Role of pressure gradients and bulk flow in dynamics of vasogenic brain edema

Abstract

The vasogenic type of brain edema was investigated using cold injury in cats as a model. The findings indicated that bulk flow and not diffusion should be considered the main mechanism for the spread of edema through the white matter. This conclusion was based on: comparison of the distances actually traveled by various substances during edema spread with those calculated theoretically for migration of the substances by diffusion; coincidence in the speed of movement by 2 substances (sucrose and albumin) with widely different diffusion coefficients; measurement of interstitial fluid pressure (IFP) at various distances from the lesion showing the presence of increased IFP in the lesion area and decreasing pressures along the edema pathway toward the normal tissue; and the fact that spreading of edema can be significantly impeded by inducing before the cold lesion an intracellular type of brain edema that reduces the size of the extracellular space (ECS) and increases the resistance to flow of edema fluid. The pressure-volume curve of the brain ECS, as derived from determinations of IFP and tissue water content, indicated that an initial steep slope in IFP probably represents the high resistance to fluid mobility through the small diameter extracellular channels and the counteracting resistance of the intermingled structures of brain parenchyma to be separated. Once the IFP exceeds these opposing forces, the ECS dilates, fluid mobility increases and the edema front advances.