Carbon Monoxide: The 'Cherry Red' Trap

If you're like me, the words "smoke inhalation" trigger an immediate mental checklist — airway, burns, hypoxia. What most residents miss is the third dimension of that scenario: the pulse oximeter is lying to you. A patient pulled from a house fire can have a pulse ox reading of 99% and a carboxyhemoglobin level of 40%. Their hemoglobin is saturated — just not with oxygen. On the anesthesiology oral boards, knowing this distinction is the difference between a consultant answer and a dangerous one.

Carbon monoxide poisoning is a perfect board scenario because it tests two things at once: your knowledge of the physiology and your skepticism of monitoring data. The examiner is watching to see if you trust the number on the screen or trust the clinical picture.

The Core Logic

CO binds hemoglobin with approximately 240 times the affinity of oxygen. The resulting carboxyhemoglobin (COHb) can't carry oxygen, but it absorbs light at the same wavelength as oxyhemoglobin. Standard pulse oximetry measures optical absorption at 660 nm and 940 nm — it reads COHb as if it were oxyhemoglobin. The displayed saturation is therefore falsely elevated by whatever percentage of hemoglobin is bound to CO.

The treatment is competitive displacement: give the patient enough oxygen to outcompete the CO for binding sites. On room air, the half-life of COHb is about 5 hours. On 100% normobaric oxygen, it drops to 60-90 minutes. On hyperbaric oxygen at 2.5 atmospheres, it drops to 15-30 minutes.

How the Examiner Tests This

The typical setup: a patient is brought in after a house fire. Their pulse ox reads 97%. They're mildly confused. The examiner asks "How are they doing?" If you say "oxygenation looks fine," you've failed the first probe.

The follow-up questions focus on: when do you use hyperbaric oxygen? Answer: COHb level above 25% (some say 20%), any neurological symptoms (confusion, syncope, focal deficits), pregnant patient, or evidence of cardiac ischemia. The examiner also wants to know what you'll do before the HBO chamber is available — and the answer is 100% high-flow oxygen immediately, no waiting.

The Board Trap

The first trap is trusting the pulse ox. In any patient with smoke exposure, the standard pulse ox number is meaningless until you have a co-oximetry reading from an arterial blood gas. CO-oximetry measures COHb, methemoglobin, and true oxyhemoglobin separately. Only this tells you what's actually happening.

The second trap is under-treating because the patient "looks okay." Early CO poisoning causes headache and confusion. Late CO poisoning causes coma and death. A patient who is mildly confused with a COHb of 35% is not "okay" — they're one step from a catastrophic neurological event. Treat aggressively, not expectantly.

Lead-In Phrases

• "I recognize that standard pulse oximetry is completely unreliable in carbon monoxide poisoning — it cannot distinguish carboxyhemoglobin from oxyhemoglobin. I will obtain a co-oximetry ABG immediately."
• "I will place the patient on 100% non-rebreather oxygen immediately — this is the most important single intervention, as it reduces the half-life of COHb from 5 hours to under 90 minutes."
• "I will evaluate for indications for hyperbaric oxygen therapy: a COHb above 25%, any neurological symptoms, evidence of cardiac ischemia, or pregnancy — any of these moves this patient to HBO."
• "My clinical assessment takes priority over the monitor number in this scenario — confusion in a smoke-inhalation patient is CO poisoning until proven otherwise."
• "I will also evaluate for concurrent cyanide poisoning in any closed-space combustion patient — if suspected, empirical hydroxocobalamin 5g IV is appropriate while I confirm."

FAQs

Why does the patient look "cherry red"?

The cherry-red skin color is a classic teaching sign caused by COHb, which gives blood a bright red appearance despite containing no usable oxygen. In reality, it's more common as a post-mortem finding than a reliable clinical sign in living patients. Don't wait for cherry-red skin to make the diagnosis.

Does HBO help in every case?

No. HBO is indicated for severe cases — high COHb, neurological symptoms, cardiac involvement, or pregnancy. For mild cases with a normal neurological exam and COHb under 20%, 100% normobaric oxygen is appropriate. The logistics and risks of HBO (barotrauma, pressure changes in an unstable patient) need to be weighed against the benefit.

What about cyanide poisoning in the same patient?

House fires with plastics and synthetics produce hydrogen cyanide in addition to CO. Suspect cyanide in any fire victim with refractory lactic acidosis — the classic "you can't use oxygen, but there's still no oxygen debt being paid" picture. Empirical hydroxocobalamin is safe and doesn't interfere with CO management. On the boards, mentioning this shows you're thinking beyond the obvious diagnosis.

CO poisoning is a test of your monitor literacy and your clinical instincts. Trust the ABG, not the pulse ox. Practice the "Smoke Inhalation at the Scene" scenario in Boards Bot until your first move is always reaching for the co-oximetry, not the pulse ox number.