Intracranial pressure is a fraction of arterial blood pressure

The relation between cerebrospinal fluid pressure (pcsf ) and arterial blood pressure (pa ) was studied in pigs and in rabbits before and after an ischemia-induced impairment of the cerebral autoregulation, in order to predict the effect of changes of pa on the pcsf . Before the ischemic brain episode no clear correlation between pa and pcsf was found. A close dependence emerged after the ischemic episode, and after each change in pa the pcsf assumed immediately a new stationary level. The results suggest that intracranial pressure is at any moment a fraction of arterial blood pressure quantitatively determined by the coordinated action of cerebrovascular resistance, i.e. arteriolar resistance Ra , regulated by vasomotor tonus, and venous resistance Rv , which is mechanically passive in the same manner as a Starling resistor device. The relation is given by pcsf = pa [1 + (Ra /pv )](-1) . Induced changes in pa have different effects on pcsf . However, this effect may be predicted to some extent because the relation between pcsf and pa may be represented mathematically as a rectangular hyperbola. This observation may be of relevance in neurological intensive care. Changes in arterial blood pressure are known to influence intracranial pressure in patients with deranged intracranial dynamics, as in the case of severe head injury. In clinical practice, both raising systemic arterial blood pressure in order to preserve cerebral perfusion pressure and induced arterial hypotension to reduce hydrostatic capillary pressure with consequent reduction of brain edema have been used in the management of patients with severe brain injury. While theoretical models of biological systems often have a limited value in practice because of inability to measure the relevant parameters from clinical data, the ratio of cerebrovascular resistances may be calculated at the bedside.