Comparing Doppler ultrasonography and cerebral oximetry as indicators for shunting in carotid endarterectomy

To determine the thresholds of selective shunting in carotid endarterectomy during general anesthesia, we compared transcranial Doppler ultrasonography and cerebral oximetry (RSO2). During carotid crossclamping, RSO2 and mean blood flow velocity in the middle cerebral artery (Vm,mca) was simultaneously monitored in 55 of 59 patients. A relative decrease in Vm,mca to <20% of preclamp velocity was the indication for selective shunting. Three patients were shunted, two because of criteria of Vm,mca and one in which Vm,mca measurements were impossible. No postoperative neurological deficits occurred. During cross-clamping, both Vm,mca (42 +/- 16 vs 26 +/- 12 cm/s; P < 0.001) and RSO2 (68 +/- 7% vs 62 +/- 8%; P < 0.01) decreased and a significant correlation between %Vm,mca and <Delta>RSO2 was found (R-2 = 0.40; P = 0.003). Decreases in RSO2 > 13% identified two patients later shunted; however, this threshold would have indicated unnecessary shunting in seven patients (false positives = 17%). Transcranial Doppler ultrasonography identified patients at risk for ischemia more accurately than RSO2. Relying on RSO2 alone would increase the number of unnecessary shunts because of the low specificity. Accepting higher decreases in RSO2 does not appear reasonable because it bears the risk of a low sensitivity.