Comparison of Brain Tissue Oxygen Tension to Microdialysis-Based Measures of Cerebral Ischemia in Fatally Head-Injured Humans

Abstract

This study investigated the relationship between brain tissue oxygen tension (PbtO2) and cerebral microdialysate concentrations of several compounds in five patients with refractory intracranial hypertension after severe head injury. The following substances were assayed: lactate and glucose; the excitatory amino acids glutamate and aspartate; and the cations potassium, calcium, and magnesium. Glucose concentrations did not correlate with PbtO2, but lactate increased as PbtO2 decreased. The lactate/glucose ratio exhibited a close relationship to PbtO2, increasing sharply only when oxygen tension reached zero. Although glucose and oxygen eventually reached very low levels and zero, respectively, in these fatally head-injured patients, the terminal decrease in PbtO2 slightly preceded that of glucose in four of the five patients. This time lag is the cause of the poor correlation between glucose and PbtO2. Glutamate and aspartate concentrations both demonstrated a close relationship to PbtO2, with sharp increases not occurring until PbtO2 was zero. Concentrations of these amino acids exhibited a similar pattern in response to decreasing glucose concentrations. Potassium concentrations began increasing at a PbtO2 of 35 mm Hg, which is not generally considered indicative of hypoxia. Sharper increases began occurring once PbtO2 dropped below 15 mm Hg, with a slight rise in the minimum potassium concentrations recorded at these low PbtO2 values. Calcium and magnesium concentrations did not vary in response to PbtO2. In summary, the most robust biochemical indicators of cerebral anoxia were elevations in the lactate/glucose ratio and in the concentrations of lactate and of the excitatory amino acids glutamate and aspartate. Furthermore, the fact that glucose concentrations continue to decrease for a short period after oxygen levels reach zero suggests that cells continue to utilize glucose anaerobically for such functions as maintenance of cellular integrity, with collapse of the cell membrane as evidenced by increases of extracellular glutamate and aspartate not occurring until both oxygen and glucose concentrations reach zero.

A prospective, randomized Phase II clinical trial to evaluate the effect of combined hyperbaric and normobaric hyperoxia on cerebral metabolism, intracranial pressure, oxygen toxicity, and clinical outcome in severe traumatic brain injury

“…Studies have shown that local brain tissue PO 2 levels are significantly correlated with ischemia and outcome. 54,55 Hyperbaric O 2 therapy clearly leads to Graph showing microdialysate L/P ratio in "uninjured" brain with posttreatment/pretreatment ratio means. A value > 1 indicates the posttreatment value is greater than the pretreatment value, and a value < 1 indicates the posttreatment value is less than the pretreatment value.…”

Hypertonic Saline In Patients With Poor-Grade Subarachnoid Hemorrhage Improves Cerebral Blood Flow, Brain Tissue Oxygen, and pH

“…5 In traumatic brain injury, episodes of low cerebral oxygenation are also associated with poor outcome. 6,7 Brain tissue oxygen tension (P b O 2 ) reflects the availability of oxygen for oxidative energy production and represents the balance between oxygen supply and demand. Ischemic thresholds for P b O 2 have been described, ranging from 1.1 kPa to 3.3 kPa.…”

Traumatic Brain Injury - Acute Care

“…For instance, in cases of cerebral ischemia, the lacate/pyruvate ratio increases markedly, whereas the glucose level declines to near zero. These findings have been shown to correlate with SjVO 2 and PbrO 2 measurements, as well as the oxygen extraction fraction as measured by PET (Robertson et al 1995;Valadka et al 1998;Hutchinson et al 2002). When used in conjunction with CPP and ICP monitoring, microdialysis may have a role in early detection of focal (as opposed to global) pathological processes, such as vasospasm or expanding contusions (Valadka & Robertson 2007;Hillered, Vespa, & Hovda 2005).…”