Pulmonary function testing: Interpretation scheme
In the last three Chalk Talk episodes, we introduced spirometry and body plethysmography
and looked at typical changes in the most important pathologies: obstruction, hyperinflation,
restriction, and mixed ventilatory defects.
In this episode, we’ll wrap things up with an interpretation scheme that enables you
to quickly come to a diagnosis by assessing only three parameters:
Start by first looking at the FEV1:FVC ratio, followed by the vital capacity and, if necessary,
the total lung capacity.
The FEV1:FVC ratio is considered pathological if it’s below 70%.
The values for the vital capacity and total lung capacity are pathological if they’re
below 80% of the normal limit
Let's begin with the FEV1:FVC ratio.
It sets the initial course towards indicating restriction or obstruction.
Keep in mind that if the FEV1:FVC ratio is lowered, the ventilatory disorder involves
obstruction in some way.
This is because a decreased FEV1:FVC ratio occurs when the FEV1 value is decreased and
the vital capacity is normal or at least less strongly decreased.
The cause of such disorders can be mainly traced back to the airways, decreasing the
airflow and lowering the volume to be exhaled forcibly in one second.
Let’s next look at the vital capacity.
If the value is within the normal range, the lung volume is normal.
In this case, the diagnosis is just obstruction.
In contrast, if the vital capacity is also reduced, the total lung capacity is required
to make a diagnosis.
In the case of a reduced vital capacity, it needs to be determined whether the total lung
volume, that is, the vital capacity and the total lung capacity, are also decreased.
This would be the case in restriction.
If only the vital capacity is reduced and the total lung capacity within the normal
range, this indicates hyperinflation.
In hyperinflation, the vital capacity is reduced at the expense of an increased residual volume.
If there’s also a decreased total lung capacity, a combination of obstruction and restriction
is present, i.e., a mixed ventilatory defect.
The reduced FEV1:FVC ratio is due to obstruction, whereas the diminished vital capacity and
total lung volume are the result of restriction.
In contrast, if the total lung capacity is within the normal range, the decrease in vital
capacity is due to an increased residual volume.
The reason for this is lung obstruction, alongside hyperinflation.
This is termed air trapping.
So, in summary, a reduced FEV1:FVC ratio can have three different causes: Together with
a normal vital capacity, the cause is obstruction; with a reduced vital capacity and total lung
capacity, a mixed ventilatory defect is present; and with a reduced vital capacity and normal
total lung volume, the cause is hyperinflation.
Let’s take a look at the types of disorders present when the FEV1:FVC ratio is within
the normal range: If the vital capacity is also normal, the finding is physiological.
In contrast, if there’s a decreased vital capacity, the total lung capacity needs to
be looked at next.
It enables us to differentiate whether the total lung volume is reduced, or just the
vital capacity as a consequence of an increased residual volume.
If the total lung capacity is also decreased, then there’s an absolute reduction in the
vital capacity.
Accordingly, the diagnosis is restriction.
If, however, the total lung capacity is within the normal range, air trapping is present.
In this case, the reduced vital capacity reflects an increased residual volume as a result of
hyperinflation.
So, as you can see, to diagnose restriction, mixed ventilatory defects, and hyperinflation,
the total lung capacity is required.
Therefore, if a decreased vital capacity is measured in spirometry, body plethysmography
should follow to determine the total lung capacity.