Decreased cerebral uptake of oxygen in coma--a consequence of decreased utilization of ATP

Glasgow Coma Scale Score Fluctuations are Inversely Associated With a NIRS-based Index of Cerebral Autoregulation in Acutely Comatose Patients

BACKGROUND

The Glasgow Coma Scale (GCS) is an essential coma scale in critical care for determining the neurological status of patients and for estimating their long-term prognosis. Similarly, cerebral autoregulation (CA) monitoring has shown to be an accurate technique for predicting clinical outcomes. However, little is known about the relationship between CA measurements and GCS scores among neurological critically ill patients. This study aimed to explore the association between noninvasive CA multimodal monitoring measurements and GCS scores.

METHODS

Acutely comatose patients with a variety of neurological injuries admitted to a neurocritical care unit were monitored using near-infrared spectroscopy-based multimodal monitoring for up to 72 hours. Regional cerebral oxygen saturation (rScO2), cerebral oximetry index (COx), GCS, and GCS motor data were measured hourly. COx was calculated as a Pearson correlation coefficient between low-frequency changes in rScO2 and mean arterial pressure. Mixed random effects models with random intercept was used to determine the relationship between hourly near-infrared spectroscopy-based measurements and GCS or GCS motor scores.

RESULTS

A total of 871 observations (h) were analyzed from 57 patients with a variety of neurological conditions. Mean age was 58.7±14.2 years and the male to female ratio was 1:1.3. After adjusting for hemoglobin and partial pressure of carbon dioxide in arterial blood, COx was inversely associated with GCS (β=-1.12, 95% confidence interval [CI], -1.94 to -0.31, P=0.007) and GCS motor score (β=-1.06, 95% CI, -2.10 to -0.04, P=0.04). In contrast rScO2 was not associated with GCS (β=-0.002, 95% CI, -0.01 to 0.01, P=0.76) or GCS motor score (β=-0.001, 95% CI, -0.01 to 0.01, P=0.84).

CONCLUSIONS

This study showed that fluctuations in GCS scores are inversely associated with fluctuations in COx; as COx increases (impaired autoregulation), more severe neurological impairment is observed. However, the difference in COx between high and low GCS is small and warrants further studies investigating this association. CA multimodal monitoring with COx may have the potential to be used as a surrogate of neurological status when the neurological examination is not reliable (ie, sedation and paralytic drug administration).

Neurotoxicity of ammonia and fatty acids: differential inhibition of mitochondrial dehydrogenases by ammonia and fatty acyl coenzyme A derivatives

In several metabolic encephalopathies, hyperammonemia and organic acidemia are consistently found. Ammonia and fatty acids (FAs) are neurotoxic: previous workers have shown that ammonia and FAs can act singly, in combination, or synergistically, in inducing coma in experimental animals. However, the biochemical mechanisms underlying the neurotoxicity of ammonia and FAs have not been fully elucidated. FAs are normally converted to their corresponding CoA derivatives (CoAs) once they enter cells and it is known that these fatty acyl CoAs can alter intermediary metabolism. The present study was initiated to determine the effects of ammonia and fatty acyl CoAs on brain mitochondrial dehydrogenases. At a pathophysiological level (2 mM), ammonia is a potent inhibitor of brain mitochondrial alpha-ketoglutarate dehydrogenase complex (KGDHC). Only at toxicological levels (10-20 mM) does ammonia inhibit brain mitochondrial NAD(+)- and NADP(+)- linked isocitrate dehydrogenase (NAD-ICDH, NADP-ICDH), and NAD(+)-linked malate dehydrogenase (MDH) and liver mitochondrial NAD-ICDH. Butyryl- (BCoA), octanoyl- (OCoA), and palmitoyl (PCoA) CoA were potent inhibitors of brain mitochondrial KGDHC, with IC50 values of 11, 20, and 25 microM, respectively; moreover, the inhibitory effect of fatty acyl CoAs and ammonia were additive. At levels of 250 microM or higher, both OCoA (IC50 = 1.15 mM) and PCoA (IC50 = 470 microM) inhibit brain mitochondrial NADP-ICDH; only at higher levels (0.5-1 mM) does BCoA inhibit this enzyme (by 30-45%). Much less sensitive than KGDHC and NADP-ICDH, brain mitochondrial NAD-ICDH is only inhibited by 1 mM BCoA, OCoA, and PCoA by 22%, 35%, and 44%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Octanoic acid-induced coma and reticular formation energy metabolism

The medium chain fatty acid octanoic acid was injected i.p. into 20-22 g Swiss-Albino mice at a dose of 15 mumol/g. This dose produced a reproducible response consisting of a 3-4 min period of drowsiness, followed by coma. These mice as well as suitable controls were sacrificed by rapid submersion in liquid N2, or by microwave irradiation in a 7.3 kW microwave oven. Tissue from the reticular formation and the inferior colliculus was prepared for microanalysis of the energy metabolites glucose, glycogen, ATP and phosphocreatine. Results from this study showed a selective effect on energy metabolism in cells of the reticular formation. Both glucose and glycogen were elevated in the coma and precoma state. In addition, ATP and phosphocreatine were decreased in the reticular formation during coma. These results show a selective effect of octanoic acid on energy metabolism in the reticular formation both in the precoma stage, and during overt coma. The selective vulnerability of the reticular formation to metabolic insult may act in a beneficial manner to the animal by inducing coma. This lowers the overall demand for energy, thereby placing the animal in a milieu in which there is an increased chance for correction of the perturbation.