Comparative studies of regional CNS blood flow and evoked potentials in the cat. Effects of hypotensive ischemia on somatosensory evoked potentials in cerebral cortex and spinal cord

Spatial specificity of the enhanced dip inherently induced by prolonged oxygen consumption in cat visual cortex: Implication for columnar resolution functional MRI

Since changes in oxygen consumption induced by active neurons are specific to cortical columns, the small and transient "dip" of deoxyhemoglobin signal, which indicates an increase in oxygen consumption, has been of great interest. In this study, we succeeded in enhancing and sustaining the dip in the deoxyhemoglobin-weighted 620-nm intrinsic optical imaging signals from a 10-s orientation-selective stimulation in cat visual cortex by reducing arterial blood pressure with sodium nitroprusside (a vasodilator) to mitigate the contribution of stimulus-induced blood supply. During this condition, intact spiking activity and a significant reduction of stimulus-induced blood volume changes (570-nm intrinsic signals) were confirmed. The deoxyhemoglobin signal from the prolonged dip was highly localized to iso-orientation domains only during the initial approximately 2 s; the signal specificity weakened over time although the domains were still resolvable after 2 s. The most plausible explanation for this time-dependent spatial specificity is that deoxyhemoglobin induced by oxygen consumption drains from active sites, where spiking activity occurs, to spatially non-specific downstream vessels over time. Our results suggest that the draining effect of pial and intracortical veins in dHb-based imaging techniques, such as blood oxygenation-level dependent (BOLD) functional MRI, is intrinsically unavoidable and reduces its spatial specificity of dHb signal regardless of whether the stimulus-induced blood supply is spatially specific.

Capsaicin differentially influences somatosensory cortical responses evoked by peripheral electrical or mechanical stimulation

The effects of capsaicin injected intraperitoneally (200 micrograms/kg) or applied locally to the cortical surface (10(-5) M) were studied on cortical potentials evoked by peripheral electrical or mechanical stimulation. Capsaicin treatment (i.p.) differentially influenced the cortical evoked potentials depending on the type of stimulation. Just after both types of capsaicin application, the responses to both kinds of stimuli decreased in amplitude. Additionally, during this time a short fall in blood pressure was observed. Half an hour later, however, only in the case of interperitoneal application the potentials evoked by electrical stimulation were facilitated, while the potentials evoked by vibrissa deflection had recovered and stayed around the control levels thereafter. In addition, the responsive cortex area activated by electrical stimulation became enlarged after the i.p. injection of capsaicin, while that of the cortex region activated by mechanical stimulation did not change significantly. Capsaicin applied locally to the cortex resulted neither in the facilitation of evoked potentials nor in the enlargement of the responsive cortical area. The present findings are the first to demonstrate that the i.p. (but not local) administration of capsaicin, in low dosage, differentially influences the cortical responses evoked by electrical and mechanical stimulation of somatosensory afferents.

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.

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.

Effect of subarachnoid hemorrhage on intracranial pulse waves in cats

The influence of vasoconstrictors of intracranial arteries on the amplitude and configuration of the intracranial pulse wave (ICPW) was investigated. Continuous pressure recordings from the descending aorta (systemic arterial pressure) and the third cerebral ventricle (intracranial pressure) were obtained from anesthetized cats. Computerized analysis of the configuration, amplitude, and frequency spectrum of ventricular wave (ICPW) and aortic pulse wave (SAPW) was performed. Artificial cerebrospinal fluid (CSF), blood, or 5-hydroxytryptamine (5-HT) was injected intracisternally. In 24 control cats, 2 ml artificial CSF was injected into the cisterna magna. This produced a significant increase in amplitude of the ICPW but no change in the SAPW. Ten animals received 14 intracisternal injections of 2 ml autologous blood which caused narrowing of the amplitude of the ICPW as well as of all its components (P1, P2, and P3), with no significant change in the SAPW's. Eight animals were also subjected to cisternal injection of 2 ml of a 10(-4)-M solution of 5-HT, resulting in findings similar to those produced by autologous blood. Frequency spectrum of the intracranial and aortic pulse waves showed a high degree of correlation between wave amplitudes and height of the fundamental wave in the FFT record. These results suggest that the cerebral vasospasm that follows cisternal injections of blood and 5-HT in cats can be diagnosed by analysis of the ICPW. This method may allow early diagnosis and continuous monitoring of cerebral vasospasm in humans.

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.

Monitoring of subcortical and cortical somatosensory evoked potentials during carotid endarterectomy: comparison with stump pressure levels

Monitoring of multichannel somatosensory evoked potentials (SEPs) has been performed in 40 cases of carotid endarterectomy (CEA). SEPs were obtained after median nerve stimulation at wrist, recording from 2nd cervical and from the scalp parietal (ipsi- and contralateral) and central (contralateral) positions. The reduction of CBF due to clamping of the carotid artery provoked SEP abnormalities in 10 of the 40 cases. None of the 30 patients with unmodified SEPs developed post-surgical neurological sequelae. SEP alterations were characterized exclusively by amplitude decrements and latency increases of the cortical components, the subcortical ones being unaffected. In 5 of these patients, SEPs returned to normal values before the end of the intervention and no neurological deficit was observed on awakening. In the remaining 5 cases SEPs retained their abnormalities and patients developed post-surgery neurological sequelae (4 immediately, 1 the day after). SEP alterations affected parietal and central components to a similar extent; however, in a few cases cerebral blood flow deficits provoked by carotid clamping modified differently the central P22 and the parietal N20-P25 waves. Comparisons with stump (back) pressure in the carotid artery revealed a higher sensitivity of the SEP technique in detecting vascularization problems due to carotid clamping. The time course of the appearance of SEP abnormalities seems to discriminate alterations secondary to collateral revascularization from those determined by embolization.

Effects of preexisting bypass graft on rCBF and SSEP's following acute stroke in dogs

Regional cerebral blood flow (rCBF) was measured with radiolabeled microspheres in a canine model of superficial temporal artery-middle cerebral artery (STA-MCA) bypass and acute ischemia. Ischemic zone flows in seven dogs with the bypass first closed and then open showed no significant contribution of bypass flow in the intact vascular system. Following acute proximal occlusion, rCBF was preserved by bypass flow. A significant flow decrease ensued when the bypass was then clipped, confirming the adequacy of the lesion and the protective effect of the bypass. Reopening the bypass after 15 minutes of ischemia restored 76% of the previous flow. This was a significant increase from the global ischemia values, and was not statistically different from preocclusive values. Preocclusion somatosensory evoked potentials (SSEP's) in these animals showed a consistent biphasic wave at 8 to 10 msec after stimulation. This wave, with some decrease in amplitude, was preserved by bypass flow following creation of the arterial lesion. Bypass clipping abolished these ipsilateral SSEP's. Variable return of SSEP's occurred following reopening of the graft, but the recordings never reached preischemic amplitudes. This experimental study shows that, in this model, a prophylactic bypass subjected to immediate demand (with no time for "maturation") can adequately augment cortical rCBF and is superior to delayed revascularization. The data lend theoretical support to placement of a prophylactic STA-MCA bypass prior to elective carotid artery sacrifice or in surgery where the risk of acute vascular injury is high.

Cortical deafferentation in cat focal ischemia: disturbance and recovery of sensory functions in cortical areas with different degrees of cerebral blood flow reduction

During and after 15-min occlusion of the middle cerebral artery (MCA) in cats, local CBF and neuronal activity were measured in cortical areas varying in the degree of CBF reduction. In an area within the ischemic center (primary auditory cortex, middle ectosylvian gyrus), CBF was severely suppressed. Click-induced auditory evoked potentials and evoked as well as spontaneous single-unit activity ceased within 1 min after occlusion. Recirculation resulted in a recovery of the different neurophysiological parameters with a time delay ranging from several minutes to 2 h. In two areas surrounding the ischemic focus (a visual area in the marginal gyrus and the forelimb representation area in the primary somatosensory cortex), CBF was reduced but remained above 30 ml/100 g/min during MCA occlusion. Visual flash-induced evoked potentials and somatosensory evoked potentials induced by median nerve electrical stimulation ceased in the corresponding areas with a somewhat slower time course as compared to the auditory responses and they recovered faster after recirculation. In another somatosensory area (hindlimb projection area in the primary somatosensory cortex), CBF stayed nearly at control levels during occlusion. Evoked potentials and single-unit activity induced by tibial nerve electrical stimulation decreased approximately 5 min after occlusion and were abolished approximately 5 min later. At that time, single-unit activity had changed to a nonresponsive pattern but persisted. However, potentials evoked transcallosally by electrical stimulation of the contralateral hemisphere were still recorded. After reopening the MCA, the recovery of neuronal functions was usually complete and occurred within approximately 5 min.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Sensory evoked responses in head injury

Head trauma is a significant source of morbidity in the United States each year. Approximately 700 patients were admitted to our surgical intensive care unit with some degree of head trauma in a 24-month period. Glasgow Coma Score (GCS) was 8 or less in 90% of this group, and 3 or 4 in 43%. Sensory evoked responses were recorded in over 500 patients. This study is reported to demonstrate that optimum care of the injured brain depends on titration of care aimed at maintaining normal neuronal function. In our series, 25% of the patients with GCS of 3 or 4 returned home or to a rehabilitation unit, a significant decrease in morbidity over other reported series. Chemical paralysis and barbiturate coma were a factor in the decision to monitor in 50-60% of the series. In these patients, the auditory brainstem evoked response (ABR), a monitor of brainstem neuroelectrical function, and the somatosensory evoked response, a monitor of brainstem and cortical function, were used to follow the effectiveness of medical and surgical management in these patients, since neurologic examination was of limited value. Case reports are presented to demonstrate that even at high barbiturate levels, access to the integrity of the central nervous system is still possible. Relations among GCS, computerized tomography (CT), intracranial pressure (ICP), ABR, pupillary response, and outcome were studied for a subgroup of 114 patients. All of these clinical parameters, except CT findings, were significantly correlated with outcome using Chi-square analysis. When the data were further analyzed with linear regression analysis, however, the only parameters that significantly correlated with outcome were pupil reactivity and ABR. The principal conclusion of this report is that the main application of serial monitoring of the sensory central pathway in the head-injured patient is not in the prediction of outcome but in the titration of care of the patient for the preservation of neuronal function.

Comparative studies of regional CNS blood flow autoregulation and responses to CO2 in the cat. Effects of altering arterial blood pressure and PaCO2 on rCBF of cerebrum, cerebellum, and spinal cord

Autoregulation and CO2 responses were investigated concurrently in cerebrum, cerebellum, and spinal cord of 19 cats by means of hydrogen clearance. Under ketamine and nitrous oxide anesthesia at normal systemic mean arterial blood pressure (MABP 123 +/- 18.4 mmHg, mean +/- standard deviation) blood flow was 86 +/- 30.0 ml/100 g/min in the cerebrum, 48 +/- 13.6 ml/100 g/min in the cerebellum, and 46 +/- 18.7 ml/100 g/min in the spinal cord. During normocapnia (PaCO2 27-33 mmHg) for every mmHg of PaCO2 variation an average flow change of 1.7 ml/100 g/min was found in the cerebrum, corresponding change rates in the cerebellum and in the spinal cord were 1.1 and 0.9 ml/100 g/min/mmHg, respectively. Thus, the effect of carbon dioxide appears to be positively correlated with the normal level of regional perfusion and metabolism. Flow values within 10% of control were recorded in the cerebrum at MABPs ranging from 79 to 123% of normal blood pressure, 53 to 146% in the cerebellum, and 83 to 128% in the spinal cord. These results suggest greater susceptibility to pressure dependent ischemia of cerebrum and spinal cord, with relative resistance of the cerebellum.