Takeaway
This clinical conundrum, related to hypoxia, underscores the importance of meticulous history-taking and the potential for unexpected complications with seemingly benign medications.
Lifelong Learning in Clinical Excellence | January 15, 2025 | 2 min read
By Drs. Edana Mann & Matt Levy, Johns Hopkins Medicine
In medicine, things generally make sense—or so my colleague and I thought after a collective 40 years of clinical practice. But this shift, we both encountered a patient that left us puzzled.
I was called to assess a surgical patient boarding in the ED. He was admitted with a small bowel obstruction but had developed acute onset unexplained hypoxia without dyspnea about 12 hours after admission. His O2 saturation was at 85-86%, and his heart rate in the low 100s. Strangely, he appeared comfortable, despite being notably pale and cyanotic as he casually flipped through TV channels. His lungs were clear, and his other vital signs were normal: no tachycardia, no hypo- or hypertension. What was going on?
I decided to go back to the basics and started a thorough review: patient history, medications, work-up, and imaging. The patient had an NG tube placed 10 hours earlier, confirmed to be in the right position. He hadn’t taken anything orally, and a repeat x-ray confirmed proper tube placement without signs of aspiration. Blood work returned normal. Nothing added up.
The differential diagnosis for a low pulse oximetry reading with minimal symptoms and normal vitals is limited. He didn’t have a hemoglobinopathy, and dyshemoglobinemias like carboxyhemoglobin or methemoglobin seemed unlikely without exposure to recent medications or toxins. His perfusion was fine, and he wasn’t wearing nail polish that could interfere with the oximeter. We even swapped out the pulse oximeter—no change. His saturation stayed fixed at 85%, despite increasing FiO2.
I transferred him to a higher acuity area for further evaluation where my colleague took over. A CTA and repeat labs were ordered, to cast a broader net and eliminate other life threats, such as PE. One of our seasoned APPs took another detailed history, asking the patient if anything had changed. The patient said no, and that his throat had been bothering him since the placement of the NG tube. He then said, “But that numbing spray has helped a lot!”
Eureka! Even though he hadn’t mentioned it before, the patient told us that he’d been using a benzocaine spray throughout the day to relieve throat discomfort from his NG tube placement. His methemoglobin level came back at 24%, confirming the diagnosis. We promptly administered methylene blue, and his oxygen saturation improved back to normal.
Lessons learned:
1. Use caution even when prescribing over-the-counter local anesthetics.
Benzocaine and similar agents (lidocaine, prilocaine) can cause methemoglobinemia, even with topical use.
2. Educate patients and monitor medication usage.
Patients often don’t understand the risks of self-administering medications, and some providers may underestimate these dangers.
3. Persistence pays off.
A thorough history and continuous reassessment can uncover the unexpected. Never give up on finding the answer.
Sir William Osler captured it best: “Listen to your patient; he is telling you the diagnosis.”
This piece expresses the views solely of the author. It does not represent the views of any organization, including Johns Hopkins Medicine.