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Current time:0:00Total duration:11:49

Heart attack (myocardial infarction) pathophysiology

Video transcript

so we know that the most common reason heart attacks happen is because of atherosclerosis and this plaque buildup will compromise blood flow to your heart muscle and when your heart muscle doesn't get access to blood flow it doesn't get access to oxygen inside that blood so that's essentially how you get a heart attack and remember in medicine we call heart attacks myocardial infarcts so myocardial referring to muscle muscle of the heart and infarct referring to lack of oxygen causing death of tissue so myocardial infarct lack of oxygen to heart muscle causing death of that heart muscle that's what a heart attack is so we know how a heart attack occurs but what exactly is happening in your body what's happening with your heart when you're actually having a heart attack well let's take a look at that and let's not let's actually start from the beginning here so let's bring in let's bring in an artery here so we can visualize what happens to the to the heart muscle cells during a heart attack so here's our artery let's draw in our cells now so there's some heart muscle cells and so you might have noticed that I drew these cardiomyocytes in a really oddly connected way so you can see a connection there and you can see connection there and and there's one there and so on and the reason that cardiomyocytes that heart muscle cells are connected in this way is because by being connected like this they can more efficiently work together to make sure that the heart pumps properly so what exactly is happening in a heart attack so let's say we're looking at this blockage right here and then let's say that the the piece of artery that we've drawn is this piece of artery right here so all these cardiomyocytes here all these cardiomyocytes are are what's surrounding this vessel here all right so it's this piece right here that we're representing so you got this plaque in your arteries upstream and it's not ruptured it's just sitting there it's not really doing too much right now but then let's say you start playing soccer right so you start running around chasing after the ball and and when you're running around blood is being forced to sort of flow faster and faster through your coronary arteries right because your your heart's pumping faster well all that blood sort of rushing through your coronary arteries because your heart's pumping faster that rushing blood will sort of bombard your plaque and and your plaque might rupture so let's say it does rupture in this case and you develop this thrombus so you develop this big clot on that ruptured plaque well in this case what do you think is going to happen to the downstream part of that artery it's not going to get that much blood right because this thrombus is blocking off the blood flow so whereas before you had lots of blood flowing through your coronary artery and therefore your your cardiomyocytes we're getting lots of oxygen out of that blood now because of that huge clot that's in the way there's way less blood flow in that coronary artery right blood flow to that heart muscle there starts to slow down so now all of a sudden these heart muscle cells aren't really getting all the oxygen they need right so now they start to become oxygen starved they start to get really hungry for oxygen and when they get really hungry for oxygen they start to send pain signals to the brain and these pain signals are basically telling the brain brain we've got like no oxygen down here you need to do something about this now and actually this this pain can feel a bit like indigestion because you you you're not really used to pain like this so your brain kind of gets confused and thinks it's it's maybe an indigestion pain so you might actually feel the pain just below your heart right above your stomach so this is actually the start of a heart attack so let's look at our clot now well it's actually still growing and it's now blocking like two-thirds of the artery so your pain will start to get worse and some people might start to get pain in their arms and and mostly we see it in the left arm and the reason you can get pain in your arms in the first place with a heart attack is because some of the nerves that are connected to the heart have the same origin as some of the ones that are connected to your arm so since your brain really isn't used to feeling pain from your heart it sort of gets confused when it does get signals from your heart and in that confusion it thinks the pain is coming from your arms and that's called referred pain so it's kind of a similar mechanism to the indigestion feeling and by the same referred pain mechanism some people even get pain radiating up to their jaw and so at this point the brain is confused right I mean it's overloaded with these increasing pain signals coming from the heart right and to add to that you got all of these cardiomyocytes that are running low on oxygen and because they're running low on oxygen the the nice normal coordinated way that your heart beats will be compromised and your brain doesn't like this so your brain senses this and and says holy crap I need to do something about this so your brain triggers this big surge of adrenaline release into your bloodstream and the adrenaline gets everywhere so it gets to your heart and it starts affecting your heart right and what does adrenaline do adrenaline will start to make your heart beat faster your heart will start to race unfortunately the adrenaline is not going to be able to do anything about the clot that's built up which is actually just growing right I mean we've sort of left it alone for a while but it's actually getting bigger and by now it's filling up basically the whole artery it's completely blocking the artery off so now our cardiomyocytes are in big trouble because now they're barely getting any blood so they're barely going to get in the oxygen and because they're barely getting any oxygen they necessarily have to slow down their rate of contract because having good access to oxygen is really key for for cardiomyocytes to produce the energy they need to do all the work they have to do so naturally if they don't have that oxygen they can't produce all the energy they need so they have to slow down so they start to slow down and then they start to stop beating altogether so because our patch of cardiomyocytes here have stopped beating well the rest of the heart has to compensate so the rest of the heart starts beating faster to compensate for our dying patch of cardiomyocytes now this point things are not looking good for our cardiomyocytes they actually can't even hold themselves together in one piece anymore their membranes actually start to break down and the cells start to rupture see cells without oxygen without blood supply they don't get the luxury of having blood carry away their toxic waste products that that sort of naturally crop up as part of their regular metabolism so these toxic waste products start to build up inside of our Maya sites and their membranes start to rupture now when our cardiomyocytes start to rupture they start to leak proteins that that only heart muscle cells contain they start to leak these proteins into the bloodstream these proteins are called troponin 'z troponin ZAR a type of structural protein that you only find in heart muscle cells and so keep that in mind because that'll become important later on when we talk about diagnosing heart attacks so now our injured heart is really starting to wear itself out and the bead is starting to get a bit weaker and you're starting to get even more effects all over your body for example it'll start to become really difficult to breathe because you can actually get some fluid buildup in your lungs and let me just quickly show you how this happens so here's your heart and here your lungs now remember blood goes out from your heart to your lungs to get oxygenated and then once it gets oxygenated it sort of comes back to your heart right and then it gets pumped out of your heart to the rest of your body well when your heart isn't pumping very well blood will sort of build up in your heart and then back up into the lungs and this build up this backflow of blood can end up making it really difficult for you to breathe so you'll often get dis mia you'll often get shortness of breath when you have you're having a heart attack so you've had your referred pain you're getting your shortness of breath your heart is racing well because your heart is not pumping efficiently not enough blood might be getting to your brain so you could start to get dizzy and disoriented so by now it's been about 15 to 18 minutes since you started having your heart attack and now things are getting really really bad you're starving heart muscle cells will actually begin to burst and die they'll actually begin to escalate from from just leaking to actually dying so I'll draw in some dead faces here but but you know this is really serious if you're not treated within about 20 minutes your heart will get damaged so badly that it won't ever be normally again because of this rate about 20 minutes after your your heart attack comes on you're losing about 500 cardiomyocytes 500 heart muscle cells per second per second and they're not like your your average skin cell or your your hair your strand of hair they can't actually be replaced so once you lose these cardiomyocytes that's it your heart will not beat normally again so you really want to limit the amount of cardio myocyte loss that happens so that's sort of the physiology behind what's happening in in a in a myocardial infarct so before we finish up there's just one more thing I want to show you so we classify myocardial infarcts into two main groups and I'll show you those groups I'll show you how we divide them up so what I'm going to do to show you this is we're going to take a cross-section here across the heart muscle okay so I'll draw that cross-section so this is a cross-section of the heart so it's sort of as if we cut away this part on the bottom here and and we're looking upward at the heart that's a little eye there and so this is the right ventricle on this side that's the chamber of the right ventricle and this on this side is the chamber of the left ventricle right because the left ventricle is here and the right ventricle is over there now let's draw in our blood vessel so let's say that right here right I'll draw a circle because remember we're cutting the left anterior descending artery this one here we're cutting that in cross-section as well left and descending artery so let's say we block off this left anterior descending let's say that that we've had a heart attack involving that artery well because it serves such a huge part of the heart wall what's going to happen is we're going to knock off a big part of the heart wall we're going to knock off a huge chunk of it right and so this type of infarct is called a full thickness infarct because it involves the entire thickness of the the thick of the heart so that's called a full thickness infarct or a transmural infarcts mural by the way it just means mural refers to a wall and trans just mean sort of crossing so transmural means it's just crossing the entire wall that's that's how big the infarct is transmural so that's one type of heart attack that's one type and the second kind let's put it up here the second kind is called a partial thickness a partial thickness infarct or a sub endocardial that's the other word for it sub endocardial infarct well how does that happen well that happens because you'll have these little arteries that come off of the big ones so for example there'll be this little one that comes off of the left anterior descending and it will actually penetrate through the heart muscle wall right because the goal of this artery is to supply blood to this little patch here okay so let's say that the supply zone the the oxygen supply zone the blood supply zone for this little penetrating artery is this area here well let's say that you get a clot that develops in this artery here then you'll still have a heart attack you'll still have a myocardial infarct but it'll be one of a much smaller region right it won't actually be a full thickness it won't be a full thickness infarct it'll just be a partial thickness so those are the two major types of myocardial infarcts