Currently accepted uses of transcranial Doppler (TCD) include all of the following EXCEPT:
Transcranial Doppler (TCD) measurements of middle and anterior cerebral artery blood flow velocity are useful for the diagnosis of cerebral vasospasm after subarachnoid hemorrhage. Qureshi et al demonstrated that an increase in the middle cerebral artery mean flow velocity as assessed by TCD is an independent predictor of symptomatic vasospasm in a prospective study of patients with aneurysmal subarachnoid hemorrhage. In addition, while some have proposed using TCD to estimate ICP, studies have shown that TCD is not a reliable method for estimating ICP and CPP, and currently cannot be endorsed for this purpose. TCD also is useful to confirm the clinical examination for determining brain death in patients with confounding factors such as the presence of CNS depressants or metabolic encephalopathy.
Regarding jugular venous oximetry in patients with TBI:
When the arterial oxygen content, hemoglobin concentration, and the oxyhemoglobin dissociation curve are constant, changes in jugular venous oxygen saturation (Sjo2) reflect changes in the difference between cerebral oxygen delivery and demand. Generally, a decrease in Sjo2 reflects cerebral hypoperfusion, whereas an increase in Sjo2 indicates the presence of hyperemia. Sjo2 monitoring cannot detect decreases in regional cerebral blood flow if overall perfusion is normal or above normal. This technique requires the placement of a catheter in the jugular bulb, usually via the internal jugular vein. Catheters that permit intermittent aspiration of jugular venous blood for analysis or continuous oximetry catheters are available. Low Sjo2 is associated with poor outcomes after TBI. Nevertheless, the value of monitoring Sjo2 remains unproven. If it is employed, it should not be the sole monitoring technique, but rather should be used in conjunction with ICP and CPP monitoring. By monitoring ICP, CPP, and Sjo2, early intervention with volume, vasopressors, and hyperventilation has been shown to prevent ischemic events in patients with TBI.
Monitoring local brain tissue oxygen tension (PbtO2) in patients with severe TBI:
While the standard of care for patients with severe TBI includes ICP and CPP monitoring, this strategy does not always prevent secondary brain injury. Growing evidence suggests that monitoring local brain tissue oxygen tension (PbtO2) may be a useful adjunct to ICP monitoring in these patients. Normal values for PbtO2 are 20 to 40 mm Hg, and critical levels are 8 to 10 mm Hg. A recent clinical study sought to determine whether the addition of a PbtO2 monitor to guide therapy in severe TBI was associated with improved patient outcomes. Mortality was significantly lower in the patients who had therapy guided by PbtO2 monitoring in addition to ICP and CPP (25%; P < 0.05). The benefits of PbtO2 monitoring may include the early detection of brain tissue ischemia despite normal ICP and CPP. In addition, PbtO2-guided management may reduce potential adverse effects associated with therapies to maintain ICP and CPP.
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