Re-defining the ischemic threshold for jugular venous oxygen saturation — a microdialysis study in patients with severe head injury

Summary

Neurological change is more likely to occur when jugular venous oxygen saturation (SjvO₂) is less than 50%. However, the value indicating cellular damage has not been clearly defined. We determined the critical SjvO₂ value below which intracerebral extracellular metabolic abnormalities occurred in 25 patients with severe head injury.

All patients received standard treatment with normoventilation and maintenance of intracranial pressure < 20 mmHg. SjvO₂ was measured from the dominant jugular bulb using a calibrated fibreoptic catheter. Intracerebral metabolic monitoring was performed by collecting perfusate from a microdialysis probe placed in the frontal lobe anterior to the intracranial catheter. Excitotoxin (glutamate) and other extracellular metabolites (lactate, glucose and glycerol) were measured frequently using enzymatic and colorimetric methods.

We observed biphasic relationships between SjvO₂ and all intracerebral metabolites. Analysis of variance showed that there were rapid increases in glutamate, glycerol and lactate when SjvO₂ dropped below 40, 43 and 45% respectively. Extracellular glucose decreased when SjvO₂ dropped below 42%.

Our findings suggested that the ischemic threshold for SjvO₂ in patients with severe head injury is 45%, below which secondary brain damage occurred.