The influence of inspired oxygen fraction and end-tidal carbon dioxide on post-cross-clamp cerebral oxygenation during carotid endarterectomy under general anesthesia.

BACKGROUND: Ten to fifteen percent of awake patients develop neurological deficits secondary to cerebral hypoperfusion after carotid artery cross-clamping. The reversal of such deficits by increasing the inspired oxygen fraction (Fio(2)) has been demonstrated, and regional cerebral oxygenation (rSO(2)) has been shown to improve during carotid cross-clamping in awake patients by increasing Fio(2). Paradoxical improvements in cerebral blood flow during carotid endarterectomy (CEA) at the time of cross-clamping and normalization of post-cross-clamp electroencephalographic abnormalities have been induced by hypocapnia. We performed this study to determine the influence of Fio(2) and end-tidal carbon dioxide (Petco(2)) on rSO(2) in patients undergoing CEA with general anesthesia during carotid cross-clamping.

METHODS: Twenty patients were recruited. Ten underwent elective shunting. Patients received standardized general anesthesia. rSO(2) was measured using the INVOS 5100B monitor (Somanetics Corporation, Troy, MI). After carotid cross-clamping, Fio(2) and minute ventilation were sequentially adjusted: 1) Fio(2) 30%, Petco(2) 30-35 mm Hg; 2) Fio(2) 100%, Petco(2) 30-35 mm Hg; and 3) Fio(2) 100%, Petco(2) 40-45 mm Hg. At each point, rSO(2) was recorded from both operative and nonoperative sides, and arterial blood gas analysis was performed.

RESULTS: Results from shunted and unshunted patients were analyzed separately. Increasing Fio(2): Administration of 100% oxygen while maintaining Petco(2) in the range 30-35 mm Hg in unshunted patients resulted in an 8% increase (P = 0.008) in rSO(2) on the operative side and a 6% increase (P = 0.011) on the nonoperative side compared with an Fio(2) of 30%. In shunted patients, administration of 100% oxygen while maintaining the Petco(2) in the range 30-35 mm Hg resulted in a 4% increase in rSO(2) on both the operative side (P = 0.008) and the nonoperative side (P = 0.011) compared with an Fio(2) of 30%. Increasing Petco(2): In unshunted patients, there was a 6% (P = 0.008) increase in rSO(2) on the operative side and a 5% increase (P = 0.024) on the nonoperative side at Petco(2) 40-45 mm Hg compared with Petco(2) 30-35 mm Hg maintaining Fio(2) at 100%. In shunted patients, there was a 3% increase (P = 0.018) in rSO(2) on the operative side and a 4% increase (P = 0.007) on the nonoperative side at Petco(2) 40-45 mm Hg compared with Petco(2) 30-35 mm Hg maintaining Fio(2) at 100%.

CONCLUSION: rSO(2) is reliably improved during carotid cross-clamping by increasing Fio(2) in patients undergoing CEA with general anesthesia. Additional improvement in rSO(2) may be gained by increasing Petco(2).