To determine the thresholds of selective shunting in carotid endarterectomy
during general anesthesia, we compared transcranial Doppler ultrasonograph
y and cerebral oximetry (RSO2). During carotid crossclamping, RSO2 and mean
blood flow velocity in the middle cerebral artery (Vm,mca) was simultaneou
sly monitored in 55 of 59 patients. A relative decrease in Vm,mca to <20% o
f preclamp velocity was the indication for selective shunting. Three patien
ts were shunted, two because of criteria of Vm,mca and one in which Vm,mca
measurements were impossible. No postoperative neurological deficits occurr
ed. 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 significan
t 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; howeve
r, this threshold would have indicated unnecessary shunting in seven patien
ts (false positives = 17%). Transcranial Doppler ultrasonography identified
patients at risk for ischemia more accurately than RSO2. Relying on RSO2 a
lone would increase the number of unnecessary shunts because of the low spe
cificity. Accepting higher decreases in RSO2 does not appear reasonable bec
ause it bears the risk of a low sensitivity.