The influence of total body hyperthermia on brain haemodynamics and blood-brain barrier in dogs.

This study was designed to examine the influence of total body hyperthermia (TBHT) using an extracorporeal circuit with a heat exchanger on the cerebral blood flow (CBF), intracranial pressure (ICP), brain tissue pH, cerebral autoregulation and blood-brain barrier (BBB) permeability in dogs. The rectal temperature of the dow was raised to 41.5 degrees C, maintained at 41.5-42.0 degrees C for 2 hours (HT period) and then reduced to normothermia by cooling. Regional CBF was measured by the hydrogen clearance method before heating, during the HT period and after cooling. ICP and brain tissue pH were monitored during the TBHT treatment. Autoregulation of the CBF during the HT period was assessed by measuring the regional CBF and the ICP in a state of induced hypo- or hypertension. The influence of TBHT on BBB permeability was examined using an immunohistochemical technique. The regional CBF increased from 38.1 +/- 6.5 (mean +/- SD) to 49.1 +/- 9.8 ml/100 g/min and the ICP from 10.3 +/- 4.2 to 16.8 +/- 3.4 mmHg when TBHT was raised. These returned to normal values after cooling. The regional CBF and the ICP changed in parallel with drug-induced changes of mean arterial blood pressure during the HT period. These changes suggest that autoregulation of the CBF is paralysed during the HT period. Brain tissue pH decreased rapidly when the rectal temperature exceeded 41.0 degrees C. The pH was 7.18 +/- 0.05 during the HT period and was relatively stable. The pH returned to a normal value after cooling. Immunopositive stain for albumin was not observed in heated brain tissue except for the normally leaky pineal gland and the choroid plexus, indicating preservation of BBB during TBHT. These results suggest that brain oedema may occur easily due to paralysed cerebral autoregulation when the arterial blood pressure fluctuates excessively, so arterial blood pressure must be controlled strictly during TBHT.