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Video transcript

- [Voiceover] Hypovolemic shock is a type of shock so low perfusion and low blood pressure that is caused by low volume, and this is low volume of the vascular space. So, to understand hypovolemic shock, let's take a look at our cardiovascular system. And of course, I'm gonna start by just drawing the heart. The heart of course delivers oxygen and vital nutrients to tissues and organs. So, I'm gonna write organs over here and between the heart and organs we have the vascular space. So we've got arteries delivering blood to organs and veins returning blood back to the heart from the organs. So I've got this oversimplified cardiovascular system right here but this will do the trick to understand hypovolemic shock. So hypovolemic shock you've got low amount of blood in the vascular space that is able to return to the heart. And the problem the heart runs into is it tries to pump blood to the organs but the organs are not getting enough oxygen, not getting enough blood because the system doesn't have a lot of blood in it. So this low return of blood to the heart is known as decreased preload. Preload is you can essentially think of as loading the heart up with blood. It's essentially the filling of the heart. Another term that's really often interchangeable is left ventricular end-diastolic volume. It's essentially saying when you're filling up the heart before it contracts. So that's preload. You're probably also familiar with the term afterload. Afterload is after the heart has already contracted and squeezed out the blood. It's how much blood is left in the heart. So then it actually stands to reason that preload minus afterload would equal to stroke volume. So think about it. How much blood is in the heart just before it contracts and how much blood leaves the heart just after it contracts. That's exactly what the stroke volume is. That's how much volume the heart has pushed out of it. So, if you lower preload, lower return to the heart, you're going to lower stroke volume. That's assuming that let's just say afterload doesn't change too much. So you lower stroke volume then that means you're going to lower cardiac output as well because cardiac output is stroke volume times heart rate. How much blood is squeezed out per beat and how many beats there are per minute. That gives you the total amount of liters squeezed out of the heart per minute cardiac output. So this decrease in cardiac ouput is what is causing the low tissue oxygenation. It's not the heart's problem. The heart is trying to beat furiously. It actually increases heart rate in response to this decreased stroke volume. Remember, that's actually one of the cardinal symptoms of shock, an increased heart rate, tachycardia. And of course, the blood pressure goes down as well as you can recall the mean arterial pressure, the blood pressure is equal to cardiac output, times resistance of the vessels. Systemic vascular resistance. So this is kind of just a quick recap of shock in general. Oh and hey, look. If we lower cardiac output which we've caused from this hypovolemic shock, we still want to try to maintain blood pressure, right? So, what do you think is gonna happen? The body is going to try to respond by increasing resistance of the blood vessels. So that's exactly what happens. Let me illustrate that right here. So there was low blood volume before and the patient has hypovolemic shock so the resistance of the blood vessels will need to increase in response. So the body does that by constricting blood vessels, vasoconstriction. Remember when the diameter of a blood vessel decreases, we have increased resistance. And this increased resistance allows blood to be squeezed forward to return to the heart. This will mostly happen in the venous system to return blood to the heart with the hope of increasing preload. And also, maintaining blood pressure with vascular resistance. And remember, this vasoconstriction is caused by the nervous system creating a sympathetic response. And the sympathetic response is really driven by this low blood pressure. If you recall, there are things called baroreceptors at key locations in the vasculature that assess the blood pressure. When the blood pressure is low, the sympathetic nervous system kicks up and not only will the blood vessels clamp down to increase blood flow to the heart but you'll also be getting sympathetic tone to the heart as well, and that's why you see this increased heart rate. Okay, so that gives you a good amount of information about the concept behind hypovolemic shock but what really causes it? Well, what's gonna cause low blood volume? I've arbitrarily divided it up into really two major concepts. Blood loss or fluid loss. So let's scroll down here. So blood loss can be really anything that you can think would cause blood loss. A patient vomiting blood, urination of blood, internal bleeding, external bleeding. And fluid loss is very similar. A patient could be excessively vomiting, having severe bouts of diarrhea, urinating too much and then some of the less commonly thought of losses of fluid. Sweating can be a cause. If a patient has an issue on the skin, so for example if they have a burn, fluid can leak out of that burn because the skin normally acts as a protective barrier and when you don't have it, fluid can escape the body more easily. And then you also have third spacing of fluid. So for example, ascites. Ascites is collection of fluid in the abdomen and you usually see it with patients who have liver failure. Proteins are normally created in the liver such as albumin and when the liver no longer can function, it no longer creates albumin. And what albumin does is maintain blood volume in the intravascular space. So if you have low albumin, plasma and blood volume can escape and collect this fluid in places such as the abdomen which is ascites. So you see, the fluid loss is very similar between these two and in fact, the symptoms will actually be very similar between these two as well. Both of them share the cardinal symptoms of shock such as tachycardia, low blood pressure and low flow to organs. But also with hypovolemic shock, you'll see cool, clammy hands and that's from the vasoconstriction that we were showing before. Blood is actually diverted away from the less essential organs such as the skin and diverted to essential organs such as the brain or the heart or the lungs. So, because blood's diverted from the skin, it will feel very cool. And next you'll see signs of dryness. A patient's mouth, they may have a dry tongue or cracked lips from all the dryness, and their skin may actually appear very dry, too. And the final symptom that I want to touch on is the patient may appear very pale. This can be especially noticed when a patient has a lot of blood loss. Blood is no longer coloring their skin so they may appear very pale. And you can look under the eyelids as well which should normally have this very pinkish color but instead it will be very pale. And that's actually known as conjunctival pallor. Paleness of the conjunctiva. Okay, so what do we do to diagnose and treat this? The diagnosis you're going to use your different shock labs. Of course serum lactate and ABG to assess oxygenation. And you might also check the CBC which is a complete blood count. And the ABG stands for arterial blood gas. So, you're assessing the oxygenation of a patient. And I might as well mention also that serum lactate shows low oxygenation of the tissues as well. So for the CBC, you might see low hemoglobin or low hematocrit particularly if blood loss is the cause of hypovolemic shock. Hemoglobin gives you the total amount of hemoglobin. Hematocrit assesses the percent of total red blood cells in the body. And once you suspect hypovolemic shock, treatment will be about maintaining blood pressure and blood volume and also blood content. So, you might be giving IV fluids to replete the blood volume. And you might give pressors which helps blood vessels clamp down as we were showing before. Or you might need to replete the patient's blood content. So for example, you might want to give them a blood transfusion or they might need platelets or coagulation factors if the patient is bleeding a lot. And coagulation factors can be given through fresh frozen plasma. Fresh frozen plasma is ripe with lots of coagulation factors. Or you might need to give albumin as we were saying before. If the patient's protein, blood protein is low albumin's a way to keep blood volume in the vasculature and out of third spaces. So you can see treatment focuses on keeping the blood volume up. And of course you want to also treat the major problem. So for example, if a patient is bleeding they may need surgery to repair damaged blood vessels and stop the bleeding.