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Current time:0:00Total duration:7:40

Video transcript

shock is the decrease of perfusion of tissues or in other words it's the decreased delivery of oxygen to different organs of the body and perfusion this delivery of oxygen to the body is essentially equal to the amount of flow so the amount of blood flow that gets to the organs over the amount of tissue that is being delivered to so for example flow could be in measurements of maybe milliliters or liters per minute so a volume per unit time and the amount of tissue is measured in mass so maybe grams of tissue and this can be really any tissue maybe a hundred grams of kidney it's the amount of blood that goes to that amount of tissue however there's another way to figure out perfusion oxygen delivery to the tissues so perfusion is actually proportional this is a proportional sign to cardiac output that's the amount of blood the heart puts out per minute it's also proportional to the systemic vascular resistance this is the resistance of blood vessels and also the amount of oxygen o2 content in the blood so let's go ahead and dive deeper and take a look at these different factors that can influence the delivery of oxygen and perfusion of the cells so cardiac output can be determined by two things it can be determined by the stroke volume so let's take a look at the heart that I have over here so stroke volume is the amount of blood that escapes the heart per beat and the other factor that influences cardiac output is heart rate so looking at the units of both of these stroke volume is measured in liters per beat so that's how much fluid escapes from the heart with each beat times the heart rate which is the number of beats per minute and so doing some simple arithmetic these two cancel out and you see that cardiac output is liters per minute so this is the measurement of cardiac output and you can see in our equation up here flow has a similar units liters per minute so cardiac output really determines the flow of the blood the more the heart squeezes to push blood out and the faster it does this leads to increased cardiac output and therefore this leads to increased perfusion of the body so a better pump leads to better delivery of oxygen to the tissues now heart rate is fairly self-explanatory this can increase or decrease based off input from the nervous system but stroke volume let's break this down a little bit further stroke volume can be broken into three different parts preload which is the amount of blood that's in the heart at the beginning of a contraction so it's blood that is loaded in the heart before or pre contraction before it squeezes stroke volume is also determined by afterload which then makes sense is the amount of blood that is remaining in the heart after it squeezes and contractility and contractility is a measurement of how well the heart can squeeze so increased contractility means the heart can squeeze a little bit better so let's look at a quick example of maybe something like hypovolemic shock where the body has low blood volume low blood volume means that blood returning to the heart is decreased there's actually a lower amount so less blood getting into the heart maybe only a little bit makes it in means less blood can be squeezed out in the same token if there's more blood let's say there's more blood remaining in the heart afterwards that means less blood was squeezed out so there was less stroke volume so the equation for stroke volume actually makes sense stroke volume is the difference between the amount of blood that started in the heart before it contracted and the amount of blood that's left in the heart so stroke volume is preload - after load that difference is the amount of blood that escaped the heart with one beat and of course if the heart has greater contractility it can squeeze harder and force more blood out now resistance of blood vessel systemic vascular resistance so that's the total resistance of vessels in the system also plays a role in perfusion as I said before and the way you can think about that is by looking at blood vessels because blood vessels are like pipes and that's how blood is delivered the rest of the body now what is resistance well resistance is the ability for the blood vessel wall to push back against the blood so essentially the blood vessel wall acts like a trampoline and just like a trampoline if you're bouncing up and down on the trampoline if the trampoline material is tighter you can bounce higher and so blood behaves similarly in blood vessels if it bounces against the wall it can bounce back forward and this pushes blood forward through the system and allows better oxygen delivery and therefore greater perfusion to the cells of the body now there are different factors that can influence resistance but really the greatest way that our body can change resistance is by changing the diameter of the blood vessels having a smaller diameter means more resistance because blood has more opportunity to bump up against the walls and bounce forward and so therefore a smaller diameter blood vessel means increased vascular resistance and therefore greater and better perfusion to the rest of the body so cardiac output systemic vascular resistance and finally oxygen content that's the other big component of perfusion if there's more oxygen content in the blood tissues can be perfused better because more oxygen can be delivered to the body so keep in mind these three parameters when you're thinking about shock and perfusion to the tissues because shock is decreased tissue perfusion now there's one final point I want to make and it's in the equation for blood pressure specifically we look at mean arterial blood pressure mean arterial blood pressure an equation for this is cardiac output times systemic vascular resistance plus central venous pressure now central venous pressure is usually low so this side of the equation is usually neglected and you can often just think here that blood pressure is equal to cardiac output times vascular resistance now it's also worth mentioning that blood pressure can also be found by sesang through a sphygmomanometer that's the little blood pressure cuff that physicians put on the arm of patient whereas this equation the mean arterial pressure equals cardiac output times systemic vascular resistance is a way to figure out blood pressure emo dynamically another way to measure blood pressures using this cuff and blood pressure is calculated by adding two thirds of the diastolic blood pressure plus one third of the systolic blood pressure so this gives the mean arterial pressure in a different way by using a blood pressure cuff and note the reason I'm going over this as you can see look cardiac output and systemic vascular resistance are factors that influence perfusion as well as factors that influence pressure so very often patients in shock will have a lower blood pressure while at the same time they aren't able to adequately oxygenate their tissues so again it's important to consider these different factors when thinking about shock and decreased tissue perfusion