Case 42: Unexplained Low Oxygen Saturation

Abstract

A 16-year-old boy (180 lb) is scheduled for a repair of hand tendons after an injury caused by a saw. His vital signs are stable, with a heart rate of 100 and blood pressure (BP) of 90/60. His mother thinks he has lost a considerable amount of blood. The hematocrit (Hct) is 28 %. The patient has a history of malignant hyperthermia (MH) after a general anesthetic for a hernia repair at age 2 years. At that time, a positive family history of MH was found. His medical history is otherwise unremarkable. The patient has no known drug allergies. After discussion with the mother and the boy, an interscalene block [1] is performed with 60 ml of 1 % prilocaine and 10 ml of 0.5 % bupivacaine. Sedation with midazolam, 2 mg intravenous (IV), is given before the procedure. The usual monitoring equipment is placed, including a liquid crystal temperature strip on his forehead and a tympanic ear probe. Twenty-five minutes after the interscalene block, there is a good sensory block, a sympathetic block, and an ipsilateral Homer’s syndrome. The surgeon infiltrates locally 10 ml of plain 0.25 % bupivacaine and the surgery commences. The pulse oximeter shows a gradual decrease over a few minutes to a saturation of 75 %. The FIO₂ is increased to 100 % via a face mask from a “clean” anesthesia machine. The saturation remains at 75 %. The patient’s other vital signs are unchanged. His temperature is normal. The breath sounds are equal bilaterally with no adventitious sounds. The respiratory rate is 14. You are concerned and take an arterial blood gas measurement from his radial line. This shows a PaO₂ of 78 mmHg and a PaCO₂ of 36 mmHg with a base excess of −1 mmol/l with a derived oxygen saturation of 96 %.