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How To Interpret Pulmonary Function Tests

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:

  1. Is there obstruction to airflow present?
    1. If there is airway obstruction
      1. How severe is it?
      2. Is it reversible or fixed?
      3. Is it possibly consistent with emphysema?
  2. Is there a restrictive process present?
    1. Is it a parenchymal process?
    2. Is it an extraparenchymal process?
      1. Is the extraparenchymal process a neuromuscular problem?
    3. Is there a combined obstructive restrictive disorder present?
  3. Is there an isolated gas exchange abnormality?
  4. Is there upper airway obstruction present
    1. 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.

  1. FEV1 < 65-80 %  mild obstruction
  2. FEV1 < 50-65% moderate obstruction
  3. 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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