Cardiac Output, Flow, Pressure and Resistance Theory – Cardiac Mechanics

00:01 Cardiac mechanics is the next topic.

00:05 Here, we need to talk a lot about how much cardiac output we have.

00:09 What is cardiac output? It’s the volume of blood ejected by the heart per minute.

00:16 How do we determine what someone's cardiac output is? You take the stroke volume, which is the bolus of blood ejected per stroke times the heart rate.

00:26 The reason why we go through this process is because we have two main variables to alter.

00:33 Chronotropy, or heart rate, governs the pump frequency.

00:38 Stroke volume governs how much push out we have per beat of the heart.

00:45 There are a number of factors that govern the control of both stroke volume and heart rate.

00:50 Some of them are going to be very similar between the two.

00:53 Other ones have some inherent differences.

00:57 Just to go through a few of the stroke volume ones.

01:01 Autonomic nervous system, such as increases in norepinephrine secretion, that will increase contractility, which causes a greater stroke volume.

01:10 Hormones, such as epinephrine, also can feed into that beta-1 adrenergic response and increase the bolus of blood ejected per stroke.

01:20 Interestingly, there are some intrinsic factors, besides nerves and hormones, that can also increase stroke volume.

01:29 And that is the heart has a very unique principle, in which the more it is stretched, the harder it contracts and we’ll go through that in more detail here in subsequent slides.

01:41 Heart rate is governed by both autonomic nerves and hormones, but doesn't have some of those intrinsic factors that stroke volume has.

01:52 So, what constitutes a stroke volume? How do you know how much bolus of blood is ejected per stroke? Well, for this, you need to know what the end-diastolic volume is, Well, for this, you need to know what the end-diastolic volume is, which is the maximum amount of volume that you can have, while the heart is filling? End-systolic volume is the most blood you can push out per beat of the heart.

02:15 You simply subtract the fill from the squeeze and you get stroke volume.

02:23 Now, what factors affect both end-diastolic volume and end-systolic volume? The primary factors that affect end-diastolic volume is a concept called preload.

02:35 Now, what is preload? Well, I will get into that in the subsequent slides, but I want to give you first a quick definition.

02:43 That’s going to be how much filling there is of the left ventricle.

02:47 It’s a little more complex than that, but that's a good place to start.

02:52 End-systolic volume is affected primarily by the afterload and inotropy.

02:58 Afterload is the resistance it's pushing against and inotropy is how hard you’re contracting.

03:06 Now, notice, though, that there are these primary factors, but nothing in the body is absolute or independent of each other.

03:15 And therefore, there are some secondary effects between preload, afterload and inotropy.