Preload and the Frank-Starling Law – Cardiac Mechanics

by Thad Wilson, PhD

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    00:02 So, now, let’s get into preload.

    00:04 Preload is very important for the filling component of the heart.

    00:09 That is your left ventricular end-diastolic volume.

    00:13 That's the maximum amount of volume you have in your left ventricle prior to a contraction.

    00:18 Interestingly, when you talk about preload, it’s actually about a fiber length that we’re most concerned about.

    00:25 How much stretch is there on the left ventricle before you contracted again.

    00:31 Those kind of thoughts can be easily seen with things like a balloon.

    00:35 You're filling a balloon until it gets to a certain point or toughness, and that's when you know it's full.

    00:42 But until that point, you have a large space to fill.

    00:46 When looking at preload, there are couple of intrinsic mechanisms associated with the muscle.

    00:52 And here, let's explain those to a greater degree.

    00:55 It depends on how much you fill the heart, how much it's going to be able to contract.

    01:01 The more it's full, the harder it can contract.

    01:04 The less it’s full, the less hard it can contract.

    01:07 So, let’s look at that diagrammatically.

    01:10 When you have a low preload, you don't fill the heart as much.

    01:15 And therefore, at any given left ventricular pressure, you can only push out so much stroke volume.

    01:22 If you fill the heart more at that same level of contractility, you can push out more blood.

    01:29 What factors affect preload? Two of the biggest ones are venous blood volume and venous compliance.

    01:37 So, the more blood volume you have, the more return of blood you have to the heart.

    01:43 If your veins are less compliant, meaning that they are constricted, the more blood you'll be able to return to the heart.

    01:52 That has to do with increasing venous pressure, and specifically, a lot depends on how compliant your left ventricle is to accept this new level of preload.

    02:03 The amount of contractility of the top portion of the heart also affects your preload because it's going to be pushing in that last little bit of blood.

    02:12 The harder the atrium contract, the more preload you get.

    02:16 Another factor that affects preload, which is a little bit more indirect – it’s one of those secondary effects – if you increase the amount of afterload, you will not be able to push as much blood out on any given stroke of the heart.

    02:32 If you don't push as much blood out, you have more blood left over after each stroke.

    02:38 Interestingly, if you have more blood left over, you can fill it to a greater degree.

    02:44 So, in fact, if there is an increase in afterload and you don't get as much blood out per stroke, on the next time the heart beats it will contract harder.

    02:53 Why? Because you filled it to a greater degree.

    02:58 The last two items that affect ventricular preload are heart rate and inotropy.

    03:06 These two items are normally associated with more cardiac output, but if you slow the rate of the heart, you don't have it beat as fast, it has more time to fill.

    03:20 So, one way to get more preload is don't beat the heart as fast and you can be on diastole for a longer period of time.

    03:29 Ventricular inotropy works on a similar principle, in which if you decrease the strength of the contraction, therefore you have a greater end-systolic volume.

    03:42 The next beat of the heart, you'll fill it to a greater degree and that increases your preload.

    03:49 So, let's look at this preload concept on a graph.

    03:52 Because this will land the principle.

    03:54 And even though this is a hard one, you'll get it if you can see it in action.

    03:59 So, when we look at a graph like this, let’s talk through the different axes.

    04:03 The first axes we’re going to have is the x-axis.

    04:07 It's going to be left ventricular end-diastolic pressure.

    04:11 On the y-axis, we’re going to have stroke volume.

    04:15 So, those are our two variables.

    04:18 If we start off in a condition, which is A, if we move from A to B, we have increased the left ventricular end-diastolic pressure.

    04:30 At that point, you’ll have a greater stroke volume that will occur.

    04:35 So, let's go through that a little bit more in detail.

    04:38 If you increase left ventricular filling pressure, that is an index of what preload? That index of preload will cause then an increase in stroke volume.

    04:50 This relationship is called the Frank-Starling mechanism or Frank-Starling principle.

    04:57 This principle or relationship allows for this inherent property of cardiac myocytes.

    05:04 It's interesting you're not actively using more ATP to generate this stronger contraction.

    05:12 You're simply letting the myocytes stretch more and then they’ll contract back harder.

    05:18 This is also referred to as a length-dependent activation.

    05:24 So, it matters at what length you are, what level of contraction you'll get.

    05:32 So how does preload affect pressure-volume loops? Everybody's favorite, pressure-volume loops.

    05:40 And with a pressure-volume loop, we have normal indices, which stack out at the top and the bottom portion of the graph.

    05:49 These are places in which we cannot go out of.

    05:52 It provides our minimal and maximal components in the figure.

    05:58 A normal pressure-volume loop looks like this, where you start off filling the heart along the bottom axis.

    06:06 You have a vertical line, which is the contraction of the heart.

    06:10 You have an ejection portion and then a relaxation portion.

    06:15 How does preload change that? Preload should allow you to fill to a greater degree.

    06:20 If you're filling to a greater degree, you should be what? Increasing that volume.

    06:26 As you do that, you make a bigger pressure-volume curve.

    06:30 You increase end-diastolic volume and you increase stroke volume.

    06:35 All because of the increase in venous return.

    06:39 Now, let's look at this if we have a decrease in venous return.

    06:44 Therefore, you’re going to have now a shorter curve that decreases left ventricular end-diastolic volume and decreases stroke volume.

    06:53 Nice ways to think of changes in the pressure-volume loop.

    06:59 How would you get such pressure-volume loop changes? Well, a good example of an increase in venous return is if you get extra volume added to your blood, maybe you were hooked up to an IV and infused in volume through, let's say, isotonic saline.

    07:17 A decrease in volume could be just the opposite.

    07:20 Maybe you've undergone dehydration and you've lost body water.

    07:24 So, those are two examples of both either an increase or a decrease in venous return.

    About the Lecture

    The lecture Preload and the Frank-Starling Law – Cardiac Mechanics by Thad Wilson, PhD is from the course Cardiac Physiology.

    Included Quiz Questions

    1. Increased atrial inotropy
    2. Increased ventricular inotropy
    3. Decreased afterload
    4. Decreased ventricular compliance
    1. Decreased atrial inotropy
    2. Decreased venous compliance
    3. Increased afterload
    4. Decreased ventricular inotropy
    5. Increased ventricular compliance
    1. Stroke volume and end diastolic volume
    2. Heart rate and preload
    3. Venous pressure and ventricular compliance
    4. Pre-load and after-load
    5. Stroke volume and end systolic volume

    Author of lecture Preload and the Frank-Starling Law – Cardiac Mechanics

     Thad Wilson, PhD

    Thad Wilson, PhD

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