OBJECTIVES—Critical vascular surgery of the brain
or the heart occasionally requires total cessation of the circulatory
system. Profound hypothermia is used to protect the brain from
ischaemic injury. This study explores the use of microdialysis to
measure metabolic indices of ischaemia: glutamate, lactate, and pH, and
cerebral temperature during profound hypothermia and circulatory arrest. METHODS—Effluent from a microdialysis catheter
placed in the cerebral cortex of three patients undergoing complete
circulatory arrest was continuously sampled. Samples were pooled over
10 minute periods and glutamate and lactate concentrations were
measured postoperatively. Brain temperature and pH were measured on
line intraoperatively. Electroencephalography and monitoring of
somatosensory evoked potentials and brainstem auditory evoked
potentials were simultaneously carried out. RESULTS—Patient 1 had normal glutamate and
lactate. PH was 6.75 to 6.85 and increased to 6.9 after warming ensued.
Patient 2 had raised glutamate and lactate during most measurements.
The glutamate concentrations peaked at 305 µM/l at the start of the
measurements and fell below 20 µM/l after warming. The lactate
concentrations peaked at 680 µM/l before cooling, rose to 1040 µM/l
during the cooling process, decreased to 212 µM/l during circulatory
arrest, and rose again to 620 µM/l after warming. The pH started at
7.06and continued a downward course until stabilising at a pH of 6.5after circulatory arrest. Patient 3 had a transient, mild increase in
glutamate and lactate during the cooling and warming period. pH was
stable throughout. CONCLUSION—Microdialysis combined
with temperature and pH measurements of the cerebral cortex
promises to be an important tool in detecting cerebral ischaemia.
Further studies are needed to validate our findings and test the
feasibility of modifying ischaemic changes.
