Pulmonary Function Test


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 Pulmonary function tests are a group of procedures that measure the function of the lungs, revealing problems in the way a patient breathes. The tests can determine the cause of shortness of breath and may help confirm lung diseases, such as asthma or bronchitis.
The tests also are performed before any major lung surgery to make sure the person won't be disabled by having a reduced lung capacity.
Purpose of Pulmonary Function Testing
Pulmonary Function Testing has been a major step forward in assessing the functional status of the lungs as it relates to : 
  1. How much air volume can be moved in and out of the lungs
  2. How fast the air in the lungs can be moved in and out
  3. How stiff are the lungs and chest wall - a question about compliance
  4. The diffusion characteristics of the membrane through which the gas moves (determined by special tests)
  5. How the lungs respond to chest physical therapy procedures
Using the results, the physician determines two important values:
  1. The forced vital capacity (FVC). FVC is the maximum volume of air that can be exhaled with force and is an indicator of the lung size, elasticity, and how well the air passages open and close.
  2. The forced expiratory volume in one second (FEV1). FEV1 is the maximum volume of air expired in one second. Airflow is considered to be limited if the outflow of forced exhalation is persistently low over the course of one second. Steady but faster than normal decline in FEV1 over time characterizes COLD.
Calculating a ratio of FEV1 to FVC is the best method for determining the presence and severity of COLD. The severity of airway obstruction may be graded by the percentage of the patient's predicted FEV1:
  1. Mild COLD is an FEV1/FVC ratio of 70% or higher, with a predicted FEV1 of less than 80%.
  2. Moderate is an FEV1/FVC ratio of 60% to 69%, with a predicted FEV1 of less than 80%.
  3. Moderately severe is an FEV1/FVC ratio of 50% to 59%
  4. Severe is an FEV1/FVC ratio of 34% to 49%
  5. Very severe is an FEV1/FVC ratio of less than 34%.


Tests for Measuring the Ability of the Lung to Exchange Gases
Arterial Blood Gas. The physician may request an arterial blood gas test to determine the amount of oxygen and carbon dioxide in the blood (its saturation). Low oxygen (hypoxia) and high carbon dioxide (hypercapnia) levels are often indicative of chronic bronchitis, but not always of emphysema. A blood gas analysis that shows very low oxygen levels (measured as PO2) is useful for determining which patients would benefit from oxygen therapy. This procedure typically draws blood from an artery in the wrist, which can be painful.

Pulse Ox Test. A less painful test for measuring oxygen in the blood is called a pulse ox, which involves placing a probe on the finger or ear lobe. When blood is fully saturated with oxygen, it forms a compound called oxyhemoglobin, which gives blood its bright red color. When blood has insufficient oxygen, it turns a bluish color (called cyanosis). This test only measures oxygen in the blood, however, and not carbon dioxide, so it is not useful in determining candidates for long-term supplemental oxygen.

Carbon Monoxide Diffusing Capacity. The lung carbon monoxide diffusing capacity (DLCO) test determines how effectively gases are exchanged between the blood and airways in the lungs. Patients should not eat or exercise before the test and they should not have smoked for 24 hours. The patient inhales a mixture of carbon monoxide, helium, and oxygen and holds his or her breath for about 10 seconds. The gas levels are then analyzed from the exhaled breath. Results can help physicians differentiate emphysema from chronic bronchitis and asthma. Patients with emphysema have lower DLCO results, indicated by a reduced ability to take up oxygen. Such results are also important in helping to determine appropriate candidates for lung reduction surgery. (Carbon monoxide levels that are 20% or less than predicted values pose a very high risk for poor survival.)