Pulse oximetry signals local anesthetic-induced methemoglobinemia

An otherwise healthy patient with a fractured mandible was scheduled to undergo an open reduction under general anesthesia. Just before transport to the operating room, bimaxillary arch bars were placed under local anesthesia with 4% prilocaine and 1:200,000 epinephrine. Although induction of anesthesia and nasoendotracheal intubation were uneventful, pulse oximetry values fell to 89% despite adequate ventilation and an inspired oxygen concentration of 50%. Inquiry by the anesthesiologist and arterial blood gas measurements revealed that methemoglobinemia had developed in response to the large amount 576 mg) of prilocaine administered. A total of 150 mg of methylene blue administered in two doses corrected the problem. The oral surgeon, having recently switched to prilocaine because of a manufacturer's recall of lidocaine, was unaware of the potential of prilocaine to cause this disorder.