Metabolic crisis without brain ischemia is common after traumatic brain injury: a combined microdialysis and positron emission tomography study

Abstract

Brain trauma is accompanied by regional alterations of brain metabolism, reduction in metabolic rates and possible energy crisis. We hypothesize that microdialysis markers of energy crisis are present during the critical period of intensive care despite the absence of brain ischemia. In all, 19 brain injury patients (mean GCS 6) underwent combined positron emission tomography (PET) for metabolism of glucose (CMRglu) and oxygen (CMRO(2)) and cerebral microdialysis (MD) at a mean time of 36 h after injury. Microdialysis values were compared with the regional mean PET values adjacent to the probe. Longitudinal MD data revealed a 25% incidence rate of metabolic crisis (elevated lactate/pyruvate ratio (LPR) > 40) but only a 2.4% incidence rate of ischemia. Positron emission tomography imaging revealed a 1% incidence of ischemia across all voxels as measured by oxygen extraction fraction (OEF) and cerebral venous oxygen content (CvO(2)). In the region of the MD probe, PET imaging revealed ischemia in a single patient despite increased LPR in other patients. Lactate/pyruvate ratio correlated negatively with CMRO(2) (P < 0.001), but not with OEF or CvO(2). Traumatic brain injury leads to a state of persistent metabolic crisis as reflected by abnormal cerebral microdialysis LPR that is not related to ischemia.

Figure 1

Mean daily LPRs over the first 8 days after injury. Mean LPR values day by day are shown in solid bars. The percentage of LPR >40 on each postinjury day are shown in hatched bars.

Figure 2

An example patient in whom the LPR was elevated above 40 for several hours. During this time, the ICP was being actively treated to maintain the ICP <20 mm Hg. The jugular venous oximetry (sjvo2) % saturation, arterial carbon dioxide pressure in mm Hg (PaCO₂), the CPP, and MAP were all within normal limits and show no evidence of brain ischemia.

Figure 3

An example patient who showed ischemic values on OEF on PET scan. There was a corresponding increase in LPR and reduction in glucose. This patient was used to define the ischemic pattern of microdialysis in the entire cohort. Values within the region of interest are shown below each PET image. Abbreviations: CMRgluc—metabolic rate of glucose, CMRO₂—metabolic rate of oxygen, CBF—cerebral blood flow, OGR—oxygen to glucose ratio, OEF—oxygen extraction fraction.

Figure 4

Region of interest cerebral metabolic rate of oxygen (CMRO₂) and the microdialysis LPR. Increasing LPR correlated with reducing CMRO₂.