Near-infrared spectroscopy (NIRS)-based cerebral oximetry measures regional cerebral oxygen saturation (rSO2). During carotid endarterectomy under general anaesthesia, a decrease in ipsilateral rSO2 by what threshold is the recommended indication for shunt placement?

• A Any fall below absolute value of 50%
• B A relative decrease of 10% from baseline at any time during carotid clamping
• C A relative decrease of 20% from baseline or absolute rSO2 below 50% ✓
• D Absolute rSO2 below 75% regardless of baseline

Explanation

Cerebral oximetry during carotid endarterectomy provides real-time monitoring of ipsilateral cortical perfusion after carotid clamping. A relative decrease of 20% or more from the patient's own pre-induction baseline, or a fall to an absolute rSO2 below 50%, is the widely accepted threshold indicating cerebral ischaemia and mandating a shunt. Baseline individualisation is crucial because patients with chronic hypoperfusion may have low baseline values. The 20% relative threshold is more sensitive than using only an absolute cut-off, because patients with good baseline rSO2 can tolerate less absolute reduction.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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