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Current time:0:00Total duration:9:08

Shock - oxygen delivery and metabolism

Video transcript

to understand the pathophysiology of shock we first need to remind ourselves what shock is now shock at its basic level mean cells are not getting oxygen they need essentially the o2 delivery the amount of oxygen that the body can deliver to itself becomes less than the amount of oxygen that is required by the body so shock is essentially a failure to deliver oxygen to the different tissues and organs and cells of the body now why do cells need oxygen well basically cells and this little box here I'm going to designate as a cell and cells need oxygen to be able to create energy this is titled aerobic metabolism creating energy metabolism with the assistance of oxygen aerobic and this is really the reason we breathe we go through respiration because we need the oxygen to help create energy however the cells of the body can also create energy through anaerobic metabolism so the cells of the body can survive without oxygen just for a little while however there's a problem with going through anaerobic metabolism versus aerobic metabolism with oxygen you're able to create a lot more energy to be able to satisfy the needs of each cell in the body yet if the body is forced to undergo anaerobic metabolism not enough energy can be created to meet the requirement to sustain life so that's one problem with undergoing anaerobic metabolism and why we need oxygen but another issue with anaerobic metabolism is a byproduct of trying to create this energy is a substance called lactic acid now come back to this point a little bit later to show what the issue is with creating lactic acid but for now it's important to remember that anaerobic metabolism creates this by-product lactic acid okay so we know now that aerobic metabolism is necessary to create the amount of energy we need to sustain cellular function to sustain life so we know that the body has to deliver oxygen and to understand how the body fails to do this in shock let's go ahead and take a look at a blood vessel so I'm going to go ahead and draw a blood vessel right here and note that this blood vessel is going to be delivering oxygen through the blood to distribute oxygen to the tissues so let's make oxygen this little light blue color so this is oxygen and as you know oxygen is carried in red blood cells in hemoglobin and so this is how oxygen is delivered to the tissues through hemoglobin through red blood cells so let's go ahead and draw some different cells and these little boxes of course are representing our cells just a smaller version then this big one up here now remember in shock the issue is tissue perfusion tissues and cells are not getting enough oxygen they're not getting enough blood that they need for oxygenation and without this oxygen they can't create the energy necessary to sustain life so in shock these cells are desperate for oxygen now there are two scenarios that we can see in shock in the first scenario there is increased extraction of oxygen and this is because there's an increased demand for the cells to have oxygen there might be an increased requirement these cells have so little oxygen in them their oxygen starved it's not really an active process but because there's so little oxygen up here and more oxygen in the blood oxygen readily diffuses into the cells so these cells are oxygen starved they pull out more oxygen therefore that would mean that there's lower oxygen return to the heart less oxygen is getting back to the heart now two types of shock that are an example of this increased extraction are cardiogenic shock and hypovolemic shock so if you think about it hypovolemic and cardiogenic shock blood is not getting pushed forward fast enough it's not getting delivered properly so the cells are using up their oxygen quicker than it's being delivered so lower delivery despite the required oxygen remaining pretty much the same the cardiovascular system is just not able to deliver that oxygen to these cells so second let's take another look let's look at these cells now these cells are still desperate for oxygen and shock that's really the definition they require more than is able to be delivered but what if there's something that actually impedes oxygen from being able to be delivered if oxygen isn't getting to the cells this is decreased extraction so this is what happens in the type of shock known as distributive shock oxygen can't be distributed to the cells so for example in septic shock when there's a lot of inflammation and swelling in the space in between the cells the interstitial space then oxygen has a tougher time diffusing through this space so oxygen can't get through all this thick fluid so this extra fluid creates a diffusion barrier so that oxygen can't be distributed so again we have increased demand of oxygen due to poor oxygen delivery so now again I'll go ahead and write these down this first scenario occurs with cardiogenic and hypovolemic shock and the second scenario occurs with distributive types of shock septic shock anaphylactic shock something that creates a barrier that prevents oxygen from getting to the cells now again if oxygen isn't delivered to the cells it's going to stay in the blood so more oxygen will return to the heart again that makes sense if there's less extraction oxygen just kind of remains in the blood which shows up as increased oxygen return to the heart now I go over this point in detail because if we can figure out how much oxygen is extracted from the tissues we can have a better idea of what type of shock it may be you know cardiogenic or hypovolemic versus a distributive type of shock so how do we measure that well that's where something called mixed venous oxygen or mixed venous oxygen saturation which is abbreviated s MV o tu saturation of mixed venous oxygen comes into play and this term is actually interchangeable with the other term which is known as central venous oxygen saturation abbreviate SCV o2 central venous oxygen saturation now what are these two terms what do they mean well it's very hard for us to measure how much oxygen is getting pulled from the tissues at each individual tissue so what we do is we look at how much oxygen is returning to the heart so we take a look at the heart and you see that blood returns to the right side of the heart through the superior vena cava so up here and the inferior vena cava so this is how blood returns for the top of the body the arms in the head versus the bottom of the body the legs the abdomen so on and so forth so when blood from both the superior and the inferior vena cava meet in the right atrium the oxygen of these two major veins these central veins mix now this is important because you know the oxygen extraction may differ between the upper part of the body and the lower part of the body so when they mix we get an average of the total oxygen coming from the upper and the lower parts of the body so now we see if the mixed venous oxygen saturation also known as the central venous oxygen saturation is lower that means less oxygen is returning to the heart and so therefore we can say that oxygen is being extracted more than normal and the same goes for if more oxygen is returning to the heart so if there's a higher mixed venous oxygen saturation than normal then we can conclude that there's decreased oxygen extraction from the tissues so again we look at the mixed venous oxygen to try to identify what type of shock the patient may be experiencing now from what we've covered so far what other tests do you think we could do to understand if the patient is experiencing shock well remember this lactic acid in anaerobic metabolism when there is low oxygen within the cells energy is created through this process and as a by-product lactic acid was created so patients who have shock may experience lactic acidosis where they have a largely increased amount of lactic acid now initially this can be overcome and doesn't cause damage to the body however over time because of this increased lactic acid I'll say la the body will have an overall decreased pH which means more acidic composition and in the presence of acid if the body becomes too acidic different proteins and structures that are normally intact in the cells start to degra day in denature which leads to a cascade of events that can eventually mean cellular death so though initially this process is reversible if shot continues long enough cells may begin to die they're starved from oxygen and starved from energy and so you can see Frenchie ating these types of shocks and understanding the basics of shock can help health practitioners prevent the potentially devastating problems that can arise in a patient who has shock