Relating cerebral ischemia and hypoxia to insult intensity

Measurement of relative cerebral blood volume using BOLD contrast and mild hypoxic hypoxia

Relative cerebral blood volume (CBV) was estimated using a mild hypoxic challenge in humans, combined with BOLD contrast gradient-echo imaging at 3 T. Subjects breathed 16% inspired oxygen, eliciting mild arterial desaturation. The fractional BOLD signal change induced by mild hypoxia is expected to be proportional to CBV under conditions in which there are negligible changes in cerebral perfusion. By comparing the regional BOLD signal changes arising with the transition between normoxia and mild hypoxia, we calculated CBV ratios of 1.5 ± 0.2 (mean ± S.D.) for cortical gray matter to white matter and 1.0 ± 0.3 for cortical gray matter to deep gray matter.

The BOLD response and vascular reactivity during visual stimulation in the presence of hypoxic hypoxia

A disproportionate increase in cerebral blood flow (CBF) relative to the cerebral metabolic rate of oxygen (CMRO(2)), in response to neuronal activation, results in a decreased oxygen extraction fraction (OEF) and hence local 'hyperoxygenation'. The mismatch is the key 'physiological substrate' for blood oxygenation level dependent (BOLD) fMRI. The mismatch may reflect inefficient O(2) diffusion in the brain tissue, a factor requiring maintenance of a steep [O(2)] gradient between capillary bed and neural cell mitochondria. The aim of this study was to assess vascular responsiveness to reduced blood oxygen saturation, using both BOLD fMRI and the CBV-weighted vascular space occupancy (VASO)-dependent fMRI technique, during visual activation in hypoxic hypoxia. Our fMRI results show decreased amplitude and absence of initial sharp overshoot in the BOLD response, while VASO signal was not influenced by decreasing oxygen saturation down to 0.85. The results suggest that the OEF during visual activation may be different in hypoxia relative to normoxia, due to a more efficient oxygen extraction under compromised oxygen availability. The data also indicate that vascular reactivity to brain activation is not affected by mild hypoxia.

Effects of hypoxia, hyperoxia, and hypercapnia on baseline and stimulus-evoked BOLD, CBF, and CMRO2 in spontaneously breathing animals

Functional magnetic resonance imaging (fMRI) was used to investigate the effects of inspired hypoxic, hyperoxic, and hypercapnic gases on baseline and stimulus-evoked changes in blood oxygenation level-dependent (BOLD) signals, cerebral blood flow (CBF), and the cerebral metabolic rate of oxygen (CMRO2) in spontaneously breathing rats under isoflurane anesthesia. Each animal was subjected to a baseline period of six inspired gas conditions (9% O2, 12% O2, 21% O2, 100% O2, 5% CO2, and 10% CO2) followed by a superimposed period of forepaw stimulation. Significant stimulus-evoked fMRI responses were found in the primary somatosensory cortices. Relative fMRI responses to forepaw stimulation varied across gas conditions and were dependent on baseline physiology, whereas absolute fMRI responses were similar across moderate gas conditions (12% O2, 21% O2 100% O2, and 5% CO2) and were relatively independent of baseline physiology. Consistent with data obtained using well-established techniques, baseline and stimulus-evoked CMRO2 were invariant across moderate physiological perturbations thereby supporting a CMRO2-fMRI technique for non-invasive CMRO2 measurement. However, under 9% O2 and 10% CO2, stimulus-evoked CBF and BOLD were substantially reduced and the CMRO2 formalism appeared invalid, likely due to attenuated neurovascular coupling and/or a failure of the model under extreme physiological perturbations. These findings demonstrate that absolute fMRI measurements help distinguish neural from non-neural contributions to the fMRI signals and may lend a more accurate measure of brain activity during states of altered basal physiology. Moreover, since numerous pharmacologic agents, pathophysiological states, and psychiatric conditions alter baseline physiology independent of neural activity, these results have implications for neuroimaging studies using relative fMRI changes to map brain activity.

Effects of glucose and PaO2 modulation on cortical intracellular acidosis, NADH redox state, and infarction in the ischemic penumbra

BACKGROUND AND PURPOSE

During focal cerebral ischemia, the ischemic penumbra or border-zone regions of moderate cortical blood flow reductions have a heterogeneous development of intracellular cortical acidosis. This experiment tested the hypotheses that (1) this acidosis is secondary to glucose utilization and (2) this intracellular acidosis leads to recruitment of potentially salvageable tissue into infarction.

METHODS

Brain pHi, regional cortical blood flow, and NADH redox state were measured by in vivo fluorescent imaging, and infarct volume was assessed by triphenyltetrazolium chloride histology. Thirty fasted rabbits divided into 6 groups of 5 each were subjected to 4 hours of permanent focal ischemia in the presence of hypoglycemia ( approximately 2.8 mmol/L), moderate hyperglycemia ( approximately 11 mmol/L), and severe hyperglycemia (>28 mmol/L) under either normoxia or moderate hypoxia (PaO2 approximately 50 mm Hg).

RESULTS

Preischemic hyperglycemia led to a more pronounced intracellular acidosis and retardation of NADH regeneration than in the hypoglycemia groups under both normoxia and moderate hypoxia in the ischemic penumbra. For example, 4 hours after ischemia, brain pHi in the severe hyperglycemia/normoxia group measured 6.46, compared with 6.84 in the hypoglycemia/normoxia group (P<0.01), and NADH fluorescence measured 173% compared with 114%. Infarct volume in the severe hyperglycemia/normoxia group measured 35.1+/-6.9% of total hemispheric volume, compared with 13.5+/-4.2% in the hypoglycemia/normoxia group (P<0.01).

CONCLUSIONS

Hyperglycemia significantly worsened both cortical intracellular brain acidosis and mitochondrial function in the ischemic penumbra. This supports the hypothesis that the evolution of acidosis in the ischemic penumbra is related to glucose utilization. Furthermore, the observation that hypoglycemia significantly decreased infarct size supports the postulate that cortical acidosis leads to recruitment of ischemic penumbra into infarction.

Simultaneous measurements of lactate and blood flow during hypoxia and recovery from hypoxia in a localized region in the brain of the anesthetized rabbit

We observed simultaneous changes in lactate level and regional blood flow (rBF) in the brain of the anesthetized rabbit by using localized proton magnetic resonance spectroscopy (1H MRS) and laser Doppler flowmetry. The volume of interest of 0.5 ml for 1H MRS contained mostly thalamic nuclei. During hypoxia peak area for lactate increased up to 57% of that from N-Acetylaspartate. While the rBF increased during hypoxia up to 260% of the control, oxygen delivery (rBF x arterial oxygen content) decreased. In the normoxic recovery period following hypoxia, the rBF recovered slowly and a consequent overshoot of oxygen delivery was observed. The multiple and stepwise linear regression analyses revealed that the averaged decrease in oxygen delivery during hypoxia was the most significant independent variable for the increase in lactate during hypoxia (correlation coefficient; r2 = 0.68) and also that the increase in lactate during hypoxia was the most significant independent variable for the time for half-recovery of rBF (r2 = 0.75). These results suggest that the increase in lactate during hypoxia is due to the deficiency of oxygen delivery and that the increase in lactate during hypoxia prolongs the period of enhancement of rBF during recovery from hypoxia.

Model for short-term intracranial pressure changes following traumatic injury

Results from primate studies show a transient increase in intracranial pressure (ICP) after a nonimpact inertial loading condition. The measured ICP increase varies linearly with the peak tangential load of these experiments. These experiments point to possible alterations in cerebral blood flow. This paper investigates the possible etiology of this particular phenomenon, and presents a simple analytical model that could explain the changes in intracranial pressure. The model combines the effects of cerebral venous constriction, arterial dilatation, and raised mean blood pressure to yield the characteristic immediate rise and exponential decay of ICP. The main contributor to the increase in intracranial pressure is believed to be vasodilation of cerebral arteries following venous constriction. Passive release of cerebrospinal fluid (CSF) is believed to mediate the long-term decay of intracranial pressure and possibly contribute to local hyperemia.

Outcome after ischemia in the developing sheep brain: an electroencephalographic and histological study

The role of seizures occurring with perinatal hypoxic-ischemic encephalopathies is unclear. We examined the relationships between the time course of parasagittal electroencephalographic (EEG) activity and pathological outcome following transient cerebral ischemia, which was induced in 33 chronically instrumented fetal sheep by occluding the carotid arteries after ligation of the vertebral-carotid anastomoses. The EEG was quantified with real-time spectral analysis. Histological outcome was assessed 72 hours later. After 10 or 20 minutes of ischemia, EEG activity was depressed and then progressively recovered and mild selective neuronal loss was seen. The length of this depression correlated with the duration of ischemia (r = 0.88). After 30 or 40 minutes of ischemia, EEG activity remained depressed for 8 +/- 2 hours, followed by a rapid transition to low-frequency epileptiform activity that reached maximum intensity at 10 +/- 3 hours. By 72 hours, EEG intensity had fallen below control levels. This sequence of prolonged depression, epileptiform activity, and then loss of intensity was associated with the development of laminar necrosis of the underlying cortex. These electrophysiological sequelae may have prognostic value. The results indicate that after a severe hypoxic-ischemic insult, the parasagittal cortex becomes hyperexcitable before the final loss of activity. Secondary neuronal death may occur in this phase.

Recovery of postischemic brain metabolism and function following treatment with a free radical scavenger and platelet-activating factor antagonists

We have studied the metabolic and functional effects of two new platelet-activating factor (PAF) antagonists (BN 50726 and BN 50739) and their diluent (dimethyl sulfoxide; DMSO) during reoxygenation of the 14-min ischemic isolated brain. Blood gases, EEG, auditory evoked potentials, cerebral metabolic rate for glucose (CMRglc), and cerebral metabolic rate for oxygen (CMRO2) were monitored throughout the study. Frozen brain samples were taken for measurement of brain tissue high-energy phosphates, carbohydrate content, and thiobarbituric acid-reactive material (TBAR, an indicator of lipid peroxidation) at the end of the study. Following 60 min of reoxygenation in the nontreated 14-min ischemic brains, lactate, AMP, creatine (Cr), intracellular hydrogen ion concentration [H+]i), and TBAR values were significantly higher and ATP, creatine phosphate (PCr), CMRglc, CMRO2, and energy charge (EC) values were significantly lower than the corresponding normoxic control values. PCr and CMRO2 values were significantly higher, and glycogen, AMP, and [H+]i values were significantly lower in the BN 50726-treated ischemic brains than in DMSO-treated ischemic brains. In brains treated with BN 50739, ATP, ADP, PCr, CMRO2, and EC values were significantly higher, and lactate, AMP, Cr, and [H+]i values were significantly lower than corresponding values in the DMSO-treated ischemic brains. TBAR values were near control levels in all brains exposed to DMSO. There was also marked recovery of EEG and auditory evoked potentials in brains treated with DMSO. Treatment with BN 50726 or BN 50739 in DMSO appeared to improve brain mitochondrial function and energy metabolism partly as the result of DMSO action as a free radical scavenger.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased cerebral lactate output to cerebral venous blood after forebrain ischemia in rats

Increased cerebral lactate levels are a well-known aspect of the sequelae of the metabolic derangements that follow cerebral ischemia. A new technique has recently become available to sample cerebral venous blood from the superior sagittal sinus on a long-term basis in conscious rats. We report the applicability of this method to assess serial biochemical responses to brain injury. Serum samples were obtained from the superior sagittal sinus, the common carotid artery, and the external jugular vein of nine anesthetized rats before and up to 7 days after 10 minutes of forebrain ischemia was produced by carotid occlusion and hypovolemic hypotension (mean arterial blood pressure 50 +/- 4 mm Hg). The cerebral venous-arterial difference in serum lactate concentration was increased for up to 3 hours after ischemia, while there was no significant change in the difference in serum lactate concentrations in the common carotid artery and the external jugular vein. This indicates an elevated output of lactate from brain tissue to blood, detectable only in the superior sagittal sinus, which underlines the usefulness of the technique. We observed a persistent elevation in brain lactate production after virtually complete recovery from the acute insult.

Relationship of calcium and adenylate cyclase messenger systems in rat brain synaptosomes

The effects of cyclic AMP on the rise in cytosolic free calcium concentration, [Ca2+]i, after stimulation with 15 mM K+ in rat brain synaptosomes were investigated. The fluorescent chelating agent Quin-2 was employed to monitor alterations of K+-evoked [Ca2+]i. Under normoxic conditions, clonidine (1, 10 microM), an alpha 2-adrenoceptor agonist, decreased the 15 mM K+-evoked [Ca2+]i. Although yohimbine (1, 10 microM), an alpha 2-adrenoceptor antagonist, had little or no effect on K+-evoked [Ca2+]i, the inhibitory effects of clonidine were blocked by yohimbine. 8-Bromo cyclic AMP, a cyclic AMP analogue, (50-500 microM), increased K+-evoked [Ca2+]i in a dose-dependent manner. The addition of cyclic AMP analogues subsequent to clonidine treatment reversed the clonidine-induced suppression of K+-evoked [Ca2+]i. On the other hand, under hypoxic conditions, K+-evoked [Ca2+]i was reduced by about 50-60%. 8-Bromo cyclic AMP and the adenylate cyclase activators, yohimbine (1-10 microM) and isoproterenol, a beta-adrenoceptor agonist, (0.1-10 microM), transiently reversed the reduction of the K+-evoked [Ca2+]i caused by hypoxia. These results indicate that the activation of alpha 2-adrenoceptor produces a rapid, sustained decrease in [Ca2+]i which may be due to a decrease in the levels of intracellular cyclic AMP. In addition, the increase in cellular levels of cyclic AMP reversed the reduction of the Ca2+ response to high K+ stimulation caused by hypoxia. If this is so, there is the possibility that increased cyclic AMP might improve the hypoxic damage.