Saturation Gap During Topical-Anesthetic Bronchoscopy

There are three different numbers, and they are answering different questions.

• PaO2 = 232 mmHg is a dissolved-oxygen tension, not a content or a saturation. From the oxyhemoglobin dissociation curve at this tension, the expected SaO2 is above 99%. So he should be saturated.
• Co-oximetry O2Hb = 64%. Functionally, 64% + 2% COHb = about 66% of hemoglobin carrying oxygen. That is the truth of the case.
• The gap of roughly 30 percentage points between expected saturation and measured O2Hb equals the MetHb fraction of 31%. That arithmetic is the diagnosis: a saturation gap with a normal PaO2 and an elevated methemoglobin.

Why the pulse oximeter sits at about 85%: methemoglobin absorbs light nearly equally at the 660 nm and 940 nm wavelengths, so the ratio-of-ratios drifts toward 1 and the displayed SpO2 converges on roughly 85% regardless of the true saturation. The 84-86% is a device artifact plateau, not a measurement of his oxygenation - and it explains why moving the probe and turning up the oxygen change nothing. Blood from this arterial sample will keep showing a handsome PaO2 while the oxygen content is low; content, not tension, is what the tissues see.

1. Stop the exposure. Remove the benzocaine; do not give more topical anesthetic. Call for help and get the proceduralist to stop.
2. Airway and 100% oxygen. He is agitated and dusky; assess for airway compromise and a depressed level of consciousness, and be ready to secure the airway. High FiO2 supports content even though it does not reverse methemoglobin.
3. Get definitive treatment moving. Methylene blue 1-2 mg/kg IV over about 5 minutes is the standard antidote; give it promptly in a symptomatic patient with a level this high rather than waiting.
4. Monitoring that is invalid: SpO2 - it will read about 85% at any true saturation and cannot be used to titrate oxygen or to judge deterioration. Trend instead with serial co-oximetry, mental status, hemodynamics, and a repeat arterial blood gas with co-oximetry. If a central or arterial line is present, use it for sampling and pressure.

The proceduralist's plan to "turn up the oxygen and recheck in 10 minutes" is the wrong move on both counts: he is already on 15 L, and oxygen does not reduce methemoglobin. Delay buys central cyanosis and possibly cardiac arrest.

Methylene blue is an oxidant that depends on NADPH generated by G6PD to reduce methemoglobin back to hemoglobin. In G6PD deficiency it is both less effective and capable of causing hemolysis, so status matters - in a population with a meaningful prevalence, some would send the test or use an alternative. But this patient is symptomatic with a methemoglobin level near a third of his hemoglobin, and the risk of delaying antidote in that setting is greater than the risk of an unrequested G6PD test. Practical answer: give methylene blue now at the standard dose, send G6PD in parallel, monitor for hemolysis, and avoid repeat doses unless the level rebounds. If methylene blue is contraindicated or ineffective, alternatives are ascorbic acid, exchange transfusion, and hyperbaric oxygen for severe cases.

To the team about the case: it stops now. The diagnosis is an anesthetic-induced methemoglobinemia, the patient's oxygen-carrying capacity is roughly a third gone, and continuing the procedure adds risk with no benefit. Reschedule once he is stable and co-oximetry has normalized, and record the reaction prominently so benzocaine and other oxidant agents are avoided on the re-do. Document the exposure, the timing, the co-oximetry values, the treatment, and the response, and report it through the institution's adverse-event process.