Cerebral blood flow during experimental epidural bleeding in swine

Regional cerebral blood flow (rCBF) was studied during an aggressive epidural bleed, using a ventilated swine model. rCBF, regional organ blood flow and cardiac output were measured using the radioactive microsphere technique. Blood flows were measured prior to the start of bleeding (Stage 1), when intracranial pressures had reached a plateau and supratentorial perfusion pressure was reduced by about 50% (Stage 2), and at isoelectric EEG (Stage 3). Supratentorial rCBF did not change significantly between stages 1 and 2 while rCVR decreased, implying autoregulatory activity. Cerebral ischaemia developed between stages 2 and 3 when rCBF values fell to levels between 20 and 50% of control values. Infratentorial rCBF changes were similar but less marked, so that adequate brain stem perfusion was maintained below the upper mesencephalon. The left temporal and left parietal cortex and upper mesencephalon suffered a greater reduction in rCBF than other regions, due to proximity to the haematoma and tentorial herniation. The supratentorial perfusion pressure at stage 2 was 60 mm Hg associated with a haematoma volume of 6% of the intracranial volume (ICV). The infratentorial perfusion pressure never fell below 60 mm Hg. The Cushing response was absent when the EEG became isoelectric. This is tentatively ascribed to the absence of hypoxia, because mechanical ventilation was used. Instead systemic arterial hypotension accompanied bleeding in this ventilated model. This hypotension was due to falling cardiac output and peripheral vasodilation.