Physiology of the cerebral circulation

The brain has ongoing, substantial energy requirements but minimal stores of energy-generating substrates. As a result, it is completely dependent on a continuous, uninterrupted supply of substrate (oxygen, glucose). Fortunately, under physiological conditions (normal intercranial haemodynamics, normal cardiorespiratory function), this requirement is provided more than adequately by the 15% of the normal cardiac output which perfuses the brain. However, even physiologically nothing is absolutely static: there are the everyday fluctuations in posture, autonomic nervous activity, wakefulness, concentration, mental/physical activity and so on which have an impact on the cerebral circulation. Once again, however, the mechanisms intrinsic to the cerebral circulation can adapt appropriately. The chapter describes the physical, and physiological, contributions which can influence, and modulate, the cerebral circulation in the normal, healthy individual. In addition, and where appropriate, the underlying mechanisms are discussed briefly.     

Posterior cerebral circulation revascularization

Posterior circulation revascularization has evolved as a method to treat selected patients with vertebrobasilar ischemia who have inaccessible atherosclerotic occlusive disease and who have failed maximal medical therapy. In addition, complex unclippable aneurysms of the posterior circulation are another indication for revascularization of the vertebrobasilar territory. Careful preoperative evaluation and meticulous attention to detail intraoperatively yield good patient outcomes with minimal morbidity and mortality. This article reviews the vascular anatomy of the posterior circulation and the indications, preoperative evaluation, operative techniques, clinical outcomes, and alternative treatments for patients requiring posterior circulation revascularization procedures.     

Angiography of collateral cerebral circulation

Zusammenfassung Während die klinischen Symptome, der Verlauf und die pathologischanatomischen Untersuchungen bei Gefäßverschlüssen uns die Lokalisation des Prozesses ermöglichen, gibt uns die cerebrale Angiographie die Möglichkeit, die Vaskularisationsverhältnisse, insbesondere die Bildung des Kollateralkreislaufes in vivo zu studieren.Die angiographischen Untersuchungen bestätigen die Befunde der Anatomen, daß die Hirnarterien nicht Endarterien sind, sondern mehrere Vaskularisationsringe aufweisen, die wieder miteinander an verschiedenen Stellen kommunizieren.Jede der 4 großen Hirnarterien ist imstande, falls keine angeborenen Anomalien vorliegen, die anderen über die zahlreich vorhandenen Kollateralen zu kompensieren.Die zuerst vonCooper (1831) vorgenommene Ligatur der 4 Hirnarterien bei Hunden und die vielen späteren Versuche bestätigen die Kollaterale-Bildung über die Haut-, Muskel- und Spinalarterien, welche bei in phylogenetisch höheren Stufen stehenden Menschen nicht in gleichem Maße vorhanden ist. Seit der ersten Ligatur beim Menschen der A. carotis (Warner-Ellis, 1775) und der A. vertebralis (Maissonneuve-Favrod, 1852), wurde das Fehlen, Aplasie und Ligatur einer oder 2 der Hirnarterien ohne Folgeerscheinungen durch die Anatomen, Kliniker und Röntgenologen in zahlreichen Fällen publiziert.Die Bildung des kollateralen Kreislaufes ist nicht allein von hydro- bzw. hämodynamischen Gesetzen abhängig, sondern von weiteren verschiedenen anderen Faktoren, welche ausführlich anhand des eigenen Materials und desjenigen der einschlägigen Literatur besprochen wurde.In der vorliegenden Arbeit werden die Kollateralen innerhalb der intracraniellen Vaskularisationsringe untereinander und derjenigen des extracraniellen Vaskularisationsringes anhand eigener Angiogramme und derjenigen der Literatur besprochen. Die Bedeutung der Kollateralen über die Äste der A. carotis externa zur A. ophthalmica und A. vertebralis wird besonders hervorgehoben.Unter Berücksichtigung dieser Tatsachen werden schließlich die gegenwärtig möglichen chirurgischen und medikamentös-therapeutischen Wege in einer Richtlinie zusammengefaßt.

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Resumen Mientras que los signos clínicos, la evolución y el examen anatomopatológico nos permiten el diagnóstico de localización de una oclusión vascular, la angiografia cerebral nos da la possibilidad de estudiar la vascularización residual y la colateral in vivo.Los examenes angiográficos confirman los estudios anatómicos, demostrando que las arterias cerebrales no son terminales, sino que por la presencia de varios anillos de vascularización intercomunican a diverses nivelés.Cada una de las 4 grandes arterias cerebrales puede compensar a las restantes a través de las multiples colaterales existentes, siempre que no exista una anomalia congenita.La ligadura de las 4 arterias cerebrales porCooper (1831) y multiples experimentos posteriores confirman la formación de colaterales a través del tejido muscular, piel y arterias espinales, que no existe en tal extension en el hombre, mas avanzado en la escala filogenética. Desde que se ligaran por primera vez la arteria carótida en el hombre (Werner-Ellis, 1775) y la arteria vertebral (Maissonneuve-Favrod, 1852) se han sucedido muchos casos publicados donde la falta, ligadura o aplasia de una o dos arterias ha sido tolerada sin trastornos.El desarrollo de la circulación colateral no depende unicamente de factores hidro- o hemodinámicos, sino de una serie de otras causas, que han sido tratadas exhaustivamente en base a nuestro material y las publicaciones.En el presente trabajo son comentadas las colaterales del anillo de vascularización intracranial y extracranial en base a angiogramas proprios y de la literatura correspondiente.La importancia de la circulación colateral a través de la arteria oftálmica y la arteria vertebral ha merecido special consideración.Respetando estos hechos se resumen las posibilidades medicamentosas y quirúrgicas actuates.

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Résumé L'angiographie cérébrale est susceptible de contribuer à l'étude de la circulation collatérale en cas d'un processus vasculaire obstructive. Les données angiographiques confirment les constatations anatomiques que les artères cérébrales ne sont pas des artères terminales.Les artères cérébrales forment des cercles vasculaires. Chacune des 4 grandes artères cérébrales peut compenser les autres — sauf les cas d'une anomalie innée.Cooper (1831) a été le premier de ligaturer les 4 artères cérébrales primitives sur le chien. Ces recherches et beaucoup d'autres prouvent l'action des branches anastomotiques des artères de la peau, des muscles et des artères spinales qui cependant ne sont pas présentes en même qualité chez l'homme. Depuis la première ligature de l'artère carotide chez l'homme (Warner-Ellis, 1775) et de l'artère vertébrale (Maissonneuve-Favrod, 1852) l'aplasie, l'atrésie et la ligature d'une ou de deux artères cérébrales sans déficit clinique a été publiée dans plusieurs travaux anatomiques, cliniques et radiologiques.Les auteurs discutent les facteurs haemo-dynamiques, vasculaires et d'autres méchanismes qui sont à base de la formation des cercles vasculaires collatéraux et intra- et extracrâniens à l'aide d'images angiographiques personnels et de la littérature. On souligne l'importance des branches anastomotiques entre la carotide externe et l'art. ophtalmique. — La conclusion est l'indication pour le traitement chirurgicale et médicale.

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Riassunto Mentre la sintomatologia e l'evoluzione clinica e l'esame anatomopatologico ci permettono di precisare la sede di occlusione vascolare, l'angiografia cerebrale ci offre la possibilità di studiare sul vivo la vascolarizzazione e in particolare la formazione del circolo collaterale.Le ricerche angiografiche confermano i reperti degli anatomici secondo cui le arterie cerebrali non sono delle arterie terminali ma presentano parecchi anelli di vascolarizzazione che comunicano di nuovo in varie località.Ognuna delle 4 grandi arterie cerebrali può, dato che non esistano anomalie congenite, compensare le altre attraverso le numerose collaterali.Le legature delle 4 arterie cerebrali praticata per la prima volta sul cane daCooper (1831) ed i numerosi sperimenti eseguiti più tardi confermano la formazione del circolo collaterale attraverso le arterie cutanee, muscolari e spinali il quale non si verifica in tale misura nell'uomo. Dacchè fu praticata per la prima volta nell'uomo la legatura della carotide (Warner-Ellis, 1775) risp. della vertebrale (Maissonneuve-Favrod, 1852), numerosi casi di agensia, aplasia e legatura di 1 o 2 arterie cerebrali senza sequele cliniche sono stati riportati da anatomici, clinici e radiologi.La formazione del circolo collaterale non dipende dalle sole leggi emorisp. idrodinamiche ma da vari fattori ulteriori. Questi vengono discussi in base al materiale proprio ed alla letteratura.Vengono trattate in base alle angiografie proprie ed a quelle della letteratura le collaterali tra gli anelli di vascolarizzazione endocranici entro loro corne pure tra questi e l'anello di vascolarizzazione estracranico.Si mette in rilievo il significato del circolo collaterale attraverso i rami della carotide esterna verso le arterie oftalmica e vertebrale.Per rispetto ai fatti sudetti le possibilità attuali di terapia chimrgica e medicamentosa vengono riassunte nel senso direttivo.

     

A clinical study of cerebral circulation during extracorporeal circulation

The objective of this study is to clarify the relationship of cerebral blood flow to extracorporeal circulation flow and mean arterial pressure during nonpulsatile extracorporeal circulation under moderate hypothermia. Cerebral blood flow was determined by an argon saturation and desaturation method after that of Pevsner and colleagues with a mass spectrometer in 21 adult patients undergoing cardiac operations. Cerebral blood flow was 25, 33, 35, and 42 ml/100 gm/min, ranging from 19 to 50 ml/100 gm/min, at extracorporeal circulation flow rates of 40, 50, 60, and 70 ml/kg/min, respectively. Cerebral blood flow increased proportionally to extracorporeal circulation flow. Cerebral blood flow scattered almost transversely to mean arterial pressure and was 31 ml/100 gm/min in a hypotensive group (mean arterial pressure 34 to 50 mm Hg) and 34 ml/100 gm/min in another group (mean arterial pressure 51 to 94 mm Hg). Mean arterial pressure did not significantly influence cerebral blood flow. Cerebral oxygen consumption did not remarkably decrease and remained in the reasonable range when cerebral blood flow was 23 to 40 ml/100 gm/min. Subsequently, we assumed that the average cerebral blood flow value of 25 ml/100 gm/min at an extracorporeal circulation flow rate of 40 ml/kg/min also would be in the safe range. All of the patients are living without cerebral complications. We conclude that (1) cerebral blood flow was extracorporeal circulation flow dependent and (2) cerebral blood flow in the safe range was maintained even in the hypotensive range, provided the extracorporeal circulation flow rate was 40 ml/kg/min or higher.     

Computerized hemodynamic evaluation of the cerebral circulation for bypass

This article describes details of the application of computer modeling of cerebral blood circulation. A brief review of the different computer modeling techniques and the current models used today for predicting surgical options for bypass surgery are presented. The use of phase contrast MR for estimating intracranial flow rates makes these models even more accurate. Two case studies are presented with computer simulation results with verification of predicted outcome both clinically and from actual flow of measurements.