A 15 year old boy visits the clinic with his mother as he is
having some breathing difficulties for the past few months.
He is very interested in sports and
wants to join the school basketball team.
When he signed up for after-school training sessions, he
found that he was getting short of breath after 15-20 minutes.
He does not experience this
symptom at any other time.
He is a healthy child and up to
date with all of his vaccinations.
There is no family history of respiratory diseases.
His vitals are respiratory rate 14, pulse 87, blood pressure
110/89 and his temperature is 36.7 degrees celsius.
His physical examination is normal including
a normal respiratory examination.
He is advised to take a pulmonary function test during
which he is asked to take in as much air as he can
and then forcefully expire all
the air into a spirometer.
The volume of expired air
represents which of the following?
Answer choice (A) - Expiratory reserve volume
Answer choice (B) - Functional residual capacity
Answer choice (C) - Tidal volume
Answer choice (D) - Total lung capacity
Answer choice (E) - Vital capacity
Now take a moment to go through the answer choices
by yourself before we go through them together.
Okay let's jump right into this question.
Let's tackle the question characteristics.
Now this is a physiology question.
It expects us to know what the lung is doing and how that
corresponds to certain categorization in pulmonary function test.
Now this is a 2-step question.
We have to think, 'Okay, what are the
parameteres that are measured by spirometry
and then what's described in the question stem
correlates to which component of spirometry measurement?
And thus the stem is absolutely required as we have to understand
the clinical situation in which we're performing spirometry,
and also be able to know what questions being
asked with the maneuver being performed.
Now let's walk through this question.
Well, step 1 - first we have to consider the
parameters that are measured by spirometry.
Now, a spirometer measures the volume of
air inspired and expired by the lungs.
Now the variables that are seen with spirometry
include FEV1, vital capacity, tidal volume,
inspiratory and expiratory reserve volumes
which can all be measured by spirometry.
Now let's refer to our image of what we can see there as
different variables included in a spirometry measurement.
Now looking first at the middle, we see the inspiratory
going upward and the expiratory going downward
between 30 and 37 ml/kg
And that correlates to as our tidal volume.
That's how much someone will breathe
when they're breathing normally at rest.
Now, if we look upward into what's called
the inspiratory reserve volume or IRV
that's when someone takes a breath in
after normal inhalation and holds it there,
that's what's called inspiratory reserve volume.
That means, and a very common way to think about it is, when
you're breathing normally and then I ask you to take a big breath,
what was your inspiratory reserve? and that's
what it is - inspiratory reserve volume.
And below we'll see the same thing, something called expiratory reserve volume,
meaning that, when you are breathing normally
and then I ask you to give me a big exhale,
how much expiration reserve did you have?
So that's expiratory reserve volume.
That makes a lot of sense.
Now we have something next to look
at below called residual volume.
That means after I've asked you to blow all the air
you can, how much air is left in the lungs?
And that's residual volume.
Now you can't blow out all the air in your lungs
because there are some amounts of air that
is needed to keep the alveoli open and that is
always going to be in the respiratory circuit.
And that is your residual volume.
Now, the next two we see are the inspiratory
capacity and the functional residual capacity.
Now, inspiratory capacity means after I've asked
you to take a breath in after your exhale,
that's your inspiratory capacity.
Now you may ask yourself, how is that
different than inspiratory reserve volume?
Well, inspiratory reserve volume
actually starts with your normal inhale.
So you're taking all breath in, then you take the rest of your
inhalation and that's how much inspiratory reserve you have left.
But inspiratory capacity starts
measurement after your normal exhale.
That is, after you normally exhale and then you take a big
inhale, that's how much inspiratory capacity you have.
Then you have what's called functional residual capacity
which means, after you've given me a normal exhale,
go ahead and give me your full exhale again.
Now, this includes residual volume.
So functional residual capacity can
actually be measured mathematically
by taking the expiratory reserve volume and adding
on the residual volume to come to that measurement.
Now, the next measurement is vital capacity.
Now if I ask you to take the biggest breath
you can in and let it all the way out,
that's the vital capacity we call or the amount of
air your lungs can handle on your full inspiration
and your full expiration which is what this
person did in the clinical question stem.
And the last variable to consider is what's called total lung
capacity which is all the air that you could expire and inspire
for the vital capacity plus the residual volume
and that's all the air that your lungs can hold.
Now, then looking back at the question stem, this
person was asked to take in as much air as he could
and then forcibly expire all
the air into the spirometry
which then correlates into what's called vital
capacity or answer choice (E) in this case.
Now, looking at that, let's go back to
understanding why he got vital capacity.
So the maximum amount of air that can be forcefully expired
after a deepest possible inspiration is called vital capacity,
so that's what they described in the question stem and
we can see in the image is what happens to this patient.
Now vital capacity, look at
your image to understand,
is the sum of the inspiratory reserve volume,
tidal volume and expiratory reserve volume.
Now, total lung capacity though is
vital capacity plus the residual volume.
And remember that residual volume is the
volume of air that is left in the lungs
after maximum forceful expiration that
cannot be measured by spirometry.
Now let's review some high-yield
facts regarding spirometry.
Okay, spirometry measures volumes of
air inspired and expired by the lungs.
Spirometry can measure FEV1, vital capacity, tidal
volume, inspiratory and expiratory reserve volumes.
Spirometry cannot however
measure residual volume,
which is the volume of the air left in the
lungs after maximal forceful expiration.
And total lung capacity is vital
capacity plus the residual volume.