Hermes A. Kontos

Validity of cerebral arterial blood flow calculations from velocity measurements.

It is obvious that a noninvasive technique that can provide continuous measurements of blood flow through a cerebral artery would be very valuable for clinical and investigative purposes. The most promising approach to the development of such a technique involves the use of ultrasound for the measurement of blood flow velocity. Since blood flow through a vessel is equal to the instantaneous average velocity times the cross-sectional area of the vessel, monitoring of velocity and vessel caliber provides the essential measurements that are needed to estimate blood flow. Busija et al showed that such a method is both accurate and useful. They measured blood flow velocity in a cerebral artery in cats and dogs with an ultrasonic Doppler probe placed around the artery, and simultaneously they measured vessel diameter by direct visualization. They found that blood flow calculated from the velocity and vessel caliber correlated exceedingly well with blood flow measured directly with radioactive microspheres. In this issue of Stroke, Aaslid and his colleagues measured blood flow velocity in the middle cerebral artery of human volunteers to investigate the responses of cerebral blood flow to rapid decreases in arterial blood pressure. The approach developed by these authors involves certain important assumptions that can have a critical influence on the validity of the results. 1 examine the soundness of these assumptions and their potential consequences on the usefulness of the technique. Two important features of the method used by Aaslid et al should be borne in mind. First, these authors did not measure vessel caliber, and they assumed that the middle cerebral artery had a constant cross-sectional area throughout the interventions they used. Second, they did not measure average velocity through the middle cerebral artery but maximum velocity. The authors justified their assumption of an unchanging vessel caliber in the

Functional, morphological, and metabolic abnormalities of the cerebral microcirculation after concussive brain injury in cats.

We induced experimental concussive brain injury by a fluid percussion device in anesthetized cats equipped with a cranial window for the observation of the pial microcirculation of the parietal cortex. Brain injury resulted in transient but pronounced increases in arterial blood pressure and in sustained arteriolar vasodilation associated with reduced or absent responsiveness to the vasoconstrictor effect of arterial hypocapnia and with reduced or absent ability of the vessels to undergo autoregulatory vasodilation in response to reductions in arterial blood pressure. Such vessels had reduced resting oxygen consumption in vitro. Electron microscopic examination of the same vessels that were studied physiologically disclosed the presence of discrete endothelial lesions consisting of either vacuolization or crater formation. Occasionally there was extensive destruction and necrosis of the endothelial cells. There was little or no morphological evidence of vascular smooth muscle damage. There was a close association between the presence of endothelial lesions and vessel dilation and unresponsiveness, suggesting a causal relationship. In cats in which the transient posttraumatic hypertensive episode was prevented, the vessels retained their normal caliber, remained normally responsive, and had no endothelial lesions. The results show that the vascular lesions in the pial microcirculation following this type of brain injury are due to the rise in arterial pressure. Circ Res 46: 37-47, 1980

Appearance of superoxide anion radical in cerebral extracellular space during increased prostaglandin synthesis in cats.

When increased prostaglandin synthesis was induced in anesthetized cats equipped with cranial windows by topical application of arachidonate (200 μg/ml) or bradykinin (20 μg/ml), there was reduction of nitroblue terrazolium, resulting in deposition of the reduced insoluble form of this dye on the brain surface. The amount of reduced nitroblue terrazolium deposited on the brain surface was measured spectrophotometrically after fixation of the brain by perfusion with aldehydes to eliminate interference from hemoglobin. Topical application of 56 U/ml superoxide dismutase or 20 μg/ml indomethatin inhibited nitroblue terrazolium reduction by 76.5%–82.5% and by 78%–85.5%, respectively. These results show that most of the nitroblue terrazolium reduction was accounted for by superoxide anion radical generated in the course of arachidonate metabolism via the cyclooxygenase pathway. No superoxide production could be detected in the absence of arachidonate or bradykinin. Histological examination showed no evidence of parenchymal cellular damage or vascular damage and no accumulation of leukocytes. Pronounced leukocyte accumulation occurred 24 hours after topical arachidonate in rabbits with chronically implanted cranial windows. Superoxide appearance was reduced severely by 4, 4′-diisothiocyano-2, 2′-stilbene disulfonate and phenylglyoxal, two specific inhibitors of the anion channel. The most likely explanation for these findings is that increased metabolism of exogenous or endogenous arachidonate via cyclooxygenase results in the appearance of superoxide anion radical in cerebral extracellular space. Superoxide crosses the membrane of undamaged cells via the anion channel.

Analysis of vasoactivity of local pH, PCO2 and bicarbonate on pial vessels.

The mechanism by which the local effect of CO2 on pial arterioles is exerted was examined in anesthetized cats equipped with a cranial window for the direct observation of the microcirculation of the parietal cortex. The dilation of pial arterioles in response to application of artificial cerebrospinal fluid with low pH was the same whether or not the Pco2 of the solution was maintained in the normal range or markedly increased. The constriction of pial arterioles in response to application of artificial cerebrospinal fluid with high pH was the same whether or not the Pco2 of the solution was maintained in the normal range or markedly decreased. Finally, pial arterioles did not change their caliber in response to application of cerebrospinal fluid with unchanged pH but markedly increased or decreased Pco2, or bicarbonate ion concentration. These results show that the action of CO2 on cerebral vessels is exerted via changes in extracellular fluid pH and that molecular CO2 and bicarbonate ions do not have independent vasoactivity on these vessels.

Calcitonin gene-related peptide mediates nitroglycerin and sodium nitroprusside-induced vasodilation in feline cerebral arterioles.

The cerebral vasodilator response induced by topical nitroglycerin and nitroprusside was examined in cats equipped with cranial windows for the observation of the cerebral microcirculation. In cats subjected to chronic unilateral trigeminal ganglionectomy, the vasodilator responses to nitroprusside and nitroglycerin were markedly depressed on the denervated side. Application of a selective calcitonin gene-related peptide (CGRP) antagonist [CGRP(8-37)] on the innervated side reduced the response to nitrodilators to the same extent as seen on the denervated side. The vasodilator response to acetylcholine was unaffected by trigeminal ganglionectomy. CGRP(8-37) almost abolished the vasodilator response to nitroglycerin and sodium nitroprusside and to CGRP, but did not affect the response to adenosine or to adenosine diphosphate. Pretreatment with LY83583, a drug that lowers cyclic GMP levels, diminished the vasodilation to CGRP and to nitroprusside but not to adenosine. We conclude that the nitrovasodilators activate sensory fibers to release CGRP, which in turn relaxes cerebral vascular smooth muscle by activating guanylate cyclase. Hence, nitrovasodilators possess a novel mechanism of action within the cephalic circulation which may explain both the occurrence of vasodilation and headache.

Oxygen radicals mediate the cerebral arteriolar dilation from arachidonate and bradykinin in cats.

Topical application of sodium arachidonate (50–200 μg/ml) or bradykinin (0.1–10 μg/ml) on the brain surface of anesthetized cats caused dose-dependent cerebral arteriolar dilation. This dilation was blocked by 67–100percent; in the presence of superoxide dismutase and catalase. These enzymes did not affect the changes in arteriolar diameter caused by alterations in arterial blood Pco2/ or the arteriolar dilation from topical acetylcholine. Enzymes inactivated by heat had no effect on the vasodilation from arachidonate or bradykinin. Superoxide dismutase alone or catalase alone reduced the dilation during application of 200 Mg/ml of arachidonate for 15 minutes; they also completely prevented the residual dilation seen 1 hour after washout, as well as the reduction in the vasoconstrictive effects of arterial hypocapnia observed at this time. The results show that superoxide anion radical and hydrogen peroxide, or radicals derived from them, such as the hydroxyl radical, are mediators of the cerebral arteriolar dilation from sodium arachidonate or bradykinin. These radicals are not the endothelium-derived relaxant factor released by acetylcholine. The presence of both superoxide anionradical and hydrogen peroxide is required for the production of the vascular damage seen during prolonged application of high concentrations of sodium arachidonate.

Detailed Description of a Cranial Window Technique for Acute and Chronic Experiments

Methods for implantation of cranial windows for the direct observation of the pial microcirculation in experimental animals are described in detail. These techniques are suitable for both acute experiments in anesthetized animals and chronic implantation permitting several months of observation in awake animals. Experience over several years shows that these techniques have an acceptably low rate of failure, are low in cost and can easily be mastered in most laboratories. They make possible observation of the microcirculation and accurate measurement of the diameter of pial vessels, and permit study of the effects on the microcirculation of a variety of maneuvers and vasoactive agents which can be studied by direct application as well as by intravascular administration. Because they preserve the integrity of the skull, the techniques permit study of the cerebral microcirculation under conditions closely approximating the normal environment of these vessels.

Oxygen radicals in CNS damage

The products of univalent reduction of oxygen, superoxide anion radical, hydrogen peroxide, and the hydroxyl radical, are capable of causing cellular damage and death. They are, therefore, logical candidates as mediators of vascular and parenchymal injury in the central nervous system (CNS). This paper reviews the sources of oxygen radicals in the CNS, their effects on cerebral vessels and on brain and spinal cord parenchyma, and the evidence which implicates oxygen radicals in various pathological conditions of the CNS.

GENERAL AND REGIONAL CIRCULATORY ALTERATIONS IN CIRRHOSIS OF THE LIVER.

Abstract The general circulation and the circulation of the upper extremity were studied in sixteen patients with cirrhosis of the liver and in eight healthy control subjects. The cardiac index was increased in ten cirrhotic patients and normal in six. In patients with normal cardiac indices, hand and forearm blood flows and arteriovenous oxygen difference for forearm deep venous blood were not significantly different from corresponding values for the control group. Based on hand and forearm blood flows and arteriovenous oxygen differences for deep and superficial venous blood in the forearm, three types of peripheral circulatory abnormalities were found among the group of patients with high cardiac index: type A was characterized by vasodilatation in skin and skeletal muscle, type B by vasodilatation in skeletal muscle only, and type C by vasodilatation in skin only. The possible causes of these circulatory abnormalities are discussed. It is suggested that multiple causes rather than a single one are probably operative. Hypocapnia could account for the increased cardiac index in patients with vasodilatation in skeletal muscle only (type B).

Cyclooxygenase Products of Arachidonic Acid Metabolism in Cat Cerebral Cortex After Experimental Concussive Brain Injury

Abstract Previous studies have suggested that following experimental fluid percussion brain injury, increased prostaglandin (PG) synthesis, with its concomitant production of oxygen free radicals, causes functional and morphological abnormalities of the cerebral arterioles. The purpose of this study was to chemically determine if PGs are altered following this injury. To facilitate interpretation of neurochemical measurements the cats were ventilated, blood pressure was measured, and a cranial window, for microscopic observation of pial arteriolar diameter was inserted. PG levels were determined in quick‐frozen cortical tissue removed from control and 3 groups of injured cats at 1.5, 8.0, and 60 min after injury. Analysis of PGE2, PGF2α, and 6‐keto‐PGFlα was performed by HPLC and GC/MS. The control levels of PGE2, PGF2α, and 6‐keto‐PGF1α, were 216 ± 44, 210 ± 48, and 48 ± 12 ng/g wet weight, respectively. Following injury, produced by a 22 ms increase in intracranial pressure, the pial arterioles dilated irreversibly and a transient hypertensive response occurred, thereby producing hyperemia. During the maximum hyperemic response, the total PGs were 75% of control. At 8 min after injury, when blood pressure returned to control level, the PGs were 158% of control and PGs fell to 111% of control at 60 min. These experiments supported our previous studies implicating increased PG synthesis in the genesis of the physiologic and morphologic sequelae of experimental concussive brain injury.