The control mechanisms activated by experimental intracranial hemodynamics changes.

The Authors describe an experimental model (rat) performed in order to evaluate the cerebrovascular changes which occur following the venous outflow impedance increase. This increase was obtained by bilateral occlusion of the external jugular veins at the neck. The rat was chosen because its intracranial and extracranial circulation is very similar to the human one, thus allowing the recording of blood flow velocity changes in the common carotid artery by means of a noninvasive technique such as Doppler ultrasound. Ligature of both the external jugular veins, performed on 35 rats, produced a decrease of common carotid artery blood flow velocity in 2/3 of the animals. The remaining 1/3 presented increased common carotid velocity. These changes were recorded within 15 minutes after ligation of the veins. On the contrary, the epidural pressure, measured in 6 cases, showed a constant increase. The results demonstrate that the different patterns of CBF and EDP do not permit univocal interpretation of the cerebrovascular reactivity control mechanisms, that is, myogenic, metabolic or neurogenic.