You are at: home > what is alpha-1? > lung info > understanding pft's

Understanding PFT's.....

"PFTs", or pulmonary function testing, is simply the evaluation of lung function. PFTs are used to determine:

• the presence of lung disease or abnormality of lung function
• the extent of abnormalities
• the extent of impairment caused by abnormal lung function
• the progression of the disease the nature of
• the physiologic disturbance
• a course of therapy and treatment, as indicated

As you may have experienced, seeing your PFT results can be quite overwhelming.  It is my hope that this simplification of what the abbreviations (i.e. FVC) mean, as well as the observed values, will help you to understand your own test results.  Obviously, not all the measured values on your test have as much significance as others, therefore, I will try to define and discuss only those which are most commonly measured and have the most significance.

PFT definitions

Normal: for comparative purposes, observed values that fall at 80% or greater than the predicted values.

Predicted Values: these are average numbers based upon one's age, height, sex, and at some institutions, race.

PFTs:

A. Pulmonary mechanics

1. FVC - Forced Vital Capacity is the maximum volume of gas that can be forcefully and rapidly expired after a maximal inspiration. This maneuver may also be called a "Flow-Volume Loop", which is the same maneuver that is graphically displayed comparing volume and flow rate. significance: May be normal or reduced in emphysema because of loss of support for small airways.

2. FEV1 - Forced Expiratory Volume after 1 second is the volume of gas expired after one second from the beginning of the FVC maneuver. significance: Usually decreased in obstructive airways due to mucus secretion, bronchospasm, inflammation, or loss of elastic support of the airways themselves, as in emphysema.

3. FEV1/FVC - The ratio of FEV1 to FVC expressed in a percentage. significance: usually decreased in obstructive airways and is independent of the relative values of FVC and FEV1.

4. FEF 25-75% - Forced Expiratory Flow from 25 to 75%, is the average rate of flow during the middle half of an FVC maneuver based upon a segment of the FVC that included the flow from medium-sized and small airways. Also known as "mid flow rates". significance: usually decreased in obstructive airways due to mucus secretion, bronchospasm, inflammation, or loss of elastic support of the airways (i.e.emphysema).

5. FEFMAX or PEFR - Forced Expiratory Flow at Maximum effort or Peak Expiratory Flow Rate, is the maximum flow rate attained during the FVC maneuver. significance: Generally, FEFMAX is a good index of the patient's effort and relative strength in forceful exhalation. Also, may have relative meaning in evaluation of the effectiveness of a patient's cough.

6. Raw - Airway resistance is the "resistance" created by the airways and the friction against the walls of the airways caused by the flow of air through these passageways. This test is performed in the so-called "body box" or Plethysmograph, where one performs "panting" maneuvers. significance: Raw is increased in emphysema because of narrowing and collapse in some of the larger airways, as well as more distal bronchioles. "Normal" would be a value which is less than the predicted value.

B. Lung volumes

"Air-trapping" - a term used to describe the state where one actually maintains a large amount of air in one's chest, even at the end of both normal and maximal exhalation.

1. SVC - Slow Vital Capacity is the volume of gas measured on a slow, complete expiration after or before a maximal inspiration, without forced or rapid effort. significance: often the SVC is significantly larger than FVC, which indicates to some degree the existence of air-trapping. Generally, with obstructive lungs, SVC is within normal limits. It may be less than normal due to the collapse of airways due to lack of support.

2. FRC or TGV - Functional Residual Capacity, or Thoracic Gas Volume, is the volume of gas remaining in the lungs after exhalation of a normal breath. This test is performed in several different ways. Perhaps the most accurate is by the "body box"; other less accurate ways (especially in light of poorly communicating airways) include the Nitrogen Washout method, Helium Dilution, and Single-Breath Nitrogen Washout. significance: relative to the predicted values, larger values of FRC or TGV indicate the degree of air-trapping (i.e. emphysema).

3. RV - Residual Volume is the volume of gas remaining in the lungs after complete exhalation. Value is obtained by any of the above methods mentioned for FRC.significance: Like FRC, RV is compared to the predicted values to determine the degree of air-trapping.

4. TLC - Total Lung Capacity is the volume contained in the lungs at the end of a maximal inspiration. This value is obtained from determination of SVC and FRC, in methods previously mentioned. significance: because of air-trapping associated with emphysema, the TLC is usually larger than predicted.

C. Diffusing capacity

DLCO - Diffusing Capacity measures the transfer of a diffusion-limited gas (Carbon Monoxide, CO 0.3%) across the alveolocapillary membrane. CO combines with hemoglobin approximately 210 times more readily than oxygen does. In the presence of normal amounts of hemoglobin and normal ventilatory function, the primary limiting factor of diffusion of CO is the status of the alveolocapillary membrane. significance: DLCO is usually decreased in emphysema because of the decrease in the total surface area, loss of capillary bed, increased distance from terminal bronchiole to alveolo capillary membrane, and the mismatching of ventilation to blood flow.

D. Other measurements

1. ABG - Arterial Blood Gasses are samples drawn from an artery (usually the radial near one's wrist), which enables us to measure the following values:

1.PH - the measurement of your arterial blood's acid-base balance. This "balance" is controlled by several factors, but primarily metabolism and ventilation. If there is an imbalance with one or the other or both, there will be a definite effect on the PH. The approximate "normal" range of PH is 7.335 to 7.45, with 7.4 being the mean. Values less than 7.4 are considered to be more "acidic", while those greater than 7.4 are more basic or "alkaline".

2.PaCO2 - Partial Pressure of Carbon Dioxide in arterial blood. This value is an indicator of how effectively your lungs are able to rid themselves of a by-product of metabolism, CO2. The "normal" range for PaCO2 is 35 to 45 mm Hg. Elevated values greater than 40-45 mainly indicate that the lungs are not able to rid themselves of the CO2 ("CO2 retainer"). This may be evident in emphysema, where much of the lungs are unable to effectively exchange gasses.

3.PaO2 - Partial Pressure of Oxygen in arterial blood. This is a measure of the actual amount of oxygen there is in your arterial blood. The "normal" for PaO2 is generally greater than 75-80 mm Hg, relative to your age. For the most part, it should be greater than 55-60 mm Hg, otherwise supplemental oxygen would be indicated. (Note of caution: More is not always better. For those of you on supplemental oxygen, please contact your physician before you significantly increase your O2 flow).

2. PULSE OXIMETRY or SaO2 - This is a non-invasive measure of one's oxygen saturation; that is, the amount of oxygen saturated in one's hemoglobin in terms of a percentage. This is not as accurate as the values obtained from an ABG and should only be used as a gauge of one's oxygenation. Normal ranges are between 95-100%. Supplemental oxygen is not generally instituted unless SaO2 is less than 88-90% at rest.

Bibliography:

Laboratory Evaluation of Pulmonary Function, William F. Miller, Robert Scacci, Lee Roy Gast, J.P. Lippincott Company, Philadelphia, PA, 1987.

Manual of Pulmonary Function Testing, Fifth Edition, Gregg L. Ruppel; Mosby Year Book, USA, 1991.

Respiratory Physiology - the Essentials, John B. West; Williams and Wilkins Company, Baltimore, MD, 1987.

with kind permission of the author: Len Moriyama, RPFT, RRT, RCP Respiratory Technician for Alpha1 Registry California Pacific Medical Center, San Francisco