Characteristics and limits of tolerance in repeated subarachnoid hemorrhage in dogs.

The effects of repeated subarachnoid hemorrhages have been investigated experimentally in dogs. The main objectives were to determine the tolerance to repeated hemorrhage and to study the changes occurring during the repeated bleeds, in intracranial pressure, EEG, ECG, systemic arterial pressure and respiration. The natural course of an intracranial hemorrhage was simulated by shunting blood from a femoral artery through a drop recorder into five different sites in the craniospinal system: the chiasmatic cistern, a lateral ventricle, the cisterna magna, the lumbar subarachnoid space and into the cerebral tissue of the left frontal lobe. The hemorrhage was allowed to continue until it stopped spontaneously. Each bleed resulted in a transient rise in intracranial pressure to the level of the arterial pressure, followed by a return to a steady state value. The time taken for the attainment of the steady state was increasingly prolonged. The final steady state pressure increased with each bleed. Ultimately, a stage was reached where the hemorrhage resulted in a sustained high pressure at the level of the arterial blood pressure, producing failure of vital functions and an irreversibly isoelectric electroencephalogram. The average number of bleeds necessary to produce this state in the case of hemorrhage into brain parenchyma was 3 (range 2-4), into the lateral ventricle, 4 range 3-5), and into the cisterna chiasmatica, 5 (range 2-7). After 5 hemorrhages into the cisterna magna and the spinal subarachnoid space, a local resistance at the bleeding site was built up which prevented further bleeding.