Experimental brain ischaemia: assessment of injury by magnetic resonance spectroscopy and histology

Phosphorus magnetic resonance spectroscopy (31P-MRS) was made to measure changes in brain high energy phosphate compounds, adenosine triphosphate (ATP) and phosphocreatine (PCr), inorganic phosphorus (Pi) and intracellular pH (pHi) during a prolonged period of incomplete brain ischaemia produced, in anaesthetized dogs, by bilateral carotid occlusion together with haemorrhagic hypotension for intervals of up to 300 min. Mean arterial blood pressure (MABP) was lowered in a stepwise fashion, until signs of metabolic decompensation (as estimated by MRS) occurred. At that point MABP was varied against further evidence of metabolic decompensation in an attempt to maintain a more constant degree of insult. At the end of the ischaemic period MABP was restored and the animals observed during a 3 h recovery period. At the end of the recovery period the brains were perfusion-fixed for histological examination. A semi-quantitative method of histological evaluation was used to determine the degree of histological damage. This permitted assignment of an 'ischaemic score' to the tissue sampled from each animal. Comparisons were then made between the magnitude of this 'ischaemic score' and the changes in metabolic and physiological variables (ATP, PCr, pHi and MABP) as well as an estimator of phosphorylation potential (PCr/Pi), which were all measured during the ischaemic insult. Histological examination showed a wide variety of neuronal alterations, including dark and pale type injury, which correlated directly with the metabolic derangements brought about by ischaemia. The degree of damage determined from this histological assessment correlated best with the duration and degree of change in PCr/Pi, supporting the use of this ratio as a critical index of cellular energy state. In particular there was a strong linear relationship between the degree of leucocyte recruitment and changes in PCr/Pi. To summarize, metabolic changes, determined by MRS, correlate with the degree of histological damage, and in turn, the classical descriptions of acute ischaemic neuronal injury appear to be validated by MRS determinations of metabolic changes during ischaemia.