Afterload – Cardiac Mechanics

00:01 Now, let's move to afterload. Afterload is a resistance.

00:06 It's how much you're pushing against the aortic valve to open it up.

00:13 So you need to open the valve up.

00:15 The heart needs to overcome the resistance that's in the cardiovascular to push out any blood.

00:21 If it couldn't open the valve up, no blood would move.

00:25 Interestingly, it is -- we talked about opening up the aortic valve but in theory or in the absolute way you quantify afterload is in the amount of pressure it generates at a certain wall stress.

00:42 A little bit more complex, but it sometimes it's important to not just think of the valve opening, but it's the amount of pressure the left ventricle needs to contract on to open that valve.

00:53 The most important thing to think about with afterload is how it affects the velocity of contractions.

01:00 This should make inherent sense to you cuz more weight you have on a muscle when you try to contract it, it will contract slower.

01:08 Simply because you're trying to overcome a greater weight.

01:12 The heart is working a very similar principle. It is trying to open up the aortic valve.

01:17 The more resistance there is to opening up the valve, the less velocity the contraction will be.

01:25 If there is an easy time to open that valve, velocity will be very rapid.

01:33 So the velocity of contraction always effected on how much afterload it needs to overcome.

01:40 You can think about this even with something like a weight.

01:43 So if you're trying to lift up, let's say, a table and when you went to lift that table up and the table was really light.

01:52 You could move it rapidly. If that table, however, was a big oak table and had stuff stacked on it and you're trying to lift it up, you would lift it using that same amount of effort, you would move slowly.

02:05 That same principle works for the heart, all on about how the resistance is to opening up the aortic valve.

02:13 So now that we know the effect on velocity of contraction, let's look at how this affects these Frank-Starling Curves.

02:21 So we have the same curve which is left ventricle and diastolic pressure on the X-axis and stroke volume on the Y-axis.

02:30 A is our normal curve and you can see two changes that happen. The first is increase in afterload.

02:39 That would be going from A to B.

02:41 When this happens, even the left ventricular and diastolic pressure increases, we get a decrease in stroke volume.

02:50 Also, this decrease in stroke volume increases preload for the next beat.

02:59 The other way is going from A to C. This is decreasing afterload.

03:06 This time, you're getting a decrease in left ventricular and diastolic pressure and you get an increase in stroke volume.

03:14 This increase in stroke volume also decreases preload.

03:19 So you notice that anytime we talk about one variable, we have to bring up the others because again, they are inter-related.

03:29 The afterload also affects how much ejection of the heart you have so not only does it affect the stroke volume, but it affects the total amount of volume of blood ejected per stroke.

03:44 It affects the velocity of shortening and this reduces the ejection velocity.

03:53 It affects the amount of end-systolic volume and it changes the amount of stroke volume.

04:02 So simply by having this fiber shortening difference affects ejection velocity, end-systolic volume, and stroke volume.

04:13 So let's look at pressure-volume loops. Everybody's favorite: pressure-volume loops.

04:20 If we put it in our two parameters with senses our maximal value and our minimal value, we put in on a normal curve.

04:28 Again, we have a ventricular filling, isovolumic contraction, ejection, and isovolumic relaxation to return back to normal.

04:36 An increase in aortic pressure means that there's an increase in afterload.

04:45 This increase in afterload makes you contract harder in isovolumic contraction before the valve opens.

04:52 So you waste a little bit of energy having to contract harder to push that valve open.

04:59 In that case, you've wasted energy.

05:02 Anytime you waste energy now you can't impart it to the rest of the circulatory system and that ends up decreasing stroke volume.

05:10 The opposite is true if you have a decrease in aortic pressure that allows you to open up that valve sooner and therefore, you can impart more energy to that blood that's within the cardiovascular system.

05:23 In that case, you increase the amount of stroke volume.