00:00
Now we're going to do some combinations, some integration of some of the aspects we've
already discussed. Adding to it, vascular function curves. So cardiac function curves are
oftentimes dealt with to things like the Frank-Starling mechanism where you usually plot either
central venous pressure or left ventricular and diastolic pressure on the X axis then you plot
cardiac output or something like stroke volume on the Y. Here, to make sure that the link
happens with vascular function curves, we're going to use central venous pressure plotted
against cardiac output. Here we have our traditional vascular function curve flipped a little bit
on its head so that we have cardiac output now on the Y axis, rather in vascular function
curves we usually put it on the X. So let's trace through this kind of curve. The normal
operating point. You usually have a cardiac output of around 5 liters. The central venous
pressure that corresponds to 5 liters is about 2 mmHg. Remember that central venous
pressure is most of the time between 0 and 4 mmHg. The intersection point here is the most
important thing. We're going to call this part here this equilibrium point. Wherever the
equilibrium point is is what the body is going to try to adjust. So, let's go through a number of
clinical scenarios that might affect either the heart or the vasculature and let's see how the
body would want to respond to that with a change in this equilibrium point. Starting with
normal, our first condition is a decrease in contractility or inotropy. So this could happen very
clinically by something like a heart attack. Someone had a myocardial infarction or a heart
attack which decrease their heart's ability to pump, it decreased their inotropy contractility. Now instead
of haviing a 5-liter cardiac output, you have a 4-liter cardiac output. That is not enough
cardiac output to perfuse all the tissues in the body, therefore some tissues are going to
become hypoxic. You can't have that, you need to fix this problem. How is the body going to fix
this problem? It may not be able to increase inotropy because of the heart attack so it's
possible that you're going to have to try to fix it through another mechanism. This decrease in
inotropy associated with a heart attack needs to be fixed. So how is the body going to do that?
One way is to volume overload and as you volume overload the heart, you get an increase in
blood volume. This increase in blood volume then moves the equilibrium point to a higher central
venous pressure. If you have a higher central venous pressure, it can move that up enough so
you have cardiac output return to closer to normal. That is a great example of how a vascular
function curve change in blood volume can fix a problem with the heart such as a heart attack.
03:58
Let's take another example for this. How about if you had an individual that lost fluid? Maybe
through something like a hemorrhage. So if they had a hemorrhage they lost a little bit of fluid
but were able to then clot their blood so that they didn't lose any more fluid. Could the heart
respond to try to maintain enough cardiac output? With these vascular function curves you
can see that occur. So the first thing that happens if we started at the equilibrium point in a
normal condition, a decrease in blood volume would have it travel down the curve to a new
equilibrium point at a lower blood volume. How do you fix that response? You need to increase
contractility or inotropy and you do that by a cardiac function curve change. What happens is
the cardiac function curve gets shifted upwards and to the right and that allows to return
cardiac output to its normal level. Notice though as you move cardiac output up to this new
equilibrium point, that central venous pressure decreases. So you have to always look at both
variables, central venous pressure and cardiac output to make sure you understand how the
body is trying to respond to either a change in a cardiac curve or a change in a vascular
function curve. I invite you to play with these scenarios so that you get a good feel for if you
have increases in blood volume, decreases in blood volume, changes in the cardiac function
curve of decreasing or increasing and how the body would then try to adapt to such a response.