Tip:
Highlight text to annotate it
X
When doing a spirometry there will be two types of data, some graphic data
which are the volume-time curve and the flow-volume curve
and some numerical data. Knowing the
basics about your data is key
in order
to successfully interpret a spirometry.
In the graphic representation of the spirometry we have to keep in mind
that there are two types of curves that are very important in order to make
a proper interpretation.
An initial curve, which is the volume-time curve, is very easy to understand
in principle, we expel a large quantity of volume
in a very short time, what is expelled in one second is the volume exhaled in the
first second
known by its acronym FEV1.
The quantity expelled from then on is slower,
a lesser quantity
until the maximum capacity of exhaled volume which is the forced vital
capacity known in British Isles
by the acronym FVC.
The next curve is the flow- volume curve which is harder to
understand from the physiological point of view,
for it represents
a flow in terms of a volume,
but
we can have a similar estimated morphology for the majority of patients
that can help us to distinguish
some acceptability errors and even make some diagnostic
estimation by assessing this curve.
In theory it tends to be very
close to the vertical axis
until it reaches a set peak, and then it follows a downward path
which must not have any artefact
and with whose morphology we can give a diagnostic orientation
until it finishes slowly, falling like a piece of paper above the
ground
slowly and gradually
on the
X-axis,
which would be the volume in this case.
After assessing the graphs it is essential to analyze
the numerical data.
Despite the apparent complexity of a spirometry with
so much numerical data
you need only assess 3 crucial aspects or three
key numerical data.
They are fundamentally the best spirometric assessment
of one's forced vital capacity
in percentage value compared to a similar population group of
age, sex, weight, height, race.
The best
exhaled volume in the first second is also represented
by one's percentage in relation to
the reference population
and one's quotient.
It is essential that we assess the observed quotient during the interpretation, the
real one between the two
absolute data
that this test offers
as a considerable test.
The forced vital capacity and the
FV1
will be deemed normal when they are above 80% of the reference
and the observed quotient is
obstructive
when it is below 70%
of the real or observable quotient.
The first fundamental criteria to properly interpret the spirometry
is to make sure that this spirometry study is acceptable,
and this can be done in two ways: either with the technician when doing
the test
or by analyzing
analytical and morphological criteria.
One must be sure that there are no acceptability errors and to do so we
must make sure
that the beginning is correct,
that the duration is sufficient,
that there are no alterations in the line and that the final part of the test is
optimal.
By evaluating the graphic data we can assess errors
that can cause
the spirometry to be deemed unacceptable.
The most common errors that we see with the spirometry are
those interconnected ones; either because the beginning of the test
is slow, what we see is that it separates
from the axis
in the flow-volume curve on the Y-axis
or because it ends early,
which we see because it does not plateau or the time of duration on the
volume-time is short.
And oftentimes what we see is that the ending is very abrupt in the
flow-volume curve
also that
the patient coughs during the first second which is seen in the shape of a spicule
at the end
of the flow-volume curve
and it would alter the interpretation of the test, which is hard to see in the
volume-time curve.
We can also see it
as soon as
the patient makes a change in effort, which is oftentimes seen in patients who
try to simulate a deficiency
during the test
what we see is that there are large ripples similar to waves or
like
humps
of a camel in the flow-volume curves
whereas
it is interpreted as a notch or assessed as the shape of a
notch on the volume-time curves.
In other instances, the patient inadvertently closes
the glottis;
therefore, we see it level off suddenly as if the volume-time curve were
a right angle
and instead we see the one in the flow-volume curve as a
very sudden
ending,
falling almost vertically.
And lastly,
one of the most common errors is also in patients who open their mouths or when
where there is some system alteration
is the loss of volume.
It is seen after reaching the plateau
on the volume-time curve,
the curve drops
over the maximum value reached
and it is seen in the
flow-volume curve as a small recoil at the end.
The next essential criteria is that the tests be reproducible,
since only the maximum efforts
are reproducible we must make sure that there is not an excessive difference
between the tests.
Therefore,
we analyze, we study various tests.
Ideally we perform three
proper tests
and based on that we make sure there are no differences over 150 millilitres
in the
forced vital capacity
nor in the exhaled volume in the first second
and we choose
those used in the interpretation. We will choose the
highest values reached in the different tests
and for the quotient
the ratio of these two.
The curves are not only used to see
if the test is acceptable or not, we can use it to obtain
data I think that is important when interpreting
the spirometry.
There are two types of
patterns, which is a simplification of what we used to know until now: or what the guides used to refer to, which are
obstructive patterns and non-obstructive patterns.
In comparison to what used to be divided into pure
obstructive pattern
mixed pattern and restrictive pattern.
In the obstructive pattern, once known as the obstructive pattern,
or those patterns that used to be mixed patterns,
the characteristic of the volume-time
curve is that it moves towards the right side
so that it is slow to plateau or never plateaus,
whereas the flow-volume
curve has a diminished peak flow
with a non dependant effort phase with an upper
concavity
and it ends very smoothly and slowly.
In the aforementioned non- obstructive patterns,
or known in many books still like
that
known as restrictive,
what we see is that they are curves similar to normal curves but everything is
smaller, reduced in size
as compared to the standard, what the blue line would be
in this case,
and the red one would be the pathology.
Once the graphic data is assessed, it is important to make a diagnosis or a
systematic interpretation by following the proper steps which are very simple, and the latest standards have simplified it even more
than the numerical data.
For this
the first thing we need to look at and analyze is the quotient between the
FEV1
and the FVC
so that
we can see if it is over 70% of that observed,
we'll say this quotient is not obstructive.
The next step is to look at the forced vital capacity;
in this case we will analyze the value
over the patient's reference and it will be normal
if over 80% of the theoretical one,
so we will say that this spirometry is normal
and it will be low if under 80% of the theoretical one,
then we will say that the patient presents a non obstructive ventilatory
alteration,
previously known as restrictive
but that has now been considered because there are patients who
can have obstruction, and due to excessive air impingement it is possible that it is not diagnosed properly with a spirometry.
In these cases
we would point out other tests like complete functional tests, volumes
or imaging studies, to be able to make a proper diagnosis.
If the quotient is under 70 per cent, always referring to what is observed,
like we've said before,
in previous steps to this
exposition,
we'll say that the patient has obstructive ventilatory alteration
and then we will make an assessment of severity
which will be mild if over 70 per cent of the FEVI,
if moderate between 60 and 69,
moderately serious
in the 50 59% of the reference,
serious if between 35 and 49 percent, and very serious if under 35%.
Let's go over all the steps
to properly interpret a spirometry
before seeing it in a very practical way with an example.
First we will assess the errors in the volume-time curve,
fundamentally the duration of the test, the scope and the maintenance
of the plateau.
We will assess the errors
regarding the line at the start, whether it reaches a peak in
its descent and in the end
and the shape of the volume-flow curve itself.
We will also assess the reproducibility,
which we clearly cannot see in the example because we would need
several tests using the same patient.
Regarding the numerical data, we will first see the quotient
between the volume exhaled in the first second and the forced vital
capacity
so that we always see what is observed and not the theoretical,
it will be normal if over 70%
or we will say there is an obstructive pattern if below 70%.
If the quotient is normal we will have to see the forced vital capacity
whose normal reference value is considered normal when over 80%,
and if it is below 80% we will diagnose the patient with a
non-obstructive pattern.
When the pattern is obstructive we will use the exhale volume in
the first second to assess severity.
And lastly we will create
a written report
that will be recorded in the clinical history.
We will finish
by representing everything mentioned so far in a
real spirometry
of any patient that you may have in tomorrow on your visits.
The first step would be
to *** the volume-time curves
where we would see the exhale time, in this case 12 seconds,
and we can already get
an idea of
his forced vital capacity, which is nearly
five litres.
The next step to perform would be step 2:
looking at the volume-time curve
to see that it is very close to the Y-axis, that it reaches a proper
peak
and that it has a morphology in its line without any incidence or alteration,
finishing off slowly
over the X-axis, as you see in this case perfectly represented.
Its morphology sometimes helps us and this upper concavity of the
non
dependent effort phase can help us make an our interpretation.
The third step is to assess the reproducibility,
an aspect that we cannot do because we would need to perform three tests
together in order to do it.
The forth step would be assessing the numerical data,
evaluating
the FEV1/FVC quotient,
as we have always said, using data we observe.
In this case it is below 70%
and the algorithm would be moved to the right.
Like we had mentioned before
we will later assess the forced vital capacity;
in this case 93 per cent is normal
and the FEV1, which is 77%, will help us define the severity.
The proper interpretation of this example would be
that the patient presents a mild obstructive pattern.
It is important to note
the total FEV1 volume to later perform studies and see the evolution.
In this case, it is at 77 per cent of what it should be.