✓ The appearance of superoxide anion radicals in cerebral extracellular space during and after experimental fluid-percussion brain injury was investigated in anesthetized cats equipped with cranial windows. Superoxide was detected by demonstrating the presence of superoxide dismutase (SOD)-inhibitable reduction of nitroblue tetrazolium (NBT). The SOD-inhibitable rate of reduction of NBT was 3.52 ± 0.72 nM/min/sq cm during brain injury and 4.11 ± 0.74 nM/min/sq cm 1 hour after injury. No significant superoxide production was detected in control animals. The sustained arteriolar dilation and reduced responsiveness to the vasoconstrictor effects of arterial hypocapnia observed 30 minutes after brain injury were eliminated by after-treatment with topical SOD (60 U/ml) and catalase (40 U/ml). The results show that experimental brain injury causes the generation and appearance in extracellular fluid space of superoxide. Superoxide production continues for at least 1 hour following injury. The sustained dilation and abnormal responsiveness of cerebral arterioles after injury are due to the continued generation of superoxide and other radicals derived from it. These functional changes can be reversed by after-treatment with appropriate scavenging agents.
Hermes A. Kontos and Enoch P. Wei
Enoch P. Wei, Robert G. Lamb, and Hermes A. Kontos
✓ Phospholipase C activity was measured in 1000 × G centrifuged cellular fractions isolated from cerebral cortical homogenates obtained from either control cats or cats subjected to experimental fluid-percussion brain injury. Phospholipase C activity was determined directly by measuring the Ca++-dependent conversion of membrane-bound, labeled phosphatidate to diacylglycerol or indirectly by measuring the diacylglycerol-dependent (brain diacylglycerol content) formation of phosphatidylcholine in the presence of labeled cytidine diphosphate (CDP) choline. Phospholipase C activity determined by either method was about two times greater in cell fractions isolated from animals subjected to brain injury than in controls (p < 0.01). The brain injury-induced rise in phospholipase C activity may be responsible, at least in part, for generating diacylglycerol that may be a source of free arachidonic acid that stimulates prostaglandin synthesis. These changes may account for the rise in brain prostaglandin levels that has been demonstrated earlier to occur after this type of brain injury.
Joseph E. Levasseur, John L. Patterson Jr., Nitya R. Ghatak, Hermes A. Kontos, and Surg C. Choi
✓ The function-specific enzyme superoxide dismutase (SOD) was tested for its protective effect in severe experimental fluid-percussion brain injury (4.45 ± 0.10 atm) in 30 of 60 randomly selected male Sprague-Dawley rats. A respirator was used only in the event of need. The number of animals with permanent resumption of spontaneous breathing (Type I respiratory response) remained essentially the same in each group. However, when Type II apnea (cannot maintain recovery) and Type III apnea (never recovers from the initial apnea) were terminated with a respirator, all rats with Type II responses from each group were successfully converted to a state of sustained spontaneous breathing. In contrast, only five (41.7%) of the 12 rats with Type III response were salvaged in the control group while five (83.3%) of six Type III rats in the SOD-treated group were saved. The results reveal the nature of the therapeutic effectiveness of superoxide radical scavengers in the overall outcome of head injury in this animal model. While SOD alone did not increase the number of spontaneous survivors, the drug shifted a number of animals from the critically injured rats with Type III respiratory response to the less critical Type II condition. Whereas induced respiration as the sole therapy in the control group lowered the mortality rate to 23.3%, respiratory assistance together with SOD treatment reduced the “mortality” to a single animal with Type III apnea (3.3%) which was alive but still required the respirator after 2 hours (p < 0.001). The results show that respiratory assistance alone accounted for a 33% decrease in mortality rate and that SOD, given in addition to induced ventilation, further decreased mortality by 20%. Since SOD enzymes are reactively specific for superoxide, the increased survival rate of the brain-injured rat must have been due either to preventing or to minimizing pathophysiological changes, probably in the brain stem, caused by oxygen free radicals.
J. Paul Muizelaar, Henk G. van der Poel, Zhongchao Li, Hermes A. Kontos, and Joseph E. Levasseur
✓ Hyperventilation reduces intracranial pressure (ICP) acutely through vasoconstriction, but its long-term effect on vessel diameter is unknown. In seven rabbits with a cranial window implanted 3 weeks earlier, the effect of prolonged hyperventilation on vessel diameter was studied. Anesthesia was maintained for 54 hours with a pentobarbital drip (1 mg/kg/hr). The pH, CO2, and HCO3 − levels were measured in arterial blood and cisterna magna cerebrospinal fluid (CSF). The diameter of 31 pial arterioles was measured with an image splitter. After baseline measurements, pCO2 was reduced from 38 to 25 mm Hg and allowed to return to 38 mm Hg for 10 minutes every 4 hours.
There was an initial vasoconstriction of 13%, which progressively diminished by 3% every 4 hours. Thus, by the 20th hour, vessel diameters at a pCO2 of 25 mm Hg had returned to slightly above baseline values obtained at a pCO2 of 38 mm Hg. The temporary return of pCO2 to 38 mm Hg every 4 hours caused vasodilation: 12% at 4 hours, gradually increasing to 16% at 52 hours. Thus, at 52 hours, the vessel diameters were 105% of baseline at a pCO2 of 25 mm Hg and increased to 122% at a pCO2 of 38 mm Hg. Arterial pH had returned to baseline at 20 hours, and CSF pH had returned at 24 hours. Bicarbonate in blood and CSF remained decreased throughout the experiments. In three control experiments during which normocapnia was maintained, vessel diameter and pH and bicarbonate levels remained unaltered over the same period. The CO2 reactivity, tested by brief periods of hyperventilation every 4 hours, also did not change.
These results indicate that hyperventilation is effective in reducing cerebral blood volume for less than 24 hours and that it should be used only during actual ICP elevations. If used preventively, its effect may have worn off by the time ICP starts to rise for other reasons, and further decreases in pCO2 cannot be obtained. Moreover, the reduction in buffer capacity with lower bicarbonate renders the vessels more sensitive to changes in PaCO2. This could lead to more pronounced elevations in ICP during transient rises in PaCO2, such as during endotracheal suctioning in head-injured patients.
J. Paul Muizelaar, Enoch P. Wei, Hermes A. Kontos, and Donald P. Becker
✓ There is no proof that osmotic agents such as mannitol lower intracranial pressure (ICP) by decreasing brain water content. An alternative mechanism might be a reduction in cerebral blood volume through vasoconstriction. Mannitol, by decreasing blood viscosity, would tend to enhance cerebral blood flow (CBF), but the cerebral vessels would constrict to keep CBF relatively constant, analogous to pressure autoregulation. The cranial window technique was used in this study to measure the pial arteriolar diameter in cats, together with blood viscosity and ICP changes after an intravenous bolus of 1 gm/kg of mannitol. Blood viscosity decreased immediately; the greatest decrease (23%) occurred at 10 minutes, and at 75 minutes there was a “rebound” increase of 10%. Vessel diameters decreased concomitantly, the largest decrease being 12% at 10 minutes, which is exactly the same as the 12% decrease in diameter associated with pronounced hyperventilation (PaCO2 30 to 19 mm Hg) in the same vessels; at 75 minutes vessel diameter increased by 12%. With hyperventilation, ICP was decreased by 26%; 10 minutes after mannitol was given, ICP decreased by 28%, and at 75 minutes it showed a rebound increase of 40%. The correlation between blood viscosity and vessel diameter and between vessel diameter and ICP was very high. An alternative explanation is offered for the effect of mannitol on ICP, the time course of ICP changes, the “rebound effect,” and the absence of influence on CBF, all with one mechanism.
Joseph E. Levasseur, John L. Patterson Jr., Claudia I. Garcia, Michael A. Moskowitz, Sung C. Choi, and Hermes A. Kontos
✓ The frequent occurrence of acute death from pulmonary failure in experimental head injury studies on Sprague-Dawley rats prompted an investigation into the manner in which acute neurogenic pulmonary edema develops in these animals as a result of an applied fluid pressure pulse to the cerebral hemispheres. Studies were performed in adult animals using histamine H1 and H2 blocking agents, or in adult animals treated as neonates with capsaicin to destroy unmyelinated C-fibers. Recordings were made of either the pulmonary arterial or the right ventricular pressure, and the left atrial and femoral arterial pressures before, during, and after injury to provide a record of the hemodynamic response throughout the development of neurogenic pulmonary edema. Head injury triggered the almost immediate development of pressure transients with and without neurogenic pulmonary edema. All rats, regardless of treatment, reacted with nearly identical systemic arterial pressure responses; however, the pulmonary responses followed a time course that was independent of systemic arterial pressure changes. Acute neurogenic pulmonary edema was always associated with a substantial increase in pulmonary arterial and left atrial pressures; conversely, pressure increases of similar magnitude were not always associated with edema. Histamine H1 and H2 blockers significantly reduced the pulmonary pressure surges only in rats free of neurogenic pulmonary edema. All capsaicin-treated rats showed suppressed pulmonary pressure responses, normal lung water content, elevated lung surface tension, and significantly reduced levels of immunoreactive substance P in the spinal cord and vagus nerve. While the pressures cannot clarify how edema influences the observed hemodynamics, they do not support the view that edema is the direct consequence of pulmonary hypertension. It is proposed that neurogenic pulmonary edema is a functional disturbance provoked by adverse stimuli from outside the lungs and that in the rat the primary afferent fiber is essential to the production of this entity.
Robert Macfarlane, Michael A. Moskowitz, Danos E. Sakas, Erol Tasdemiroglu, Enoch P. Wei, and Hermes A. Kontos
✓ Cerebral hyperperfusion, a state in which blood flow exceeds the metabolic needs of brain, may complicate a number of neurological and neurosurgical conditions. It may account for the propensity with which hemorrhage, cerebral edema, or seizures follow embolic stroke, carotid endarterectomy, or the excision of large arteriovenous malformations, and for some of the morbidity that accompanies acute severe head injury, prolonged seizures, and acute severe hypertension. Hyperperfusion syndromes have in common acute increases in blood pressure, vasodilatation, breakdown of the blood-brain barrier, and the development of cerebral edema. These common features suggest the possibility that they share the same pathogenic mechanisms. It was believed until recently that reactive hyperemia was caused primarily by the generation of vasoactive metabolites, which induced vasodilatation through relaxation of vascular smooth muscle. However, the authors have recently established that the release of vasoactive neuropeptides from perivascular sensory nerves via axon reflex-like mechanisms has a significant bearing upon a number of hyperperfusion syndromes. In this article, the authors summarize their data and discuss possible therapeutic implications for blockade of these nerves or their constituent neuropeptides.
Yuji Ueda, Enoch P. Wei, Hermes A. Kontos, Eiichi Suehiro, and John T. Povlishock
Object. In the experimental setting, hypothermia has been demonstrated to attenuate the damaging consequences of stroke and traumatic brain injury (TBI). Laboratory studies of TBI have focused primarily on the use of early hypothermic intervention, with little consideration of the potential efficacy of more delayed but prolonged hypothermia, which would constitute a more clinically relevant approach. In this investigation, the authors evaluated whether delayed, prolonged hypothermia after TBI protected the cerebral microcirculation.
Methods. Male Sprague—Dawley rats were equipped with cranial windows for direct visualization of the pial arterial circulation and then subjected to impact-acceleration brain injury. The rats were randomly divided into four experimental groups: Group 1 consisted of normothermic animals; in Group 2 the rats received a 1-hour period of hypothermia (32°C) 30 minutes posttrauma, followed by slow rewarming (32–37°C/90 minutes); and in Groups 3 and 4 the rats received a more delayed induction (at 1 hour postinjury) of either 1 hour (Group 3) or 2 hours (Group 4) of hypothermia, followed by the slow rewarming. The pial arteriolar responses to acetylcholine (ACh) or hypercapnia were measured until up to 6 hours postinjury. With this approach the authors found that the normothermic group demonstrated severely impaired vasoreactivity in terms of ACh-dependent dilation and CO2 reactivity in comparison to baseline values (p < 0.001). In contrast, hypothermia of short duration that was initiated early (30 minutes postinjury) conferred significant cerebrovascular protection (p < 0.001), yet this protection was reduced when the onset of this 1-hour hypothermic period was postponed to 1 hour postinjury. Nevertheless, reduced protection could be significantly improved (p < 0.001) with prolongation of the hypothermic period to 2 hours.
Conclusions. The results of this study show that early as well as delayed but prolonged hypothermia attenuate the impaired vascular responsiveness seen after TBI, indicating the potential clinical usefulness of this treatment.
Douglas S. DeWitt, Larry W. Jenkins, Enoch P. Wei, Harry Lutz, Donald P. Becker, and Hermes A. Kontos
✓ The effects of two levels of fluid-percussion brain injury on cerebral blood flow (CBF) and pial arteriolar diameter were investigated in cats. Regional CBF was measured using the radioactive microsphere technique. Experimental brain injury resulted in changes in arterial blood pressure, CBF, and pial arteriolar diameter that were related to the severity of the injury. Low-level injury (1.88 ± 0.11 atm, mean ± standard error of the mean) resulted in a slight transient increase in CBF which had returned to preinjury levels by 30 minutes. High-level injury (2.68 ± 0.19 atm) resulted in larger, statistically significant (p < 0.01) increases in whole-brain CBF, decreases in cerebrovascular resistance, and increases in pial arteriolar diameter 1 minute postinjury. One hour after injury, CBF had returned to preinjury levels while cerebral perfusion pressure was significantly (p < 0.01) reduced. There was no evidence of reduced CBF in any region studied. Pial arterioles dilated during the posttraumatic hypertensive period and then returned to control diameters within 1 hour after injury. Changes in the diameter of pial arterioles were significantly correlated with posttraumatic changes in CBF.