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Current time:0:00Total duration:12:19

Video transcript

I want to take a moment to really acknowledge the beauty of this molecule right here this is a heme molecule and this little molecule allows us to deliver oxygen all throughout our body through a protein called hemoglobin hemoglobin is a protein with four different subunits and I'm drawing right here and at the core of each subunit is a heme molecule and these heme molecules have this iron group that is essential for binding oxygen this iron is Fe 2 plus and Fe 2 plus is also known as ferrous iron so at the core of each of these subunits is an Fe 2 plus molecule that allows for binding of oxygen and delivery of this oxygen to the tissues and the reason I colored this hemoglobin molecule slightly differently is because it's composed of two different types of subunits so in each hemoglobin molecule there are two alpha subunits and two beta subunits the protein structure of these subunits of hemoglobin are just slightly different which allows the formation of an entire hemoglobin molecule now this little molecule this hemoglobin makes a big impact on the body each red blood cell has close to 270 million hemoglobin molecules so you can see that it plays a very large role in delivering oxygen throughout the body and of course oxygen first comes from the lungs to jump on the hemoglobin and then it's delivered to the tissues so what if we somehow impaired hemoglobins ability to deliver oxygen to the tissues well what is impaired oxygen delivery to the tissues called it's called shock so if something impairs the ability of hemoglobin to dissociate from oxygen and drop oxygen off in the tissues it can lead to shock so there's an issue with oxygen dissociating or coming off of hemoglobin so this is known as dissociative shock so let me go ahead and show you something you might be familiar with known as the oxygen hemoglobin dissociation curve and on the x-axis I have written here pao2 and what that stands for is the concentration of oxygen and here on the y-axis we're going to put saturation of oxygen and so this is really oxygen saturation of hemoglobin how saturated it is so it makes sense as we increase oxygen concentration oh two will probably be more likely to bind on to hemoglobin there will be more oxygen readily available for binding on to hemoglobin and so that can actually be seen in the dissociation curve as the concentration of oxygen increases the saturation on to hemoglobin increases and as we continue to increase the oxygen concentration the pao2 hemoglobin will become completely saturated so the curve will sort of plateau out at a hundred percent oxygen saturation so now let's consider the issue of dissociative shock if something causes decreased delivery of oxygen to the tissues from hemoglobin so in other words hemoglobin does not let go of oxygen it doesn't deliver it to the tissues hangs on to it right so this would be increased binding of oxygen on to hemoglobin so what would this look like on this curve here well as we increase the concentration of oxygen we're saying that hemoglobin binds oxygen more readily so it will become saturated more quickly and so we see what's called a left shift in the oxyhemoglobin curve and so intuitively again as we increase oxygen concentration hemoglobin quickly grabs up and binds on to oxygen so we have this increased binding of oxygen and you might think that would be a good thing but consider this for a moment we can agree that the pao2 the oxygen concentration in the lungs is very high so at a very high level we're pretty close to a hundred percent saturation of hemoglobin now in the tissues of course concentration of oxygen is lower so we'll see what this dotted line here the saturation of hemoglobin in the tissues is very low a lot lower it is in the lungs so essentially as red blood cells travel from the lungs to the tissues they're able to drop off a lot of oxygen they let go of the oxygen they're hanging on to so we have this delivery of oxygen however let's take a look at when oxygen is bound more tightly you'll see that the saturation of hemoglobin is all the way up here so when you go from the lungs to the tissues on this increased binding curve this left shift curve we see decreased oxygen delivery right only so much of the hemoglobin is letting go of its oxygen and so that's shock now a pneumonic I like to use to remember this left shifting of the curve is that oxygen is left on hemoglobin now let's get back to the topic at hand what can cause a left shift in the oxyhemoglobin curve well there are two main causes of dissociative shock and the first cause is met hemoglobin and if you have met hemoglobin in your blood it's known as methemoglobinemia amia means in the blood so what what the heck is met hemoglobin well before I emphasize that the fe 2 plus form or ferrous is the type of heme group that you see in hemoglobin however there's another form of iron known as ferric iron now ferric iron is when the iron molecule is oxidized to the three plus state iron oxidized to the three plus state does not bind oxygen readily so oxygen will not bind on to this ferric heme group if hemoglobin has a ferric iron it's known as met hemoglobin so met hemoglobin has this fe 3 plus and as I said fe 3 plus cannot bind oxygen now that in itself could be considered pretty bad however having this fe 3 plus produces a conformational change or change in shape of this hemoglobin molecule that allows oxygen to bind more readily onto these other sites think of it this way there's no seat right here for oxygen so these other oxygen molecules rush over and want to take up these other seats because there's very limited seating so in methemoglobinemia we see increased ferrous iron binding of oxygen and as mentioned before this decreases the tissue perfusion and so we see a left shift in oxygen binding curve so what causes methemoglobinemia Mahima globe anemia is caused by nitrates now nitrates can be found in certain medications particularly antibiotics so an example of a common antibiotic that's rich in nitrates is bactrim which is also known as trimethoprim TMP and sulfamethoxazole SMX another common antibiotic is dapsone certain anesthetics can do it too it's an example of an anesthetic that causes methemoglobinemia is benzocaine but medications are not the only cause of methemoglobinemia some pesticides have been noted to cause it so sometimes people who drink water from wells where the ground is saturated with a lot of pesticides they can get poisoned and have methemoglobinemia now what symptoms can you see in a patient who has methemoglobinemia the symptoms are going to include signs of oxygen starvation so beginning symptoms often include a headache or dizziness and as symptoms progress to more severe patients may exhibit fatigue or confusion and potentially even loss of consciousness and you can also see the typical symptoms of shock such as a rapid heart rate tachycardia difficulty breathing now also known as dyspnea and other signs of organ dysfunction now that the treatment of methemoglobinemia will be with a medication known as methylene blue so you'll give IV methylene Ballu and what IV methylene blue does is aids in the conversion of Fe three-plus back to the fe 2 plus state oh and I forgot a really good mnemonic to remember this oxygen will bind readily to the fair Russ state because it's a togetherness hemoglobin and oxygen makes us however oxygen does not want to bind to Fe three-plus because it's disgusting ick it doesn't like the Fe three-plus now make hemoglobin emia is especially dangerous for newborns there's an enzyme that adults have known as cytochrome b5 reductase now what cytochrome b5 reductase does is it converts Fe three-plus to the fe 2 plus state just like we see for the treatment in methemoglobinemia so converting it back will allow oxygen to bind and dissociate correctly like it normally should now newborns have a decrease in this enzyme up until about 4 months old so if they're exposed to perhaps certain anesthetics or medications or pesticides in drinking water newborns may experience its associated of shock now to finish this up met hemoglobin is not the only cause of dissociative shock another major cause is carbon monoxide poisoning so the molecule carbon monoxide has one carbon and one oxygen carbon monoxide so carbon monoxide actually binds hemoglobin HB a hundred times greater than oxygen so carbon monoxide actually prevents oxygen from binding on to hemoglobin in the lungs and similar to met hemoglobin if carbon monoxide is bound to hemoglobin then oxygen will bind more tightly to these other subunits so it creates a conformational change a change in shape of hemoglobin preventing the release of oxygen into the tissues so going back down here we have decreased oxygen delivery which is due to increased strength of binding however even though oxygen is strongly binding if another carbon monoxide molecule comes closer it will kick off the oxygen because again it has a hundred times more binding affinity than oxygen so again all of this causes a left shift in the oxyhemoglobin curve now the causes of carbon monoxide poisoning really have to do with fires wood stoves house fires or any sort of combustion engine creates carbon monoxide so long-term exposure to the smoke and fires can cause carbon monoxide poisoning and like methemoglobinemia the symptoms are very similar you'll start out with a headache and dizziness and progress to fatigue confusion loss of consciousness difficulty breathing so on and so forth the treatment for carbon monoxide will be a hundred percent oxygen the idea is if you over saturate the patient with oxygen it can hopefully cause carbon monoxide to be kicked off of the hemoglobin molecule so that's dissociative shock in the inability for oxygen to dissociate from hemoglobin