Question

Patient receives topical anesthetic (benzocaine) and develops O2 sat of ~85%, ABG with O2 sat 99%:

a) Methemoglobinemia
b) Hypoxemia
c) Hypercapnia
d) Carboxyhemoglobinemia

Answer 1

a) Methemoglobinemia is the most likely diagnosis for a patient with an apparent discrepancy between pulse oximetry (showing low O2 sat) and arterial blood gas (showing high O2 sat) after receiving benzocaine, as benzocaine can cause oxidation of hemoglobin iron, leading to a form of hemoglobin that cannot effectively bind oxygen.

Step-by-step explanation:

A patient who receives topical anesthetic like benzocaine and then develops an oxygen saturation (O2 sat) of approximately 85%, but has an arterial blood gas (ABG) showing an O2 sat of 99%, is likely experiencing methemoglobinemia. This condition occurs when the iron in hemoglobin is oxidized to its ferric form (Fe3+), rendering it unable to bind oxygen effectively. As methemoglobin increases, it imparts a chocolate-brown color to the blood. Despite the ABG indicating a high O2 sat, the pulse oximeter may show a lower reading because it cannot differentiate methemoglobin from oxyhemoglobin or deoxyhemoglobin. Topical anesthetics such as benzocaine are known to increase the formation of methemoglobin, disrupting the normal functions of hemoglobin.