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Shock - oxygen delivery and metabolism

Video transcript

- [Voiceover] To understand the pathophysiology of shock, we first need to remind ourselves what shock is. Shock at its basic level means cells are not getting oxygen they need. Essentially the O2 delivery, the amount of oxygen that the body can deliver to its cells becomes less than the amount of oxygen that is required by the body. Shock is essentially a failure to deliver oxygen to the different tissues and organs and cells of the body. Why do cells need oxygen? Basically cells, in this little box here I' going to designate as a cell, and cells need oxygen to be able to create energy. This is titled "aerobic metabilism." 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. 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 by-product of trying to create this energy is this substance called "lactic acid." I'll 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. 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. I'm gonna go ahead and draw a blood vessel right here. 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. This is oxygen. As you know, oxygen is carried in red blood cells in hemoglobin, so this is how oxygen is delivered to the tissues, through hemoglobin, through red blood cells. Let's go ahead and draw some different cells. These little boxes, of course, are representing our cells. Just a smaller version than 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. In shock, these cells are desperate for oxygen. There are two scenarios that we can see in shock. In the first scenario, there is increased extraction of oxygen. 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. They're 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. 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. Two types of shock that are an example of this increased extraction are cardiogenic shock and hypovolemic shock. 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. 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. Second, let's take another look. Let's look at these cells. These cells are still desperate for oxygen and in 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. This is what happens in the type of shock known as "distributive shock." Oxygen can't be distributed to the cells. For example, in septic shock when there's a lot of inflammation and swelling in the space in between the cells, the interstitial space, the oxygen has a tougher time diffusing through this space, so oxygen can't get through all this thick fluid. 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. 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. 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. 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: cardiogenic or hypovolemic versus a distributive type of shock. So how do we measure that? That's where something called mixed venous oxygen, or mixed venous oxygen saturation, which is abbreviated SMVO2, saturation of mixed venous oxygen, comes into play. This term is actually interchangeable with the other term, which is known as central venous oxygen saturation, abbreviated SCVO2, central venous oxygen saturation. Now what are these two terms, what do they mean? 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. 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. This is how blood returns from the top of the body, the arms and the head, versus the bottom of the body, the legs, the abdomen, so on and so forth. 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. This is important because 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. 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. 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. 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. Patients who have shock may experience lactic acidosis where they have a largely increased amount of lactic acid. 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 degredate and denature, which leads to a cascade of events that can eventually mean cellular death. So though initially this process is reversible, if shock continues long enough, cells may begin to die, they're starved from oxygen and starved from energy. You can see differentiating 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.