I realize reading this in March may make you question 10 percent of the patients you've seen in the past two months, but this is a reminder to break out of your it's-all-influenza-to-me winter doldrums.
Modern technology often ruins the punchline of carbon monoxide poisoning, especially when the paramedics roll in and say, “The fire department was on scene, and confirmed an elevated CO level,” but there's probably a number of these cases that still slip through the cracks when there's no home detector or proactive fire department around. (One study suggests we miss up to 23 percent of CO cases. [Ann Emerg Med 1987;16(7):782.]) Your CO Spidey sense should tingle a little when you seeing multiple patients with flu-like symptoms, especially if they started at the exact same time (one person starts feeling sick before the other with influenza, in my experience).
Besides reminding you to keep CO poisoning in your differential, I thought I'd provide a quick summary for those of you who aren't recently graduated from residency. The symptoms are vague and flu-like. Most people get a headache but not everyone. I just saw a couple with CO poisoning, and one had a headache while the other did not but did have significant nausea, vomiting, and myalgias.
Ignore cherry red skin. You have to have had a lethal amount of CO to get that red. Keep in mind that greater than 3-4% carboxyhemoglobin levels (COHb) is considered elevated in nonsmokers, and more than 10% is abnormal in smokers. (One paper reported that the COHb rises approximately 2.5% for each pack of cigarettes smoked per day. [J Am Assoc Nurse Anesth 1987;55(5):421.]) If you suspect CO poisoning, just start non-rebreather oxygen (we all know this isn't actually 100% oxygen), which will reduce the half-life of CO in the blood from 320 minutes on room air to 74 minutes.
Then there's hyperbarics, which for CO seems to be a favorite controversy of toxicologists and “altitudists” alike, with conflicting data and studies to boot. I'm going to try to simplify it: Consult your local toxicologist.
You should think about hyperbarics in anyone with severe symptoms that are worse than garden-variety flu symptoms: loss of consciousness, ischemic changes or severe anginal symptoms, neurological deficits, significant metabolic acidosis, or a level more than 25% (most people with severe symptoms are going to have a high level). Remember, the goal of hyperbarics is to improve neurocognitive sequelae like memory function.
And who gets to go home? It's probably reasonable to discharge patients whose symptoms have completely resolved, who did not have loss of consciousness or acute coronary syndrome symptoms, and have no sign of end-organ damage (brain, heart). Also send home those whose COHb level is normal (some would argue an initial, only mildly elevated level plus time on high-flow oxygen doesn't require repeating) and who feels better and who road tests and PO challenges well.
I often get confused about CO and cyanide, and in fires, just think about both. Cyanide is formed from the combustion of a ton of materials in the home: cotton, wool, paper, and plastic, for example. You inhale both in a closed-space fire, and both impede oxygen delivery to your tissues. Some experts suggest sending lactates on patients with significant smoke inhalation because a lactate greater than 10 is highly suspicious for cyanide toxicity.
You may have learned about the Lilly Cyanide Antidote Kit containing sodium nitrite, sodium thiosulfate, and amyl nitrite to bind up cyanide, but, unfortunately, it also makes methemoglobin, which you can imagine might be a bit suboptimal for the patient with oxygenation issues. Luckily, there's now the CYANOKIT, which is some incredibly clever pharmacology. It's hydroxocobalamin (Vitamin B12a), which binds up cyanide to form cyanocobalamin (vitamin B12). The CYANOKIT is a dark red, so it will mess up your pulse oximetry and can even shut down hemodialysis machines by triggering the machines to think there's a blood leak. (Blood Leak would be a great name for a death metal band.)
We can make it through this flu season! I believe in us! At some point, the season will end, and we can remove our masks from our chapped faces and put lotion on our cracked, alcohol-burned hands. If it looks like the flu, smells like the flu, and moans like the flu, remember, it's probably the flu. But bragging rights will be yours if you consider CO in your differential and make the diagnosis. And hopefully now, you'll feel a little more confident in treating it, too.