On the Reversibility of the Function of the Severely Damaged Brain

It is conceivable that the severely damaged brain might have a "no return point" of the brain function. The severe brain damage was prepared by elevating intracranial pressure (injection of the saline though the cisterna magna at the rate of 100 mmH_2 O/min) in adult dogs. Cerebral perfusion pressure was extremely reduced and EEG became flat along with increased intracranial pressure. This satge was termed Stage 3b in our previous paper. Cerebrovascular response to CO_2 inhalation was examined by detection of the increased count of RISA which was injected intravenously or by changes in intracranial pressure (ICP). At various points of Stage 3b, ICP released. In order to evaluate the cerebral microcirculation at this time, 200 ml of colloidal carbon was injected into the vertebral artery at the systolic blood pressure. At Stage 3b, the ICP of 16 dogs were held at this level for 40 minutes or less, and released. At this time, 10 per cent CO_2 inhalation resulted in increased ICP. The mortality rate within 24 hours after the experiment was 50 to 60 per cent. The microcirculation in the brain tissue was impaired moderately in this group. The ICP of 5 dogs were held at a high level for more than 40 minutes at Stage 3b, and then released. In these cases, no reactivities of cerebral vesseles to CO_2 inhalation were observed. The mortality rate within 24 hours after the experiment was 80 per cent. The cerebral microcirculation was damaged remarkably and "no-reflow phenomenon" of the cerebral vesseles was found. In human cases, vascular reactivity to CO_2 inhalation was examined successfully using 5μc/kg RISA injection. It was speculated that impairment of cerebral vasoreactivity to CO_2 inhalation was compatible with the no return point of the brain function.