CARBON MONOXIDE
Carbon Monoxide (CO) is a greedy bitch. It steals all the Hemoglobin binding sites (x240 more affinity for Heme).
CO plays by its OWN rules. Once it binds to Hgb, it never leaves. And we know that when one “site” is occupied on a Hgb, the rest of the sites like to stay bound also (peer pressure). Oxygen isn’t dumped into the tissues. The stupid Dissociation Curve shifts left AND down.
CO is an invisible odorless gas. Toxic levels of CO are found with combustion (car exhaust, gas heaters, fire). I saw a case once where a family brought a charcoal grill inside their home (it was cold outside), and the entire family of 6 was found a few minutes later passed out on their front lawn. They all survived, but they all showed up to the ER unconscious.
CO Poisoning causes very vague symptoms: HA, malaise, nausea, confusion, dizziness and sometimes syncope. A classic (but rare) sign is cherry red Lips. You can also see bilateral globus pallidus lesions on an MRI (also seen with Cyanide poisoning). There’s no cyanosis with CO poisoning, because it isn’t the absence of oxygen that causes cyanosis, as cyanosis occurs only when CO2 is bound to Heme.
The standard pulse ox machine doesn’t differentiate between CO and O2 (some newer models can, however), so their oxygen saturation is 100%. You have to get a blood test called the carboxyhemoglobin level. The level is normally less than 3%, but smokers / urbanites have levels around 10%. Over 15% suggests CO poisoning.
Treat poisoning with oxygen. Some studies suggest that hyperbaric oxygen is especially effective.
METHEMOGLOBINEMIA
Fe3+
Methemoglobin is a disease where the iron inside of heme goes from the 2+ state to the 3+ state, rendering it less functional. It’s sort of like the heme rusts over.
Heme needs iron. Specifically, the iron needs to be in the Fe2+ state. Some oxidizing drugs can change iron to Fe3+. Heme that contains ferric (3+) iron is totally useless -- it can’t bind O2 and it can’t release O2 into the tissues. Can’t bind, can’t release…. it’s as if the heme wasn’t even there. This creates a functional anemia! The drugs capable of oxidizing heme include the local anesthetics (Lidocaine, Benzocaine), Nitric oxide and Dapsone (an infrequently used antibiotic). Yes, I know it’s a pretty random group of drugs, but it is important to learn them.
Here are some diagnostic features. Methemoglobin gives the blood a “chocolate” hue. It can cause cyanosis. Methemoglobin interferes with the pulse oximeter, so the oxygen saturation is variable (often around 80%). The PaO2 is perfectly normal (the lungs are working normally).
The treatment is Methylene Blue, which reduces the methemoglobin back to the Fe2+ state (fascinatingly, Methylene Blue can also induce Methemoglobinemia in a healthy person! No clue why). Another helpful treatment is Vitamin C.
I think one of the take home points about this chart is that SaO2, Hgb and PaO2 (three very common diagnostic tools for dyspnea) can be normal in the dyshemoglobinopathies, i.e., CO poisoning, Cyanide poisoning and Methemoglobinemia.
CYANIDE POISONING
C≡N
Cyanide breaks the e- transport chain. Cells have to use the lactic acid pathway instead. The heart and brain are heavily reliant on the e- transport chain, making Cyanide toxic as fuck. Cyanide usually just kills you (rapid cardiovascular collapse), but a buzzword is almond-scented breath. The oxygen saturation, PaO2 and Hgb are usually normal, making the diagnosis difficult. House fires are perhaps the most common cause -- Cyanide is in the smoke. Nefarious murder, Nitroprusside (a powerful vasoactive drug) or Amygdalin (a pseudoscience drug made out of apricot pits) are other causes.
There are three treatments for Cyanide poisoning. They each work by binding to Cyanide, transforming it into a less toxic byproduct.
Sodium Thiosulfate binds to Cyanide, turning it into thiocyanate, which is easily excreted in urine.
Hydroxocobalamin also binds to Cyanide, turning it into cyanocobalamin, also excreted in pee.
Nitrite also works by binding up Cyanide, but it does so in an indirect way. In essence, you use the Nitrite to induce Methemoglobinemia. Sounds crazy, right? The point is that 3+ iron neutralizes Cyanide (while 2+ iron does not). So by inducing one disease, you can treat another more serious one.
