ABIM Exam Review Practice Question of the Week: A helpful review of pulmonary function tests

You are given a set of pulmonary function test (PFT) readings that show:

FEV1 is low
FEV1/FVC ratio is elevated
TLC is decreased
DLCO is normal
Residual volume is increased

Which of the following conditions does this patient likely have?

A. COPD

B. Asthma

C. Sarcoidosis

D. Obesity

E. Interstitial fibrosis

Explanation:

PFTs can be interpreted into obstructive and restrictive lung diseases.

Obstructive lung disease (characterized by airway obstruction) could be conditions such as COPD and asthma.
Restrictive lung disease (characterized by reduced lung volume) can be further broken down into intrathoracic and extrathroacic diseases.

The first way to differentiate between obstructive and restrictive disease is to look at the TLC (Total Lung Capacity). With obstructive diseases, TLC would be increased. The way to understand this concept is the following:

With obstructive diseases, there is bronchoconstriction or air trapping that occurs. As a result there is so much air in the lungs that total lung capacity and residual volume are increased.
Restrictive diseases are just like the name sounds….they “restrict” the amount of air in the lungs which means TLC would be decreased.

This example shows that the TLC is decreased so we can eliminate Choice A (COPD) and Choice B (Asthma) since both are obstructive diseases. Now we have to choose among the other 3 choices. This is where we see if the condition is an extrathoraic or intrathoracic restrictive disease.

Intrathoracic restrictive diseases consist of sarcoidosis, interstitial fibrosis, or pulmonary fibrosis. With these diseases, the DLCO and residual volume levels are decreased.
Extrathoracic restrictive diseases consists of disorders such as: kyphosis, obesity, Guillain-Barre syndrome, Myasthenia Gravis, and muscular dystrophy. This would show a normal DLCO and increased residual volume levels.

Of the above, only Choice D (Obesity) could match the criteria for this patient.

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