A 12-year-old boy is brought to the emergency department due to a peritonsillar abscess. In preparation for incision and drainage, the patient is given a topical anesthetic, an anxiolytic, pain medications, and antibiotics. He develops a headache and becomes lethargic. Temperature is 38 C (100.8 F), pulse is 110/min, and respirations are 16/min. The lungs are clear to auscultation bilaterally. The lips and fingertips appear blue and the oxygen saturation is 84%. Supplemental oxygen is started but does not improve peripheral oxygen saturation. Which of the following is the most likely cause of this patient's hypoxemia?

Question

A 12-year-old boy is brought to the emergency department due to a peritonsillar abscess.  In preparation for incision and drainage, the patient is given a topical anesthetic, an anxiolytic, pain medications, and antibiotics.  He develops a headache and becomes lethargic.  Temperature is 38 C (100.8 F), pulse is 110/min, and respirations are 16/min.  The lungs are clear to auscultation bilaterally.  The lips and fingertips appear blue and the oxygen saturation is 84%.  Supplemental oxygen is started but does not improve peripheral oxygen saturation.  Which of the following is the most likely cause of this patient's hypoxemia?

Quizplus · Accepted Answer

11ec160b_3c41_5b11_9249_4118ba2d490b_MD0001_00 This patient with cyanosis that does not improve with supplemental oxygen has acquired methemoglobinemia due to the topical anesthetic (ie, benzocaine). Although methemoglobinemia is most commonly caused by dapsone, the most severe cases are seen after administration of benzocaine, which is found in many throat sprays or teething-relief medications. Acquired methemoglobinemia occurs when heme iron atoms are oxidized from the ferrous (Fe²⁺) to the ferric (Fe³⁺) state. Fe³⁺ heme is unable to bind to oxygen; furthermore, the oxygen affinity of the remaining heme moieties in the hemoglobin is increased. This results in a functional anemia (due to reduced oxygen binding) and a left shift of the oxygen dissociation curve (due to increased oxygen affinity), which leads to impairment in oxygen delivery despite adequate partial pressure of oxygen in arterial blood. Blood appears chocolate brown and does not change color when exposed to air. Due to the wavelength of light absorbed by Fe³⁺ heme, pulse oximetry converges on 85% as the proportion of methemoglobin increases, regardless of the true oxygen saturation status. Treatment with methylene blue or ascorbic acid reduces iron back to the Fe²⁺ state. (Choice A) This patient is at risk for bronchial occlusion due to aspiration because of difficulty swallowing (ie, peritonsillar abscess) and medications that impair his ability to protect his airway (ie, pain medication, anxiolytic). However, the lungs would likely sound abnormal, and supplemental oxygen would improve cyanosis. (Choice B) Carbon monoxide binds to hemoglobin with greater affinity than oxygen and also causes a left shift of the oxygen dissociation curve, impairing oxygen delivery to tissues. On pulse oximetry, carboxyhemoglobin reads like oxyhemoglobin, resulting in erroneously high (ie, falsely normal) readings. In addition, this patient does not have exposure risk factors for carbon monoxide (eg, house fire, car exhaust). (Choice C) Opioid pain medications and benzodiazepine anxiolytics can lead to a decreased central respiratory drive that, if severe, can lead to hypoxia. However, cyanosis would improve with supplemental oxygen. (Choice D) Medications can precipitate acute hemolysis in patients with glucose-6-phosphate dehydrogenase deficiency. However, it presents with sudden-onset jaundice, pallor, and dark urine; supplemental oxygen would improve pulse oximetry. Educational objective: Acquired methemoglobinemia can be precipitated by benzocaine and result in the conversion of Fe²⁺ to Fe³⁺, which results in functional anemia and a left shift of the oxygen dissociation curve. Patients have cyanosis that does not improve with supplemental oxygen. __________ References: Methemoglobinemia in the operating room and intensive care unit: early recognition, pathophysiology, and management. (http://www.ncbi.nlm.nih.gov/pubmed/32193811) Methemoglobinemia related to local anesthetics: a summary of 242 episodes. (http://www.ncbi.nlm.nih.gov/pubmed/19224791)

A 12-Year-Old Boy Is Brought to the Emergency Department Due