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Pulmonary Function Tests

Pulmonary Function Tests (PFT's) consist of several tests designed to provide objective information about lung function. Common pulmonary function tests ordered are spirometry, lung volumes, lung diffusion capacity, and methacholine challenge.

The basic pulmonary function test is the spirogram or spirometry. The spirogram is obtained by seating the patient comfortably in a chair with soft nose clips gently holding the nostrils closed. The patient is asked to take his/her deepest breath, then form a tight seal with his or her mouth around a disposable plastic tube, and finally exhale (blow out) into the plastic tube as hard and as long as possible. To obtain consistent, reliable results, this test may need to be repeated several times. This test measures the total volume exhaled (termed the FVC or forced vital capacity), the amount exhaled in the first second (termed the FEV1 or the forced expiratory volume in the first second), and finally the FEV1:FVC ratio). Patients with obstructive lung diseases (such as asthma, chronic bronchitis, emphysema or COPD) often have low FEV1:FVC ratios. Patients with restrictive lung diseases (such as pulmonary fibrosis) often have normal FEV1:FVC ratios, but the FVC itself is low. Not only does the spirogram identify patients who might have obstructive or restrictive lung disease, but also helps the physician objectively determine the severity of the diseases and provides a benchmark for future comparison should the patient show either improvement or deterioration.

Lung volume determination is used to measure the total volume of the lung in the chest (termed the total lung capacity). This total lung capacity includes the size of the "largest" breath one can take (from the lungs maximally expanded to maximally emptied) as well as the gas that still remains in the chest even after the patient makes his/her effort to fully "empty" the lungs; this remaining gas is called the "residual volume" and can be measured indirectly by using either a Helium tracer dilution technique or a body-box technique. A total lung capacity less than about 80% of predicted likely indicates pulmonary restriction (small lung size) either from scarring of the lung tissue, extrinsic compression of the lung by fluid or chest/abdominal wall abnormalities, or weakness of the diaphragms. A high total lung capacity usually indicates "trapping" of air in the lungs by obstructive lung diseases such as asthma, chronic bronchitis, emphysema or COPD.

Lung diffusion capacity determination essentially measures how well the lungs are working in their basic function - that is, transferring gas between the air and the blood stream. During this maneuver, a deep breath containing a tracer gas is held for about ten seconds. The amount of this tracer gas absorbed through the lungs is measured and reflects the efficiency of gas transfer. A score of less than 80% of predicted usually indicates poor gas transfer and less than 40-50% usually reflects a more severe problem with gas transfer. Gas transfer deficits are seen in many pulmonary disorders, most commonly pulmonary scarring disorders and emphysema.

A methacholine challenge may help diagnose asthma when the spirograms are unremarkable. In this study, repeated spirograms are performed after increasingly higher dose inhalational "challenges" of methacholine are administered. Non-asthmatics will show very little change in their spirograms even at relatively high doses of inhaled methacholine where asthmatic patients will generally show worsening spirogram scores at relatively low doses of methacholine.

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