Mean Arterial Pressure (MAP) – Blood Vessels and Pressure

00:00 So now let's take a step back, use Ohm's Law and try to use this across the whole body instead of just one blood vessel. To do this, we use the determination of mean arterial pressure. We'll have to determine then what is systemic vascular resistance and then we can determine what cardiac output is. So these are going to be the three variables that we're going to deal with. Okay, so let's talk through which ones are which. So MAP is abbreviation for mean arterial pressure. That is going to be the cardiac output times systemic vascular resistance plus central venous pressure. This can be seen quite well over here on the diagram on your right. If central venous pressure is low such as 0, you can utilize this equation by dropping the central venous pressure term. You might ask, is central venous pressure usually 0? It's in between 0 and 4. Mean arterial pressures are much higher, maybe in the order of 95 mmHg. So when you look at a central venous pressure, it is very low and oftentimes people will drop that portion of the equation just to make things simpler. So in this case, we have mean arterial pressure approximates cardiac output times systemic vascular resistance.

01:38 Knowing what cardiac output equals allows us to do another substitution. Cardiac output is simply the product of heart rate times stroke volume. That's the number of beats of the heart times the stroke volume or the output per beat. You multiply those two together times systemic vascular resistance and again you can approximate mean arterial pressure. So now how do you use mean arterial pressure to get to the flow that you want to look at? First thing about mean arterial pressure to kind of grab hold and that is really what is mean arterial pressure and how we're calculating it. Because when you get a blood pressure measurement, let's say with a sphygmomanometer and a stethoscope on your upper arm, you usually get a systolic blood pressure and a diastolic blood pressure. Your systolic blood pressure is your maximal pressure that you hear, the diastolic blood pressure is when you no longer hear any Korotkoff sounds through the stethoscope. You would think that you could use these simple two pressures to be able to determine what mean arterial pressure is but it's a little bit more complex than that and that is because there is a time component to blood pressure. So if we look at this diagram over here where we have time on the X axis and we have pressure on the Y. Time spent in diastole is longer than a time spent in systole. So I will approximate that here with my voice. You go through systole and then diastole, systole, diastole. The diastolic portion lasts longer and that longer-lasting portion means that you have to wait it to a greater degree. To do that, you utilize this particular formula where you take a third of pulse pressure which is systolic minus diastolic pressure and you add it back in the diastolic pressure. So diastolic pressure is waited to a greater degree than systolic pressure. Why? Because you spend a longer duration in diastole than you do in systole. So mean arterial pressure is not simply the mean of this systolic and diastolic pressure, you have to wait it to a greater degree for diastolic pressure.