Traumatic Brain Injury in the Rat: Alterations in Brain Lactate and pH as Characterized by1H and31P Nuclear Magnetic Resonance

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Abstract
Application of both phosphorus (31P) and proton (1H) magnetic resonance spectroscopy (MRS) to the study of brain metabolism permits the noninvasive measurement of intracellular pH and brain lactate level. We have used water-suppression 1H MRS with novel lactate-editing techniques, together with 31P MRS, to characterize sequential changes in brain lactate level and pH in vivo over an 8-h period following fluid-percussion brain injury of graded severity in the rat. A transient fall in intracellular pH (from 7.09 ± 0.07 at baséline to 6.88 ± 0.09 at 40 min postinjury) occurred in animals subjected to moderate-(1.5–2.2 atm) and high-(2.5–3.3 atm) but not low-level (0.1–1.2 atm) injury; intracellular pH returned to baseline by 90 min postinjury. Transient elevations in brain lactate level were observed that temporally paralleled and were significantly correlated with the pH changes for all injury levels (r= 0.93, p < 0.001). Postinjury alterations in intracellular brain pH and lactate level were identical in magnitude in animals subjected to either moderate or high-level injury. However, animals subjected to moderate injury had a moderate chronic neurological deficit that persisted up to 4 weeks postinjury, whereas animals subjected to a high level of injury showed greater histopathological damage and a more severe chronic neurological deficit. These data suggest that the extent of posttraumatic intracellular cerebral acidosis in our model of experimental head injury is not directly related to the severity of functional neurological deficit.