00:01
Okay, now, let’s go to
talk through the different
amounts of ventilation
that a person has.
00:07
So the classic formula
for ventilation,
this is minute inhalation or
total ventilation is simply
taking the breathing frequency
times the tidal volume.
00:20
If we want to account for
alveolar ventilation,
now we need to account
for dead space.
00:28
So here, we have the volume of air
that reaches out of the mouth.
00:34
It’s the breathing frequency times the
tidal volume minus the dead space.
00:39
So alveolar ventilation accounts for dead
space while minute ventilation does not.
00:45
Alveolar ventilation is more important in
pulmonary medicine than minute ventilation.
00:52
But minute ventilation is
easier for us to measure
because dead space is a
little bit hard to measure.
01:00
So let’s take this example here of a
14-year-old girl with a history of asthma.
01:06
She does a number of
pulmonary functions tests
after taking a short-acting
beta-adrenergic agonist.
01:13
A great example of
this is albuterol.
01:16
Her tidal volume is
400 milliliters.
01:19
Her breathing frequency
is 10 breaths per minute.
01:23
So if you were calculating
per minute ventilation,
you simply take 10 times 400 and that
would yield 4000 milliliters per minute.
01:35
If you wanted to calculate
her alveolar ventilation,
we’re going to have to
account for dead space.
01:41
So in this case, you’ll
take her tidal volume, 400,
minus the dead space
volume, which is 200,
which would yield 200 multiply that by
10, which is her breathing frequency,
and you’d get 2000
milliliters per minute.
01:57
So alveolar ventilations will always
be less than minute ventilations,
but alveolar ventilation tells
you the amount of gas that
actually gets to the point
where gas exchange can occur.
02:11
So there are ways to calculate
this in a more complex manner
and this is shown with
this particular formula.
02:20
But we don’t have to worry about
this formula to the same extent
because we just want to make
sure that the relationship
between alveolar ventilation and
CO2 is inverse of each other.
02:32
And I’ll show you this from
some teeter-totter examples.
02:36
If alveolar ventilation goes
up, CO2 has to go down.
02:40
The alternate also occurs as the CO2 goes
up, alveolar ventilation rate goes down.
02:48
And where you can use this clinically
is all you have to do is
either measure the CO2
or measure alveolar ventilation and
you’ll be able to predict the other.
02:59
So someone has a high
CO2 in their blood,
you automatically know that their
alveolar ventilation rate is low.
03:07
And the inverse of
that is also true.
03:09
If you see that CO2 is low,
you automatically know that
alveolar ventilation rate is high.
03:18
Ventilation also relies
on lung compliance.
03:22
So compliance is the inverse of
stiffness or how stiff the lungs are.
03:29
Normally, you have a
nice compliant lung.
03:34
However, it can change in
certain disease states
such as interstitial
pulmonary fibrosis.
03:40
In this case, there is scar
tissue that forms in the lungs,
increasing the amount of collagen fibers
and making the tissue less
pliable or less stretchable.
03:52
And therefore its
compliance has decreased.
03:54
There are other clinical
conditions such as emphysema .
03:57
And in emphysema, there’s
an increase in compliance.
04:02
And this done because
the emphysema process
of pathophysiology breaks
down elastin fibers.
04:08
And therefore, it’s increased its
compliance and it’s less stiff.
04:15
You can think of it as more floppy.
04:19
So if you want to think
of these three examples,
probably the best way to do that is
to think it’s overall compliance.
04:27
Now, with compliance, there
are changes that occur
even in the lung itself
depending upon how much stretch
is produced by
particular forces.
04:39
So in this case,
gravity can cause a decrease in lung
compliance in the top portions of the lung.
04:46
There is an increase in compliance
at the base of the lung.
04:50
So if you’re a visual person, I think the
easiest way to think about compliance
is think about two rubber bands.
04:57
If a rubber band is not stretched,
it has a higher compliance.
05:04
If you’ve already fully
stretched a rubber band,
it is hard to stretch
it to a greater degree.
05:10
In this case, its compliance has decreased
when it is already pretty stretched.
05:16
Now, why is this important
for overall ventilation?
Because ventilation has
a greater propensity
to go to areas that
have high compliance.
05:27
So an area that is highly compliant,
such as the base of the lung,
will get more ventilation than
areas that are lower compliant
such as the apex of the lung.
05:39
If we want to look at this
in terms of pathophysiology,
if you have fibrotic areas of
the lung with low compliance,
ventilation or airflow will not
want to go to those areas,
but will in fact go to others.