Tasty Morsels of Critical Care

Welcome back to the tasty morsels of critical care podcast.

We’re going to cover a bit of an environmental/tox topic today and look at carbon monoxide poisoning from Oh’s manual chapter 83 on burns. I have previously covered this on the old tasty morsels of EM series back when i was doing my EM fellowship exams.

As you no doubt remember from school chemistry classes, carbon monoxide is a colourless, odourless, tasteless gas produced when combustion occurs with insufficient oxygen.

We’re likely to see this in a couple of contexts.

1) the house fire victim, pulled from the fire unconscious and sick

2) the sub acute or chronic poisoning in a patient presenting with headaches and flu symptoms that seem to get better when they leave the problem environment. The classic EM example is the whole family who present with flu symptoms and no fever and even the dog is sick. We’re much less likely to see this cohort in the critical care side of things.

How does it make people sick? Haemoglobin is a fickle little protein, while evolved to carry oxygen to needy tissue beds it actually has a distinct preference not for our beloved oxygen but for carbon monoxide. Introduce some carbon monoxide at the alveolus and the haemoglobin molecule will bind to CO with an affinity 240 times that than for oxygen. I take that number of 240 somewhat at face value but I presume someone got a PhD from working that out. In visual form my preferred means of explanation for this would be the distracted boyfriend meme where the haemoglobin boyfriend looks longingly over his shoulder at the carobon monoxide while his oxygen girlfriend looks on in horror. Hopefully you get the idea.

So instead of having lots of circulating oxyhaemoglobin we’re instead left with lots of not especially useful carboxyhaemoglobin. Let’s imagine 50% of our Hb is now carboxyHb and 50% is OxyHb we’re left with a sort of severe fucntional anaemia where half of our Hb is out of action. One might be inclined to think that this is the major cause of morbidity and mortality in CO poisoning but in fact this is only a small portion of the problem. CoHb actually has a direct cytotoxic effect on things cytochrome oxidase and myoglobin function. As such it interrupts the whole process of oxidative metabolism and life as we know it.

We can measure the level of CO fairly easily, any blood gas machine worth its salt should be able to give you a break down of the types of Hb present in the sample. This is co-oximetry and typically it’ll show you oxy, deoxy, carboxy and met haemoglobins. All these different forms of Hb absorb different wavelengths of light. The lowly pulse oximeter does not have the subtlety to distinguish the different wavelengths as it only functions at wavelengths of 940 and 660nm. Indeed the pulse ox often demonstrates a non diagnostic number somewhere in the 80s rather than a true reflection of the CarboxyHb or OxyHb present.

Severe CO poisoning resulting in obtundation is going to have high level of COHb on our cooximeter. >10% is quoted but it’s more often over 30%. Patients are going to be pretty sick often from multiple pathologies but COHb on its own is enough to produce severe neurological injury, shock and even cardiac injury is also quite prevalent. Expect a high lactate given the disruption of oxidative metabolism. Resuscitate and investigate as you would any sick patient.

Treatment is nice and simple in that we just give loads of oxygen. Oxygen reduces the half life of CO in the blood quite dramatically, commonly quoted numbers are

• the haf-life of COHb in an FiO2 of 0.21 is 300 minutes


• the half-life of COHb in an FiO2 of 1.0 is 60-90 minutes

There is a substantial rationale and literature on the use of hyperbaric oxygen as a means of accelerated clearance of COHb. But the RCTs that have been done don’t seem (to me at least) to give a clear benefit. The Lindell Weaver NEJM RCT in 2002 did suggest a neuro benefit but only 8% of the patients in this trial were intubated. A follow up trial in 2011 by ICU steroid guru Djilalli Annane did not find a benefit . So if anyone should get this it might be the non intubated isolated COHb poisoining. This is not really our cohort. Our cohort is likely to be tubed, shocked, with multiple injuruies and not someone you want to transport cross county to put in a single person hyperbaric chamber for hours at a time.

Reading

Oh Manual Chapter 83