One of the first questions in interpreting pulmonary function testing
is the definition of what is "normal". A great deal of data has been
amassed in an attempt to determine what is normal for an individual of a
given height, race, sex, and age. Despite the large amount of data
gathered, many questions and interpretation problems still exist.
However, we must do the best job with the data we have available.
Currently, the most commonly used method of deciding whether a measured
value falls outside of the normal range is to take the measured value for
that individual and compare it with a mean value measured for a group of
similar individuals. If the individual's value falls outside of the
predicted value by 20% or more, then it is said to be abnormal. For
example, if an individual's TLC is predicted to be 8 liters (100%) and the
measured value is 6 liters (75%), then this is an abnormally low value.
Obviously values immediately around the "magic" 80% mark must be
interpreted with caution and will need to be interpreted in the light of
other measurements. Some authors use the concept of the 95%
confidence interval for those values falling within the normal range.
The questions which we will be able to answer with a complete set of
pulmonary function tests are:
- Is there obstruction to airflow present?
- If there is airway obstruction
- How severe is it?
- Is it reversible or fixed?
- Is it possibly consistent with emphysema?
- Is there a restrictive process present?
- Is it a parenchymal process?
- Is it an extraparenchymal process?
- Is the extraparenchymal process a neuromuscular problem?
- Is there a combined obstructive restrictive
disorder present?
- Is there an isolated gas exchange abnormality?
- Is there upper airway obstruction present
- Is it variable or fixed and intra or extrathoracic?
1. Is Obstruction Present?
In all cases of obstruction there will be a reduction in expiratory
flow as noted on the spirogram. The FEV1 will be reduced.
However, this value might also be reduced in restrictive lung disease.
How do we deal with this problem? Two strategies have been
devised. It has been noted for some time that in obstructive lung
disease, although all indices of flow decrease, the FEV1 tends
to decrease more than the FVC. Thus in individuals with obstruction,
the FEV1/FVC tends to be reduced to a value below that
predicted for normal individuals. If one has only spirometric data
available, the diagnosis of obstructive lung disease can be made by a
finding of a reduction in the FEV1 and FEV1/FVC.
The ATS has defined the lower limit of normal (LLN) for the FEV1/FVC
as the predicted value for that individual – 9 for women and predicted
value – 8 for men. Some athletes and older people will have an
abnormally low FEV1/FVC ratio. This does not indicate an
obstructive ventilatory defect. There is no reduction in FEV1.
If the full set of lung volumes has also been measured, then other
clues to an obstructive process will be available. A very sensitive
indicator of obstruction to airflow is an increase in the RV which has
been referred to as airtrapping. With more severe obstruction to
airflow, increases in FRC and TLC can also be seen. Thus the
characteristic findings of an obstructive defect on pulmonary function
testing include a reduction in FEV1, a reduction in the FEV1/FVC,
and an increase in RV with either a normal or increased TLC.
Occasionally, in mild obstructive lung disease, the only defect
which may be seen is a reduction in FEF25-75.
2. How Severe is the Obstruction?
The severity of obstruction is graded on the basis of the reduction in
FEV1 and has been determined by agreed on standards from the
American Thoracic Society.
- FEV1 < 65-80 % mild obstruction
- FEV1 < 50-65% moderate obstruction
- FEV1 < 50% severe obstruction
3. Is the Obstruction Fixed or Reversible?
In some obstructive airways diseases, a part or all of the obstruction
will be reversible with bronchodilators. Therefore in all cases
where the technician notes obstruction, two inhalations of a
bronchodilator will be given to the subject. An improvement of 12%
in the FEV1 or FVC is considered a significant response with an
increase of at least 200ml. Asthma is considered the prototypical disease
reactive to bronchodilators. However, more "fixed" types of
obstruction such as emphysema and chronic bronchitis may also show
reversibility. In addition, because asthma is a variable disease, at
times pulmonary function tests may appear entirely normal. One will
therefore make the diagnosis by clinical history or attempt to provoke
obstruction using a "bronchoprovocational" agent such as methacholine or
cold air which can illicit bronchoconstriction which might not otherwise
be seen.
4. Is the Disease Consistent with Emphysema?
Emphysema is a diagnosis made by the pathologist examining lung
tissue and now more recently with a typical pattern on thoracic CT scan.
However, there are certain findings on pulmonary function testing which
can point towards a diagnosis of emphysema. A reduction in FEV1,
FEV1/FVC as well as an increase in RV are seen. The TLC
is elevated consistent with a reduction in inward elastic recoil of the
lung because of destruction of elastic tissue. Frequently, a
reduction in DLCO reflecting destruction of the alveolo-capillary bed is
also seen. The flow-volume loop may also show findings of dynamic
airway collapse.
5. Is Restrictive Lung Disease Present?
The defining factor for restrictive lung disease is the
reduction in the TLC. TLC, RV, VC, and FRC all tend to be reduced,
though not in all cases. Measurements of expiratory flow tend to be
preserved including the FEV1/FVC and FEF25-75.
6. What Type of Restrictive Pattern is Present?
Parenchymal processes result in a restrictive pattern by reducing the
compliance or "stretchability" of the lung. Frequently in these
processes there is a destruction of the alveolo-capillary bed which is
seen as a reduction in the DLCO. A reduction in the TLC coupled with
a reduction the DLCO points to a parenchymal cause of restrictive disease.
Diseases outside of the lung which prevent maximal expansion of the
respiratory system including neuromuscular, skeletal, and even
extrathoracic processes such as ascites or pleural effusion can lead to
restrictive ventilatory defects. A neuromuscular disease such as
Duchenne's muscular dystrophy affects the muscles of expanding the chest
wall. All lung volumes will be reduced in a nearly proportionate
way. The DLCO will usually be normal because there is no intrinsic
problem with the lungs. One lung volume, expiratory reserve volume
(ERV) may actually be greater than predicted because of weak expiratory
muscles. The finding of a reduction in maximal inspiratory and
expiratory pressures confirms the cause of restrictive defect.
Abnormalities in the skeletal system or chest wall itself can result in
a restrictive ventilatory defect. The kyphoscoliosis can result in
reductions in TLC with a preserved DLCO as can such unusual entities such
as fibrothorax, massive ascites, or obesity. In these cases muscle
strength and DLCO may appear normal.
How Severe is the Restrictive Defect?
Based on American Thoracic Society criteria, restrictive lung disease
is based on the criteria of TLC.
| 65-80% |
mild restriction |
| 50-65% |
moderate restriction |
| < 50% |
severe |
7. Is there a combined disorder
(obstructive and restrictive) present?
On occasion there can be a combination of obstruction and restrictive
processes occurring simultaneously. For instance, a patient who
smokes and has developed emphysema and later presents with a neuromuscular
cause of restrictive lung disease. Some diseases can intrinsically
have both a restrictive and an obstructive component such as sarcoidoisis
in which there may be an endobronchial component as well as an
interstitial component causing restrictive lung disease. In these
cases, the finding will be a combination of a reduction of TLC associated
with reduction in flow, namely a decrease in FEV1 and FEV1/FVC
ratio.
8. Is there an isolated gas exchange
abnormality present?
Sometimes the only abnormality noted on pulmonary function testing is
a reduction in DLCO. This test is quite variable and difficult to
perform so that in general concern is not raised until the DLCO is
approximately 60% or less than that of predicted. Isolated
reductions in DLCO may be an early sign of interstitial lung disease, a
vasculitis, pulmonary emboli, or anemia. The DLCO can be corrected
for anemia to rule out the latter.
9. Is Upper Airway Obstruction Present?
Upper airway obstruction may be suggested by the clinical findings of
stridor on physical examination. Reductions in flow are usually seen
on the forced expiratory maneuver. However, when flow is plotted
against volume evidence of upper airway obstruction can be readily
appreciated. Abnormalities in the flow volume cure are immediately
appreciated. Intra and extrathoracic variable and fixed lesions can
be lesions can be identified, ranging from mediastinal tumor to an
enlarged thyroid. (See figure 5 below Q: is this fig 5
above or another fig?)
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