with the Wechsler Preschool and Primary Scale of intelligence, the Beery-Buktenica Test of Visual-Motor Integration, the Griffiths Locomotor Subscale, and neurologic examination. Chi-square analyses were carried out on neonatal and outcome measures, and sensitivity, specificity, and predictive values for each of the neonatal tests were ascertained.

RESULTS

The ENNAS and the ABR had good negative predictive value for cognitive (85.7% to 93.3%), locomotor (83.3%), and visual-motor (91.4% to 100%) performance. The ABR had good specificity, whereas the ENNAS was more sensitive. The SEP was an excellent prognostic tool, with high sensitivity (100%) and specificity (80% to 81.3%) for motor as well as cognitive domains. All infants with normal SEPs had favorable outcomes, whereas those with absent potentials did poorly. The SEP abnormalities were associated with findings on neurologic examination in all cases.

CONCLUSIONS

Normal neonatal evoked potentials and ENNAS are associated with favorable outcomes. The SEP most accurately predicts neurodevelopmental status at school entry.

Hyperbaric oxygen after global cerebral ischemia in rabbits does not promote brain lipid peroxidation

OBJECTIVE

To determine whether hyperbaric oxygen administered immediately after global cerebral ischemia increases free radical generation and lipid peroxidation in the brain or alters neurophysiologic recovery.

DESIGN

Prospective, randomized, controlled trial.

SETTING

Animal research laboratory.

SUBJECTS

Adult male New Zealand white rabbits.

INTERVENTIONS

Anesthetized rabbits were subjected to 10 mins of global cerebral ischemia by infusing a mock cerebrospinal fluid into the subarachnoid space and increasing intracranial pressure equal to mean arterial pressure. Immediately upon reperfusion, one group of rabbits (n = 9) was treated with hyperbaric oxygen at 2.8 atmospheres absolute for 75 mins while the control group (n = 9) breathed room air for an equivalent period of time. At the end of the reperfusion period, oxyradical brain damage was determined by measuring brain levels of oxidized and total glutathione and free malondialdehyde. Neurophysiologic brain injury was assessed with cortical somatosensory evoked potentials.

MEASUREMENTS AND MAIN RESULTS

Both oxidized glutathione and the ratio of oxidized glutathione to reduced glutathione (total minus oxidized) were higher (p < .05) in the hyperbaric oxygen group, indicating that hyperbaric oxygen increased free radical generation. Nonetheless, brain malondialdehyde content, an index of lipid peroxidation, was similar (p > .05) in the two groups. Cortical somatosensory evoked potential recovery at the end of reperfusion was 50% higher (p < .05) in the hyperbaric oxygen-treated animals compared with controls.

CONCLUSIONS

Treatment with hyperbaric oxygen after ischemia increased the amount of oxygen free radicals in the brain. However, this increase in free radical generation was not associated with an increase in lipid peroxidation or a reduction in neurophysiologic recovery when measured after 75 mins of recirculation. These results suggest that hyperbaric oxygen administered immediately after global ischemia does not promote early brain injury.

Experimental study on the optimum flow rate and pressure for selective cerebral perfusion

The optimum flow rate and pressure for selective cerebral perfusion during moderate hypothermia (25 degrees C) were investigated in 36 mongrel dogs. Cerebral perfusion was performed for 90 minutes at a flow rate of 100% (the physiologic flow rate), 50%, 25%, and 0%, or no flow (cerebrocirculatory arrest). Somatosensory evoked potentials were monitored to assess brain function. An excess lactate level was considered an index of anaerobic cerebral metabolism, and histopathologic evaluation was performed. Somatosensory evoked potentials showed no abnormalities at flow rates of 100% and 50%, but became abnormal in some dogs at 25% and in all dogs under no-flow conditions. The excess lactate level only increased at a no-flow rate, but not significantly. Histopathologic evaluation showed no ischemic changes at flow rates of 100% and 50%, but there were slight ischemic changes at 25% and severe ischemic damage at no flow. The mean carotid arterial pressure was 63.1 +/- 5.9, 39.8 +/- 6.2, 24.9 +/- 6.0, and 11.3 +/- 3.5 mm Hg at a flow rate of 100%, 50%, 25%, and no flow, respectively. These results suggest that the safe range of flow rates for cerebral perfusion during moderate hypothermia is more than 50% of the physiologic level with a carotid arterial pressure of about 30 mm Hg or more.

Somatosensory evoked potentials in cerebral ischemia of rabbits

Twenty-one rabbits were used in the ischemic group and six in the control group. Cerebral ischemia of variable degree was induced by Fe particle injection method. Somatosensory evoked potentials (SEPs) and cerebral blood flow (CBF) were compared when the CBF levels decreased to their minimum. The latency of the SEPs increased along with the decrease of the CBF when it was lower than 20 ml/100 g/min (68% of the pre-ischemic control level). This may be related to the ischemic change of the white matter. The amplitude showed diphasic changes. When the CBF decreased below 20 ml/100 g/min, the amplitude increased; when the CBF was lower than 11 ml/100 g/min (38% of the pre-ischemic level), it decreased. These results indicate that the functions of the cerebral cortex might be excited in mild ischemia, and be suppressed in severe ischemia.

Nonlinear changes in brain's response in the event of injury as detected by adaptive coherence estimation of evoked potentials

Injury-related changes in evoked potentials are studied with the aid of the coherence function, which effectively measures the degree of linear association between a pair of signals recorded during normal and abnormal states of the brain. The performance of an adaptive algorithm for estimating coherence function is studied, and the effects of additive noise on the estimated coherence function is discussed. Further, a linearity index is formulated and, through analysis and simulations, the index is shown to respond in a predictable manner to increasing nonlinearity while maintaining the robustness to the observation noise. Somatosensory evoked potentials are shown to be sensitive to injury resulting from acute cerebral hypoxia. We analyze the somatosensory evoked potentials recorded from anesthetized cats during inhalation of 8-9% oxygen gas mixtures and during recovery with 100% oxygen. Analyses of the experimental data show a very sharp drop in the magnitude coherence estimates during hypoxic injury and a corresponding rapid decline in the linearity index at the very early stages of the hypoxic injury. Thus, injury may lead to nonlinearities in the electrical response of the brain, and such measurements analyzed by the adaptive coherence estimation method may be used for diagnostic purposes.

High-rate sequential sampling of auditory brain-stem and somatosensory evoked responses in hypoxia

We developed a high-rate sequential recording technique that allowed simultaneous measurements of both auditory brain-stem response (ABR) and somatosensory evoked potential (SEP) every 10 sec. Using this method, a transient increase in amplitude of all the ABR and SEP components in response to hypoxia in dogs could be detected. The increase in amplitude preceded the prolongation of latency. Our study showed that there were successive changes of evoked potentials in response to hypoxia. A transient increase in amplitude is the first to occur, followed by a latency prolongation and an amplitude decrease for both ABRs and SEPs.

Effects of hypobaric hypoxia on the human auditory brainstem responses

Auditory brainstem responses (ABRs) were recorded in six volunteers before, during and after 90-min exposure to hypobaric hypoxia (5,184 m; barometric pressure = 405 mmHg) in an altitude chamber. Waves I, III and V absolute and interpeak latencies were analysed. The main result of the experiment was a significant shortening of the brainstem transmission time (I-V interval) in the recovery from hypoxia compared with the basal condition. This finding could be explained with a slow decay of the compensatory mechanisms acting during hypoxia and/or a transient neuronal hyperexcitability at the end of the hypoxic stress.

Automated evoked potential monitoring

We have developed a computer system for use in the operating room and intensive care unit for the automated acquisition and classification of flash evoked potentials. The project goals were to: (1) acquire evoked potentials without user intervention; (2) recognize, label, and trend important parameters; and (3) alarm if parameters deviate too far from normal. The system consists of a Nicolet Pathfinder, programmed for periodic flash visual evoked potential acquisition and a personal computer for analysis. Analysis software utilizes the C+ +language. The system has been tested on sixteen cases, determinations of specific waveforms were validated ex post facto by two experts. Three parameters in each of three channels were correctly recognized in more than 87% of all waveforms acquired. No signals at all were obtained in only 0.8% of all samples.

Orthonormal (Fourier and Walsh) models of time-varying evoked potentials in neurological injury

Estimation of time-varying changes in evoked potentials (EP's) has important applications, such as monitoring high-risk neurosurgical procedures. We test the hypothesis that injury related changes in EP signals may be modeled by orthonormal basis functions. We evaluate two models of time-varying EP signals: the Fourier series model (FSM) and the Walsh function model (WFM). We estimate the Fourier and Walsh coefficients with the aid of an adaptive least-mean-squares technique. Results from computer simulations illustrate how selection of model order and of the adaptation rate of the estimator affect the signal-to-noise ratio (SNR). The FSM results in a somewhat higher steady-state SNR than does the WFM; however, the WFM is less computationally complex than is the FSM. We apply these two orthonormal functions to evaluate transient response to hypoxic hypoxia in anesthetized cats. Trends of the first five frequencies (Fourier) and sequencies (Walsh) show that the lower frequencies and sequencies may be sensitive indicators of hypoxic neurological injury.

Monitoring of somatosensory evoked potentials during carotid endarterectomy

Somatosensory evoked potentials (SEPs) were monitored in the course of 368 carotid endarterectomies (CEAs) carried out in 312 patients. In an initial group of 26 patients the shunt was used routinely while in a second group, involving 342 CEAs, it was applied selectively on the basis of modifications which the SEP underwent during clamping. The criterion for shunting was the progressive reduction, up to 50%, of the N20-P25 amplitude. New postoperative neurological deficits appeared in 6 patients, all of whom displayed a transitory SEP flattening. The SEPs of 2 of these returned to normal by the time they awoke and both showed a clinical deficit homolateral to the operated side. In only 2 cases did the deficit fail to regress completely and their postoperative CT scans revealed ischaemic lesions. A positive relationship emerged between SEP changes and back pressure values; nonetheless, as many as 75% of the patients with low residual back pressure values (less than 25 mmHg) tolerated the clamping. SEP monitoring appears to provide a reliable basis for selectively applying a shunt when there is a high risk of haemodynamic ischaemia during clamping.

Presyncope caused by central hypovolaemia is not preceded by evoked potential alterations

The mechanism(s) responsible for the onset of presyncope during a central hypovolaemic challenge have gone undefined for many years. It has been speculated that a decrease in cerebral blood flow initiates presyncopal responses, which in turn lead to greater decreases in cerebral oxygen delivery and unconsciousness. Somatosensory evoked potentials (SEP) were monitored as a measure of cerebral functioning in ten subjects during presyncopal symptom limiting lower body negative pressure (a central hypovolaemic challenge). SEP latency and amplitudes have been correlated with cerebral oxygen uptake, so SEP activity can serve as an indirect indicator of cerebral homeostasis. SEPs were generated by electrically stimulating the median nerve and recoding the resulting potentials over the contralateral cerebral cortex. While heart rate and mean blood pressure both fell at presyncope, there were no changes noted in either SEP latency or amplitude at any point before (latency = 22.9 +/- 9 ms; amplitude = 2.86 +/- 0.24 microV), during (22.6 +/- 0.9 ms; 2.68 +/- 0.2 microV), or after (22.7 +/- 0.9 ms; 2.37 +/- 0.23 microV) the occurrence of presyncope. We conclude that the onset of presyncope is not associated with a decrease in cerebral function.

Effect of graded hypoxia on cortical and spinal somatosensory evoked potentials

Cortical somatosensory evoked potential (CSEP), spinal somatosensory evoked potential (SSEP), and electroencephalogram were recorded in rats under pentobarbital anesthesia. After baseline recordings in room air (21% O2), animals were subjected to a graded hypoxia at 15.75%, 10.5%, and 5.25% oxygen levels for 10 minutes. Each level of hypoxia was followed by a 15-minute reoxygenation period. With a moderate hypoxia (15.75% O2), measured latencies for the CSEP and the SSEP were not significantly different compared with baseline (p greater than 0.05). The CSEP amplitude showed a significant increase (p = 0.02) during reoxygenation after the moderate hypoxia. Change in the latency or amplitude of SSEP at 15.75% hypoxia or during the reoxygenation period was not significant compared with the room air (p greater than 0.05). No change in the electroencephalogram was noticed with the moderate hypoxia. At severe hypoxia (10.5% O2), 80% of the animals lost CSEP within 2 minutes. The loss of CSEP was concomitant with significant attenuation of the electroencephalogram waves. The SSEP was resistant to the severe hypoxia and was present in all animals. We concluded that hypoxia affects CSEP with the tendency to increase the amplitude at moderate hypoxia (15.75%) and loss of the latency and amplitude with severe (10.5%) and extreme (5.25%) hypoxia.

Prognostic significance of multimodality evoked response testing in high-risk newborns

Exposure to hypoxic-ischemic events in fetal or neonatal life may lead to permanent brain damage and subsequent neurodevelopmental deficits. Clinical and diagnostic tools have been somewhat helpful in identifying an at-risk group, particularly those patients sustaining significant neurologic sequelae. In this prospective study, the prognostic significance of multimodality evoked responses in high-risk newborns was examined. A group of 44 high-risk newborns, as well as 14 healthy newborns, were tested during the newborn period with auditory brainstem responses and somatosensory evoked responses; these tests were repeated at 2 and 6 months corrected age. A neonatal neurologic examination, the Einstein Neonatal Neurobehavioral Assessment Scale, was also conducted. At 1 year corrected age, both groups were assessed in a blind fashion by a pediatric neurologist and a psychologist to determine neurodevelopmental outcome. Results indicated that somatosensory evoked response abnormalities in particular predict an abnormal neurologic status at 1 year of age. Abnormalities that persisted or worsened correlated with severe neurologic impairment, whereas an abnormal somatosensory evoked response that improved or normalized in infancy was associated with mild to moderate neurologic sequelae. Increased brainstem conduction in the auditory brainstem responses was also associated with neurologic sequelae. Normal findings from auditory brainstem responses and somatosensory evoked responses predicted normal developmental scores in all areas, as well as a normal neurologic outcome at 1 year with negative predictive powers ranging from 85-100%. Evoked response testing appears to be an important adjunct to the neurologic investigation of high-risk newborns.

Frontal hypermetabolism and thalamic hypometabolism in a patient with abnormal orienting and retrosplenial amnesia

A patient with verbal amnesia and a propensity to direct his attention to the right following a retrosplenial area lesion was studied with positron emission tomography using [F-18] fluorodeoxyglucose. These studies showed that the left thalamus was hypometabolic, and the anterior 2/3 of the left hemisphere was hypermetabolic when compared with the right. There were no significant differences seen in the medial temporal lobes. Based on this study, it is posited that interruption of hippocampal input into the anterior thalamus was responsible for the amnesia, and the left frontal hyperactivity was associated with the propensity to attend contralaterally.

Considerations after intraoperative monitoring of somatosensory evoked potentials during carotid endarterectomy

Somatosensory evoked potentials (SEPs) following median nerve stimulation were used to monitor cerebral function during 26 carotid endarterectomies. The patients with minor SEP variations had no neurological deficits on regaining consciousness while the one with more serious SEP variations had a transient deficit. The method thus seems useful in the early detection of ischemic brain impairment.

Somatosensory evoked potentials of the dog: recording techniques and normal values

Median and tibial nerve somatosensory evoked potentials (SSEPs) of 5 sedated dogs were studied to determine their normal features and optimal stimulation and recording techniques. Cortical potentials were mapped from an extensive array of skull electrodes as each limb was independently stimulated with subdermal needles. The effects of bandpass and stimulus intensity and rate were also assessed. Three cortical components (P1, N1, P2) were evoked by median or tibial nerve stimulation and were localized along the coronal suture at lateral and medial electrodes, respectively. SSEP voltage varied much more than morphology, topography, or latency. The inion was a stable, indifferent reference site. Cortical SSEP frequency content was mostly below 250 Hz. Maximal SSEP voltage was achieved only at stimulus intensities 2-3 times motor threshold. Appropriate methods minimize technical difficulties and consistently yield legible SSEPs.

Extended latency of the cortical component of the somatosensory-evoked potential accompanying moderate increases in cerebral blood flow during systemic hypoxia in cats

In 24 adult cats, the somatosensory-evoked potential (SEP) and cerebral blood flow (CBF) were measured under paralyzed, anesthetized conditions during exposure to two different ventilatory regimens. Group I cats (ventilated from 20 to 2% oxygen) responded with a significant increase in white matter blood flow from 25.0 +/- 7.8 to 43.8 +/- 10.5 ml/100 g/min recorded at 7% O2. Gray matter blood flows in these animals increased but not to significant levels above the control blood flow measured at 20%. No significant changes in blood flow were observed in group II animals ventilated over the range of 25-3% oxygen as gray matter rose slightly (but not significantly) with hypoxia and white matter flows remained at levels of 25-30 ml/100 g/min. The latency of the cortical component of the SEP was related to the degree of hypoxia. For both groups, significant extensions in the latency to the occurrence of the cortical component of the SEP (normalized to the % of control SEP) occurred in each case (P less than 0.05). An inverse, linear relationship existed between the latency to the appearance of cortical component (ms) and the percentage oxygen concentration of the ventilatory mixture. No significant changes in thalamocortical conduction times were found, which indicates that hypoxia may have generalized effects on the synaptic pathways supporting the conduction of the SEP. The variation in blood flow and the latency of the cortical component observed between groups I and II may reflect the oxygen concentration used at the beginning of the experiment (25 vs 20%) and the gradations between them vs 3 and 2%.(ABSTRACT TRUNCATED AT 250 WORDS)

Loss of the somatosensory evoked response as an indicator of reversible cerebral ischemia during hypothermic, low-flow cardiopulmonary bypass

We assessed somatosensory evoked response (SSER) as a monitor of cerebral protection during nonpulsatile, hypothermic cardiopulmonary bypass (CPB). In 13 dogs under CPB, extracorporeal flow rate (EFR) thresholds for loss of SSER were determined by stepwise reduction of the EFR from 2.0 to 0.25 L/min/m2 at perfusion temperatures of 35 degrees C, 30 degrees C, 25 degrees C, and 20 degrees C. Testing began at 35 degrees C in Group 1 (N = 6) and at 20 degrees C in Group 2 (N = 7). Immediately on loss of SSER (denoted as a decrease of 80% or more in the amplitude of the somatosensory evoked potentials), EFR was restored to 2.0 L/min/m. Thresholds for loss of SSER ranged between 0.75 and 0.25 L/min/m2. SSER was always restored on return of EFR to 2.0 L/min/m2; thus loss of SSER was a reversible ischemic change. Both groups had similar threshold values at 35 degrees C, but at lower temperatures, Group 1 thresholds were significantly higher than those in Group 2. Since 35 degrees C was the first test temperature for Group 1 but the last for Group 2, EFR reduction at 35 degrees C apparently caused neurophysiological changes (depletion of cortical energy reserves), which diminished subsequent tolerance to ischemia, but EFR reduction at 20 degrees C did not. Our findings show that loss of SSER warns of reversible cerebral ischemia, and support SSER monitoring as a useful measure of cerebral function during low-flow, hypothermic CPB.

Rat middle cerebral artery occlusion: use of evoked potentials and tetrazolium staining to assess chronic ischaemia

This study examined the effects of permanent, unilateral cerebral artery occlusion on the somatosensory evoked potential (SEP) recorded from the ipsilateral cortex in the anaesthetised rat. Ten days after artery occlusion the SEP was absent in the majority of rats tested and in the remainder the wave amplitude was reduced compared to the potential recorded from the normal hemisphere but latency was unaffected. Histochemical staining with Tetrazolium for infarct size has shown that loss of the SEP correlated with ischaemic damage to the cortex. SEP recording can be used to assess the extent of cortical ischaemia in this small animal model.