A 56-year-old man with a recent diagnosis of cirrhosis comes to the hospital for elective upper gastrointestinal endoscopy to evaluate for esophageal varices. The patient has elevated liver aminotransferase levels, and a hepatitis serology was positive for chronic hepatitis C. Abdominal imaging studies revealed mild ascites, a nodular-appearing liver, and splenomegaly. He has no other medical problems. The patient has a remote history of injection drug use and does not use tobacco or alcohol. Vital signs are within normal limits. Benzocaine throat spray is used for topical anesthesia, and procedural sedation is obtained with midazolam and fentanyl. During the procedure, the patient's oxygen saturation by pulse oximetry gradually decreases to 85% on ambient air and does not improve with application of supplemental oxygen via face mask. Physical examination demonstrates bluish discoloration of the lips and fingertips. Lung fields are clear to auscultation, and heart sounds are normal. Results of urgent laboratory tests are as follows: Which of the following is the most likely cause of this patient's decrease in oxygen saturation?

Question

A 56-year-old man with a recent diagnosis of cirrhosis comes to the hospital for elective upper gastrointestinal endoscopy to evaluate for esophageal varices.  The patient has elevated liver aminotransferase levels, and a hepatitis serology was positive for chronic hepatitis C.  Abdominal imaging studies revealed mild ascites, a nodular-appearing liver, and splenomegaly.  He has no other medical problems.  The patient has a remote history of injection drug use and does not use tobacco or alcohol.  Vital signs are within normal limits.  Benzocaine throat spray is used for topical anesthesia, and procedural sedation is obtained with midazolam and fentanyl.  During the procedure, the patient's oxygen saturation by pulse oximetry gradually decreases to 85% on ambient air and does not improve with application of supplemental oxygen via face mask.  Physical examination demonstrates bluish discoloration of the lips and fingertips.  Lung fields are clear to auscultation, and heart sounds are normal.  Results of urgent laboratory tests are as follows:  Which of the following is the most likely cause of this patient's decrease in oxygen saturation?

Quizplus · Accepted Answer

Explanation: This patient has new-onset hypoxia by pulse oximetry (85%) during an endoscopic procedure, with a significant difference between the oxygen saturation value estimated on blood gas analysis and that obtained on pulse oximetry (ie, large oxygen saturation gap). He likely has acquired methemoglobinemia due to anesthetic use. Some medications - most commonly topical anesthetics (eg, benzocaine), dapsone, and nitrates (in infants) - cause the iron component of hemoglobin to be oxidized, thereby forming methemoglobin, which cannot bind oxygen. The remaining normal hemoglobin also has an increased affinity for oxygen, resulting in less oxygen delivery to tissues. Because methemoglobin absorbs light at distinct wavelengths, pulse oximetry commonly is ~85% (as seen in this patient) regardless of the true oxygen saturation. In parallel, blood gas analysis frequently returns a falsely elevated result for oxygen saturation (eg, 99% in this patient) as it provides an estimate based only on the PaO₂, not on effective hemoglobin-oxygen binding. These inaccurate readings create the large oxygen saturation gap. oxidized Cyanosis can occur when methemoglobin comprises ~10% of total hemoglobin, but hypoxia symptoms (eg, headache, lethargy) occur only when levels surpass 20%. At levels >50%, there is risk of severe symptoms (eg, altered mental status, seizures, respiratory depression) and death. Treatment involves discontinuing the causative agent and administering methylene blue, which helps reduce iron to its normal state. (Choice A) Anaphylaxis (eg, due to medication) can lead to respiratory compromise and hypoxia but is associated with symptoms such as wheezing, stridor, and dyspnea. (Choices C and G) Although atelectasis and mucus plugging can lead to transient hypoxemia due to a collapsed portion of lung, the hypoxemia is commonly reversible with supplemental oxygen and would not explain the oxygen saturation gap. Procedural hypoventilation should also correct with supplemental oxygen. (Choice D) This patient has clear lung fields and normal heart sounds, making pulmonary edema less likely. Pulmonary edema also typically improves with supplemental oxygen and is not characterized by a large oxygen saturation gap. (Choice E) Patients with liver disease can have large-volume ascites and severe muscle wasting, which may lead to decreased diaphragmatic excursion and resultant hypoxia. However, this patient has only mild ascites on ultrasound and no evidence of muscle wasting. (Choice F) Hepatopulmonary syndrome results from arteriovenous shunting in the lungs and can occur after years of liver disease. Patients frequently exhibit platypnea (induced dyspnea in the upright position) and orthodeoxia (hypoxia in the upright position). This syndrome would not explain the patient's large oxygen saturation gap. Educational objective: Acquired methemoglobinemia results from the oxidization of iron in hemoglobin, which is most commonly due to topical anesthetic agents or dapsone. It presents with hypoxia, a characteristic pulse oximetry reading of ~85%, and a large oxygen saturation gap. References: Methemoglobin--it's not just blue: a concise review. (http://www.ncbi.nlm.nih.gov/pubmed/16986127)

A 56-Year-Old Man with a Recent Diagnosis of Cirrhosis Comes