A 15-month-old boy is brought to the emergency department with cyanosis. Over the past week, the patient has been teething and has had mild nasal congestion and low-grade fevers. He has been eating and drinking normally. The patient was extremely fussy today, despite his mother giving him acetaminophen and a topical anesthetic that she used when she had a toothache. He was hospitalized for bronchiolitis 6 months ago but is otherwise healthy, and immunizations are up to date. Blood pressure is 90/60 mm Hg, pulse is 158/min, and respirations are 50/min. Pulse oximetry is 85% on room air. Physical examination shows a sleepy child with blue discoloration of the skin and nails. The lungs are clear to auscultation bilaterally. Cardiac examination reveals no murmurs or gallops. The abdomen is soft, nontender, and has no hepatosplenomegaly. Blood samples, including arterial blood gas on room air, are obtained for laboratory testing. Subsequently, 100% oxygen is administered but fails to increase the pulse oximetry reading or the cyanosis.
Which of the following laboratory findings would most likely be seen in this patient?

Question

A 15-month-old boy is brought to the emergency department with cyanosis.  Over the past week, the patient has been teething and has had mild nasal congestion and low-grade fevers.  He has been eating and drinking normally.  The patient was extremely fussy today, despite his mother giving him acetaminophen and a topical anesthetic that she used when she had a toothache.  He was hospitalized for bronchiolitis 6 months ago but is otherwise healthy, and immunizations are up to date.  Blood pressure is 90/60 mm Hg, pulse is 158/min, and respirations are 50/min.  Pulse oximetry is 85% on room air.  Physical examination shows a sleepy child with blue discoloration of the skin and nails.  The lungs are clear to auscultation bilaterally.  Cardiac examination reveals no murmurs or gallops.  The abdomen is soft, nontender, and has no hepatosplenomegaly.  Blood samples, including arterial blood gas on room air, are obtained for laboratory testing.  Subsequently, 100% oxygen is administered but fails to increase the pulse oximetry reading or the cyanosis. 
Which of the following laboratory findings would most likely be seen in this patient?

Quizplus · Accepted Answer

11ec25c8_d706_b2d0_bc81_13e97cd2eb88_MD0003_00 This patient has methemoglobinemia, an uncommon complication that can occur after excessive exposure to an oxidizing agent (eg, dapsone, nitrites, local/topical anesthetics) and results in cyanosis. Additional clinical features may include lethargy, respiratory depression, seizures, and death. In normal hemoglobin, iron is in the ferrous (Fe²⁺) state. When exposed to an oxidizing agent, at least one of the four iron molecules is oxidized to the ferric (Fe³⁺) state, resulting in acute methemoglobinemia. Ferric iron is unable to bind oxygen; in addition, the ferric state changes the hemoglobin structure and causes ferrous sites to have an increased affinity for oxygen (ie, "left shift" on oxygen-dissociation curve). The increased oxygen affinity prevents oxygen release in peripheral tissues (ie, decreased oxygen delivery). ²⁺ Because methemoglobin absorbs light at a different wavelength than hemoglobin, patients have pulse oximetry readings of ~85% regardless of the true oxygen saturation level (as seen in this patient). Supplemental oxygen has no effect on the altered methemoglobin structure; therefore, it does not improve cyanosis, blood color, or pulse oximetry readings. In contrast, arterial blood gas testing analyzes only unbound arterial oxygen (as opposed to hemoglobin-bound oxygen) and displays a falsely elevated oxygen saturation level, shown as normal PaO₂. (Choice A) PaCO₂ increases in respiratory failure such as in severe bronchiolitis or asthma. In a tachypneic individual with healthy lungs, as in this case, PaCO₂ will be low due to hyperventilation from tachypnea. (Choice B) An elevated plasma osmolal gap is seen after ethanol, methanol, or ethylene glycol ingestion. Patients with this condition may have signs of alcohol intoxication but are not cyanotic. (Choice C) Elevated transaminase levels signify liver inflammation and can be due to an overdose of acetaminophen, but these patients have abdominal pain, nausea, and vomiting, not cyanosis. (Choice D) Co-oximetry testing analyzes hemoglobin absorption wavelengths and can identify hemoglobin, methemoglobin, and carboxyhemoglobin. Elevated levels of methemoglobin would confirm the diagnosis in this patient, and therefore co-oximetry testing would not be normal. Educational objective: Methemoglobinemia presents with cyanosis after exposure to an oxidizing agent (eg, dapsone, nitrites, anesthetics). Pulse oximetry saturation is typically low and does not improve with the administration of oxygen; however, PaO₂ is normal. References: A 12-day-old boy with methemoglobinemia after circumcision with local anesthesia (Lidocaine/Prilocaine). (http://www.ncbi.nlm.nih.gov/pubmed/27747692) Lidocaine-induced methemoglobinemia: a clinical reminder. (http://www.ncbi.nlm.nih.gov/pubmed/25637615)

A 15-Month-Old Boy Is Brought to the Emergency Department with Cyanosis