So, mechanical ventilation complications are
things that you want to keep in mind. Okay,
so what is this Auto-PEEP that I told you
we will come to? Let's set this up. The
top portion, both A and B represent Auto-PEEP.
Now, what this means is the fact that as a
goal, what is occurring here is that you are
increasing positive pressure where? The
alveoli and the lung volume is increasing
like crazy. I want you to take a look at B
first. You will notice here, as you go from
left to right, you are increasing in a staircase
manner. The fact that you are increasing a
staircase manner and that arrow that you are
seeing there then represents increased amount
of volume in your lung due to Auto-PEEP. In
other words, positive end expiratory pressure
is introducing and actually retaining further
amounts of air in your lung volume. So B
is something that you want to take a look
at first. Easy to understand, because that
Y parameter represents lung volume. Let's
take a look at A.
A couple of things in A that are very important
for you to pay attention to. Let's begin.
First, I want you to move from left to
right, this is flow. So, as you move from
left to right, at first, you are going to
have a spike. This means increase in flow.
What does this mean to you in terms of action?
Breathing. Good. It’s inspiration. Clear.
Then how are you going to maintain a positive
pressure where it’s plateau? Okay, what
is my next step? Expiration. So, flow therefore
while expiration it's going to drop, isn’t
it? Now, with every point of expiration which
is positive and expiratory pressure and say
that it’s increasing in staircase manner,
then I want you to now, go from the bottom
of that graph in figure A and I want you to
come up slowly towards where it says flow,
and where you have a black solid line horizontally.
So, as you come up to that dashed line, interesting
enough, what just happened here? During expiration,
you are not getting rid of all of this air
because of Auto-PEEP. There is increased amount
of air at the end of expiration, positive
end expiratory pressure. This is not good,
ladies and gentleman. Mean to say that, as
a consequence, often times and something that
you want to keep in mind. Remember, with
mechanical ventilation one of the things
that you are trying to do is what? Minimise
PEEP, so that you would not increase the amount
of air at the end of expiration within your
lungs, therefore, introducing new lesions.
I want you to compare this to what we have
in normal. I want you to take a look at figure
C. Compare figure C to figure A.
Please compare figure C to figure A. You will
notice that figure C, this C and D will be
normal patients. In figure C, as in Charlie,
you will notice that there is no dashed line.
Meaning to say that in a normal individual,
upon expiration, you are going to come back
up to that horizontal solid black line, thus
there is no further retention of air and volume
within your lung. This is obviously not Auto-PEEP
this is normal. I want you to compare C and
D, both are normal individuals. C is flow,
D is measuring lung volume. You will notice
here please, that at the end of expiration,
there is no further increase in lung volume,
you have come back down to baseline. What
I like for you to compare here, which you
probably have already done is compare D, as
in delta, with B, as in boy. You will notice
that B and boy with Auto-PEEP, the lung volume,
you see that arrow there? This then represents
increased amount of air left in your lung
at the end of expiration and it has continued
to increase in a staircase manner. That is
B. In D, you see no such space or discrepancy.
Every single time, with expiration, it has
now come back to baseline. Welcome to Auto-PEEP
and this is a major issue clinically. Something
that you want to keep in mind so that you
know how to manage your patient properly and
so you know how to avoid this. Auto-PEEP can
result in hypotension or pneumothorax. The
pneumothorax is easy enough to understand
because the lung volume is increasing, therefore,
may introduce a tear in the parenchyma and
may result in tension pneumothorax. Okay.
What about the hypotension? Is that a typo?
No, this is physiology in absolute clinical
application. The hypotension, the reason your
patient has decreased blood pressure. If that
alveoli is getting really big, once again,
what are you doing to that adjacent structure,
which is a pulmonary capillary? Compressing
it. Good. If you compress it long enough,
what happens to, let me ask this question.
What happens to venous return to the right side? Decreases.
What happens to venous return to the left side? Decreases.
Good. Where are you going
to measure blood pressure? At the brachial
artery. So you have decreased venous
return to left side, you have decreased cardiac
output. Result in what? Hypotension. Good.
If you haven’t understood that, good time
to integrate the physio in respiratory with
what is going on here clinically.