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 Pulmonary Function Laboratory |
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Pulmonary Function Laboratory |
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| For more information about pulmonary function tests and pulmonary physiology, be sure to check out the Johns Hopkins Interactive Encyclopedia of Pulmonary Physiology |
| Most Common Tests |
| Less Common Tests |
| Spirometry Spirometry is a measurement of forced expiration. The patient inhales maximally, filling his or her lungs to Total Lung Capacity, and then exhales forcefully into a device called a spirometer. The spirometer measures volume and time, and from this several important parameters may be calculated:
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| Spirometry after Bronchodilator Spirometry, repeated after the administration of a bronchodilator (most often albuterol [Ventolin, Proventil]). Generally, an increase of more than 10% in either FVC or FEV1 is considered a positive response. Back To List |
| Flow-Volume Loops The same general test as spirometry, except the data collected are plotted in a different way, showing flow vs. volume. The patterns thus revealed may indicate the site and nature of any airways obstruction. Back To List |
| Single Breath Diffusing Capacity The single breath diffusing capacity, or DLCO, is a measure of the ability of the lungs to diffuse oxygen into, and carbon dioxide from, the bloodstream. The test is performed by having the patient complete the following steps:
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| Helium Dilution Lung Volumes This test measures the total amount of gas in the lungs after a complete inspiration. The patient is connected to a spirometer containing a known concentration of helium. Initially, the gas in the patient's lungs dilutes the helium present in the system, and the helium concentration falls rapidly. After a few minutes, however, the patient and the spirometer equilibrate, and the helium concentration reaches a steady value. By measuring the initial and final concentrations of helium present, and by knowing the volume of the spirometer, the amount of gas in the patient's lung at the start of the test may be calculated. Back To List |
| Body Plethysmographic Lung Volumes This test measures the same things as does Helium Dilution Lung Volumes, but in a very different way. The patient sits in a clear rigid chamber, breathing through a valve. At some point in the breathing cycle, the valve is closed for a few seconds and the patient is asked to pant (although no breathing will occur since the valve is closed). Each time the patient tries to pant out, the gas in the lungs is decompressed slightly and the gas in the box surrounding the patient is compressed slightly. When the patient tries to pant in, the opposite occurs. By measuring the pressure changes in the lungs and in the box, the amount of gas in the patient's lungs may be calculated. Back To List |
| Arterial Blood Gas Analysis This test measures the amounts of oxygen and carbon dioxide and the pH of an arterial blood sample. Most often the blood sample is obtained in the laboratory via either the radial artery (near the wrist) or the brachial artery (near the inside of the elbow). A blood gas analyzer measures and reports the pO2 (the amount of oxygen present), the pCO2(the amount of carbon dioxide), and the pH of the blood. Back To List |
| Pulse Oximetry This test estimates the amount of oxygen in the arterial blood by shining a light through a fingertip. Because blood that holds a lot of oxygen is a different color than less oxygenated blood (oxygenated blood is bright red, otherwise it is purple), the device is able to estimate the blood's oxygen content. Back To List |
| Sputum Induction In this test, the patient is asked to breathe from a nebulizer a mist containing hypertonic saline solution (essentially, water with salt in it). When the mist reaches the lungs, it draws water out from the surrounding tissues. Eventually, this causes the patient to cough in an effort to eliminate this excess fluid. The sputum thus expectorated is collected and transported to the Pathology laboratories for further analysis (generally to identify any microorganisms that may be present). Back To List |
| Pulmonary ExerciseTest Any one of a family of tests in which measurements are made while a patient is asked to perform increasing amounts of exercise. In the simplest type of exercise test performed in our laboratory, a desaturationtest, measurements of heart rate and oxygen saturation from pulse oximetry are recorded while the patient is asked to pedal a stationary bicycle. The bicycle's resistance is increased as the test progresses and the patient is asked to continue until either the patient's oxygen levels fall or until the patient is unable to continue. If the test is ended due to a fall in oxygen levels then it is repeated, after an appropriate rest and recovery period, while the patient is given extra oxygen via a nasal cannula (a small oxygen tube placed beneath the nose). This cycle continues until the patient is able to exercise to exhaustion without a fall in oxygen levels or until the maximum amount of supplemental oxygen has been used. This test is used primarily to detect and quantify the need for supplemental oxygen. In a more sophisticated form of exercise testing, which our laboratory terms a gas collection test, laboratory staff collect measurements about a patient's breathing, as well as heart rate and oxygen saturation from pulse oximetry, while the patient is asked to pedal a stationary bicycle. The collection of expired gases permits the laboratory to caclulate various quantities that are useful in assessing a patient's response to external work. Among these quantities are minute ventilation (amount of breathing per minute); tidal volume (the size of each individual breath); respiratory rate (the number for breaths taken in each minute); oxygen consumption (the amount of oxygen fuel consumed each minute); and carbon dioxide production (the amount of waste carbon dioxide produced each minute). The test progresses with increasing resistance on the bicycle until the patient is unable to continue. This test may be helpful in evaluating causes of unexplained shortness of breath or decreased exercise capacity and may be helpful in distinguishing between cardiac and pulmonary limitations. The most sophisticated form of exercise testing peformed in our laboratory adds an arterial catheterization to the gas collection test described above. The arterial catheter is a very small tube placed in the radial artery, the blood vessel in the wrist supplying blood to the hand. Blood samples are drawn periodically and analzyed as described above. The additional information obtained from the blood analysis may be helpful in detecting subtle impairments in lung function. In all forms of exercise testing, patients are monitored by a physician for changes in blood pressure and heart function (via electrocardiograph). Back To List |
| Static Lung Compliance A test in which laboratory staff measure the elasticity, or stiffness, of the lung. The test is performed by inserting a very small catheter into the esophagus, which is used to estimate pleural pressure (the pressure surrounding the lung). Once this catheter is in place, the patient is asked to exhale slowly through a mouthpiece and into a device used to measure volume. The mouthpiece is blocked periodically during exhalation, for a fraction of a second each time, permitting an estimate of the pressure across the wall of the lung (static transpulmonary pressure). A graph of lung volume versus static transpulmonary pressure produces a curve whose slope is the compliance (compliance is defined as the change in volume divided by the change in pressure). Patients with emphysema may have elevated compliance while patients with diseases such as fibrosis may have decreased compliance. Back To List |
| Trans-diaphragmatic Pressure (Pdi) This is a test of muscle strength designed to assess the function of the main breathing muscle, the diaphragm. In this test, two very small catheters are inserted, one into the stomach and one into the esophagus. The patient is then asked to make maximal inspiratory efforts, usually by sniffing forcefully, while the pressures at the tips of the two catheters are recorded. The maximum pressures are representative of the strength of the diaphragm and may be helpful in assessing possible weakness of this muscle. Back To List |
| Ventilatory Dead Space Ventilatory dead space is a measure of the amount of each breath that is "wasted" by filling areas of the respiratory system that do not exchange gas with the blood. The test is performed by having the patient breathe into a large bag via a special breathing valve while an arterial blood sample is collected and analyzed. Back To List |
| Methacholine Challenge In a Methacholine Challenge, a patient performs repeated spirometry tests following inhalation of increasing concentrations of the chemical methacholine. In some patients with hyperreactive airways, methacholine may cause a change in airways function which is detected by the spirometry testing. Those patients with a significant change in spirometry following inhalation of methacholine will have a positive test result. Following this test, patients are given albuterol, a drug which counteracts the effects of the methacholine. This test may be helpful in evaluating unexplained cough or possible asthma. Back To List |
| Maximal Inspiratory/Expiratory Pressures This is a test of total respiratory muscle strength. In one part of the test, the patient is asked to attempt to inhale as forcefully as possible against a blocked tube. In the other part of the test, the patient is asked to attempt to exhale as forcefully as possible against a blocked tube. In both cases, the pressure the patient generates while attempting to inhale or exhale is recorded. This pressure is correlated with the overall strength of the breathing muscles and may be helpful in assessing possible respiraotry muscle weakness. Back To List |
| Oxygen Shunt Study An oxygen shunt study estimates the portion of a patient's blood flow which does not participate in effective gas exchange within the lungs. The test involves performing an arterial blood gas analysis after the patient has breathed 100% (pure) oxygen from a large bag for a few minutes. Back To List |
Please email questions or comments to pftlab@www.med.jhu.edu. |
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Pulmonary Function Laboratory
601 N Caroline Street Suite 7012, Baltimore, Maryland 21287 USA