Primary brain injury has limited treatment (it has already happened), so efforts have concentrated on preventing secondary brain injury. Secondary injury has a profound influence on outcome. It occurs as a result of the complications of primary brain injury - raised intracranial pressure, ischaemia, oedema and infection. It is important to realise that these complications are usually delayed and are therefore amenable to intervention. In TBI, the main precipitants of secondary injury are hypotension and hypoxaemia. Hypoxaemia, as defined by oxygen saturations < 90%, and hypotension, as defined by a systolic blood pressure of < 90 mmHg, are associated with a statistically significant worse outcome and are common at the scene of injury. Patients with severe TBI who are intubated in the prehospital setting have a better outcome.
Following brain injury, specific cells are rendered dysfunctional although not mechanically destroyed. If the subsequent environment is favourable, many of these cells can recover. Hypoxia or ischaemia can facilitate irreversible damage.
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