Derangement of regional cerebral blood flow and of its regulatory mechanisms in acute cerebrovascular lesions

Microregional blood flow changes in experimental cerebral ischemia. Effects of arterial carbon dioxide studied by fluorescein angiography and xenon 133 clearance

✓ A branch of the middle cerebral artery on the convexity of the dog brain was occluded to produce an area of focal cerebral ischemia which could then be defined by fluorescein angiography of the brain. Repeated fluorescein angiography and measurement of microregional cerebral blood flow by xenon133 injected into the carotid artery and monitored by miniature lithium-drifted silicon detectors for gamma activity demonstrated that the ischemic zone was reduced in size by better collateral flow when the animals were allowed to breathe 5% carbon dioxide and 95% oxygen. Conversely, hyperventilation reducing the pCO2 made the ischemic zone larger by reducing collateral flow. No evidence was found to indicate that hypercapnia preferentially deprived the ischemic zone of perfusion flow. Retrograde collateral flow in the surface arteries appeared effective in terms of microcirculatory perfusion.

Physiological modification of immediate ischemia due to experimental middle cerebral occlusion--its relevance to cerebral infarction

In cats subjected to middle cerebral artery occlusion, the resulting ischemia was unaffected by the blood pressure at the time of occlusion when arterial P CO CO 2 was normal or low. At normal and elevated blood pressures, hypercapnia established prior to occlusion minimized the ischemia and hypocapnia aggravated it. Re-occlusion during postischemic reactive hyperemia resulted in ischemia of the same severity as during the initial occlusion, provided P CO CO 2 and blood pressure were not changed. These observations suggest that a general determinant of the severity of immediate ischemia in this preparation is the competence of the collateral circulation.

This conclusion suggests that proper analysis of the pathogenesis of cerebral infarction requires consideration of immediate ischemia separately from the subsequent course of the ischemic lesion. There are so many variables bearing on the pathogenesis of cerebral infarction that a rational therapy of acute stroke can only be visualized when there is knowledge of the specific arterial lesion, frequent or continuous measurement of regional cerebral blood flow, regional metabolism or neuronal activity, intracranial pressure, and some way of distinguishing paralyzed but viable tissue from infarction.

Increase in regional cerebral blood flow following experimental arterial air embolism

✓ Acute intracranial hemodynamic alterations consequent to arterial air embolism were studied in the dog using the radioxenon clearance technique. In eight dogs, the mean pre-embolic (control) hemispheric flow was 28.3 ml/100 gm/min. Following the injection of varying amounts of air into the right vertebral artery, there was an augmentation in the measured cerebral blood flow in all animals. Statistical analysis revealed the flow response to be independent of the amounts of air used in these experiments. The mean post-embolic cerebral blood flow was 39.3 ml/100 gm/min, representing a statistically significant increase of 11.0 ml/100 gm/min. Although the post-embolic supernormal flow may be due to the interaction of multiple pathophysiologic factors, air-induced traumatic vasodilatation is advocated as the most important pathogenetic mechanism. Prolonged vasodilatation with loss of autoregulation results in physiological shunting of blood through the affected capillary beds. Alterations in the intracerebral vasculature due to arterial air embolism are compared with studies by other investigators who have observed the effects in extracerebral vessels.

The relation between cerebral oxygen consumption and cerebral vascular reactivity to carbon dioxide

The mechanisms whereby CO 2 affects cerebral vessels are not as simple as once thought, 1, 2 and are probably both directly on cerebral vascular walls 3, 4 and indirectly by action on brain stem neurones. 5 Furthermore, cerebral vascular reactivity to CO 2 has been reported to be altered by a number of physiological and pathological circumstances. The capacity to dilate to increased Pa CO CO2 is decreased in cerebral vascular lesions, 6-8 and is affected by changes in cerebral perfusion pressure. 9 Cervical sympathectomy is said to increase CBF response to Pa CO CO2 changes, whereas sympathetic nerve stimulation abolishes reactivity to CO 2 . 10 Deep anesthesia, hypothermia, or trauma to brain reduces reactivity to cerebral vessels to CO 2 , the one common denominator for these states being reduced cerebral metabolism. This report demonstrates that the capacity of cerebral vessels to dilate or constrict in response to changes in Pa CO CO2 is influenced by cerebral oxygen consumption.

Induced seizures as therapy of experimental strokes in dogs

✓ Intracerebral pO2, as measured in normal dog brains by a modified mass spectrometer, was found to increase following seizure activity and remain elevated at least 2 hours. These results were found with both drug- and electrically-induced seizures. The pO2 increased to a greater degree in brain tissue rendered ischemic by middle cerebral artery occlusion. A transient reflex hypertension was observed with seizure activity, but hypertension alone failed to produce significant pO2 changes. Since oxidative metabolism has been shown by other investigators to proceed at an elevated rate during seizure activity, the increased pO2 must reflect improved collateral circulation following seizure activity.

Effect of Pa CO2 on blood flow and microvasculature of ischemic and nonischemic cerebral cortex

The right middle cerebral artery (MCA) was occluded in cats; after subsequent craniectomy cortical blood flow (CBF) was measured bilaterally with 85 Kr, and photographs of the superficial microvasculature were made. Ventilation was controlled, and Pa CO CO2 was altered by changing the concentration of CO 2 in the inspired air. In nonischemic cortex CBF varied as an exponential function of Pa CO CO2 , and the caliber of superficial arterial vessels (50 to 200µ in diameter) increased with increasing Pa CO CO2 . In ischemic cortex, changes of Pa CO CO2 produced no change of CBF in six of ten animals studied within one day of occlusion; in four of these six, there was no change in the caliber of arterial vessels. In the four other animals of this group, there was a paradoxical response (an increase of Pa CO CO2 produced a decrease of CBF of ischemic cortex), and in two of these four animals, there also was a paradoxical response of the caliber of arterial vessels. In eight animals allowed to survive 5 to 12 days after MCA occlusion, the arterial vessels of ischemic cortex regained some reactivity: a normal response of CBF to changes of Pa CO CO2 was found in four, and appropriate changes of vessel caliber were found in all eight. The ischemia-induced impairment of the reactivity of cortical vessels to changes of Pa CO CO2 casts doubt on the usefulness of CO 2 inhalation for the treatment of strokes of humans.