Hypertension artérielle contrôlée et protection cérébrale

Abstract

Among the techniques of cerebral protection, the use of controlled arterial hypertension is based on the following arguments : 1) Cerebral ischaemia is the final common pathway of any insult to the brain, particularly through secondary lesions. Causes of secondary cerebral lesions include pressure under the brain retractors, temporary clipping, arterial hypotension, hypoxaemia, anaemia and hypercapnia. 2) In the brain, the critical lower value for cerebral blood flow is around 25 mL · 100 g −1 · min −1, under which two types of ischaemic areas can be defined : the penlucida type where cerebral function is abolished, without permanent cerebral lesion and the penumbra type where cerebral tissue recovers only if flow is rapidly restored. In the latter case the duration of ischaemia is very important. 3) Cerebral blood flow is maintained stable within a large range of variations of mean arterial pressure through the autoregulation mechanisms, which is based on vasomotricity of the cerebral circulation, which implies major variations in cerebral blood volume. However, autoregulation needs several dozens of seconds to be achieved. Therefore, sudden variations in mean arterial pressure are associated with short lasting but major variations in cerebral blood volume. 4) In case of increased intracranial pressure, a decrease in cerebral perfusion pressure causes cerebral vasodilation through the autoregulation mechanism, with an increase in cerebral blood volume which will, in turn, increase intracranial pressure and thus decrease cerebral perfusion pressure, and so on. This is the vasodilatory cascade. The therapeutical increase in mean arterial pressure will correct this phenomenon and decrease intracranial pressure. This is called the vasoconstrictive cascade. 5) In case of vascular occlusion by vasospasm, extrinsic compression or during temporary clipping, cerebral protection may be based on the collateral vessels near the ischaemic area and also on vascular anastomoses inside the circle of Willis. Following Poiseuille's law, as vasodilation is already maximal distally to the stenosis, the major factor of vascular resistance is no longer the radius of the vessel, but its length. Accordingly, the increase in perfusion pressure will improve the local flow and participate in prevention or treatment of an ischaemic event. Indeed, the increase, although modest, of the local flow may be sufficient to switch from an ischaemia of penumbra type to a penlucida type. Finally, controlled arterial hypertension 1) implies to achieve the optimal cerebral perfusion pressure in order to create the vasoconstrictive cascade under cover of intracranial pressure monitoring and 2) helps to decrease the ischaemic risk secondary to vasospasm, for example by achieving a mean arterial pressure between 70 and 100 mmHg before clipping and between 100 and 120 mmHg after clipping. To obtain this level of pressure, phenylephrine, noradrenaline and dopamine are the agents of choice, as they are free of harmful effects on the cerebral circulation.