Fig. 4
From: The critical care management of poor-grade subarachnoid haemorrhage
Approach to low brain tissue oxygen. Consider the combined used of PtiO2 and microdialysis catheter to detect non-hypoxic patterns of cellular dysfunction [97]. According to the manufacturer, an equilibrium time as long as 2Â hours may be necessary before PtiO2 readings are stable, because of the presence of the tip surrounding microhaemorrhages. Sensor damage may also occur during insertion. Increase inspired fraction of oxygen (FiO2) to 100Â %. If PtiO2 increases, it confirms good catheter function. Oxygen challenge to assess tissue oxygen reactivity. FiO2 is increased from baseline to 100Â % for 5Â minutes to evaluate the function and responsiveness of the brain tissue oxygen probe. A positive response happens when PtiO2 levels increase in response to higher FiO2. A negative response (lack of PtiO2 response to higher FiO2) suggests probe or system malfunction. Another possibility if there is a negative response is that the probe placement is in a contused or infarcted area. Follow-up computed tomography might be necessary in this situation to ensure appropriate probe position. Mean arterial pressure (MAP) challenge to assess cerebral autoregulation. MAP is increased by 10Â mm Hg. Patients with impaired autoregulation demonstrated an elevation in ICP with increased MAP. When the autoregulation is intact, no change or a drop in ICP levels follows the elevation in blood pressure. Another way to assess cerebral autoregulation is the evaluation of the index of PtiO2 pressure reactivity. When autoregulation is intact, PtiO2 is relatively unaffected by changes in CPP, so the index of PtiO2 pressure reactivity is near zero [163]. The threshold haemoglobin (Hgb) of 9Â mg/dl to indicate blood transfusion was based on a previously published PtiO2 study [164]. CPP cerebral perfusion pressure, CSF cerebrospinal fluid, CT computed tomography, ICP intracranial pressure, PaCO 2 arterial partial pressure of carbon dioxide, PaO 2 partial pressure of oxygen in arterial blood, P ti O 2 brain tissue oxygen pressure, RASS Richmond Agitation-Sedation Scale, SAH subarachnoid haemorrhage, SBP systolic blood pressure