The influence of systemic arterial pressure and intracranial pressure on the development of cerebral vasogenic edema

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Abstract
The influence of intracranial pressure (ICP), systemic arterial pressure (SAP), and cerebral perfusion pressure (CPP) upon the development of vasogenic cerebral edema is largely unknown. To study their relationship, the authors have produced an osmotic disruption of the blood-brain barrier unilaterally in rabbits by injecting 1 cc/kg of 2M NaCl into the left internal carotid artery. The amount of vasogenic edema produced was assessed by quantitation of the extravasation of Evans blue dye into the area of maximum blood-brain barrier breakdown by means of optical densitometry following formamide extraction. The ICP was measured using a cisterna magna catheter into which mock cerebrospinal fluid could be infused at a predetermined pressure. The SAP was controlled by exsanguination from a femoral artery catheter. In 18 animals in which blood pressure was not controlled, no significant relationship between the ICP and the degree of Evans blue dye extravasation was noted. In these animals, however, a direct relationship between CPP (defined as mean arterial pressure minus mean ICP) and extravasation of Evans blue dye was found (correlation coefficient 0.630; p less than 0.001). When ICP was held constant at 0 to 5 mm Hg in another group of 16 animals and different levels of blood pressure were produced by exsanguination, a significant direct relationship between extravasation of Evans blue dye and the SAP was found (correlation coefficient 0.786; p less than 0.001). In a third group of 20 animals, the blood pressure was held constant at 90 to 100 mm Hg and the ICP was varied between 0 and 75 mm Hg. There was a highly significant result indicating increasing Evans blue dye extravasation with lower levels of ICP (p less than 0.001). Cerebral blood flow determinations by the hydrogen clearance method indicated loss of autoregulation in all animals in the areas of brain injured by intracarotid hypertonic saline. These results indicate that high SAP and low ICP (that is, a large CPP) promote Evans blue dye extravasation in this model of blood-brain barrier disruption. This finding has implications for the management of patients with vasogenic edema.