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
Pulmonary Function Tests are used for the following reasons:
1. Screening for the presence of obstructive and restrictive diseases
2. Evaluating the patient prior to surgery - this is especially true of patients who: a. are older than 60-65 years of age b. are known to have pulmonary disease c. are obese (as in pathologically obese) d. have a history of smoking, cough or wheezing e. will be under anesthesia for a lengthy period of time f. are undergoing an abdominal or a thoracic operation
3. Evaluating the patient's condition for weaning from a ventilator. If the patient on a ventilator can demonstrate a vital capacity (VC) of 10 - 15 ml/Kg of body weight, it is generally thought that there is enough ventilatory reserve to permit (try) weaning and extubation.
4. Documenting the progression of pulmonary disease - restrictive or obstructive
5. Documenting the effectiveness of therapeutic intervention
EQUIPMENT
The primary instrument used in pulmonary function testing is the spirometer. It is designed to measure changes in volume and can only measure lung volume compartments that exchange gas with the atmosphere. A device is usually always attached to the spirometer which measures the movement of gas in and out of the chest and is referred to as a spirograph. Sometimes the spirograph is replaced by a printer. The resulting tracing is called a spirogram. Many computerized systems have complex spirographs or printouts that show the predicted values next to the observed values (the values actually measured). The unit will have in memory all of the prediction tables for males and females across all age groups. In sophisticated spirometers, there may be special tables of normal values programmed into the machine for selection when Blacks, children or other groups are being tested who may vary from the normal PFT tables established for Caucasian adults.
NORMAL VALUES
Normal values are used to compare the patient's PFT results with those measured on thousands and thousands of "normal" adults. By having tables of normal values, it is then easy to compare the severity of the disease process or the rate of recovery taking place in the patient's lungs. There are a few variables such as age, gender, and body size which have an impact on the lung function of one individual compared to another.
• Age: As a person ages, the natural elasticity of the lungs decreases. This translates into smaller and smaller lung volumes and capacities as we age. When determining whether or not your patient has normal PFT findings, it would be important to compare the patient with the PFT results of a normal person of the same age and gender.
• Gender: Usually the lung volumes and capacities of males are larger than the lung volumes and capacities of females. Even when males and females are matched for height and weight, males have larger lungs than females. Because of this gender-dependent lung size difference, different normal tables must be used for males and females.
• Body Height & Size: Body size has a tremendous effect on PFT values. A small man will have a smaller PFT result than a man of the same age who is much larger.
• Race: Race affects PFT values. Blacks, Hispanics and Native Americans have different PFT results compared to Caucasians. Therefore, a clinician must use a race appropriate table to compare the patient's measured pulmonary function against the results of the normal table written for that patient's racial group.
• Other factors such as environmental factors and altitude may have an affect on PFT results but the degree of effect on PFT is not clearly understood at this time
TERMINOLOGY AND DEFINITIONS
FVC - Forced Vital Capacity - after the patient has taken in the deepest possible breath, this is the volume of air which can be forcibly and maximally exhaled out of the lungs until no more can be expired. FVC is usually expressed in units called liters. This PFT value is critically important in the diagnosis of obstructive and restrictive diseases.
FEV1 - Forced Expiratory Volume in One Second - this is the volume of air which can be forcibly exhaled from the lungs in the first second of a forced expiratory maneuver. It is expressed as liters. This PFT value is critically important in the diagnosis of obstructive and restrictive diseases.
FEV1/FVC - FEV1 Percent (FEV1%) - This number is the ratio of FEV1 to FVC - it indicates what percentage of the total FVC was expelled from the lungs during the first second of forced exhalation - this number is called FEV1%, %FEV1 or FEV1/FVC ratio. This PFT value is critically important in the diagnosis of obstructive and restrictive diseases.
FEV3 - Forced Expiratory Volume in Three Seconds - this is the volume of air which can be forcibly exhaled in three seconds - measured in Liters - this volume usually is fairly close to the FVC since, in the normal individual, most of the air in the lungs can be forcibly exhaled in three seconds.
FEV3/FVC - FEV3% - This number is the ratio of FEV3 to the FVC - it indicates what percentage of the total FVC was expelled during the first three seconds of forced exhalation. This is called %FEV3 or FEV3%.
PEFR - Peak Expiratory Flow Rate - this is maximum flow rate achieved by the patient during the forced vital capacity maneuver beginning after full inspiration and starting and ending with maximal expiration - it can either be measured in L/sec or L/min - this is a useful measure to see if the treatment is improving obstructive diseases like bronchoconstriction secondary to asthma.
FEF - Forced Expiratory Flow - Forced expiratory Flow is a measure of how much air can be expired from the lungs. It is a flow rate measurement. It is measured as liters/second or liters/minute. The FVC expiratory curve is divided into quartiles and therefore there is a FEF that exists for each quartile. The quartiles are expressed as FEF25%, FEF50%, and FEF75% of FVC.
FEF25% - This measurement describes the amount of air that was forcibly expelled in the first 25% of the total forced vital capacity test.
FEF50% - This measurement describes the amount of air expelled from the lungs during the first half (50%) of the forced vital capacity test. This test is useful when looking for obstructive disease. The amount of air that will have been expired in an obstructed patient is smaller than that measured in a normal patient.
FEF25%-75% - This measurement describes the amount of air expelled from the lungs during the middle half of the forced vital capacity test. Many physicians like to look at this value because it is an indicator of obstructive disease.
MVV - Maximal Voluntary Ventilation - this value is determined by having the patient breathe in and out as rapidly and fully as possible for 12 -15 seconds - the total volume of air moved during the test can be expressed as L/sec or L/min - this test parameter reflects the status of the respiratory muscles, compliance of the thorax-lung complex, and airway resistance. Surgeons like this test value because it is a quick and easy way to assess the strength of the patient's pulmonary musculature prior to surgery - a poor performance on this test suggests that the patient may have pulmonary problems postoperatively due to muscle weakness. MVV can therefore be viewed as a measure of respiratory muscle strength. One major cautionary note is that this test is effort dependent and therefore can be a poor predictor of true pulmonary strength and compliance.
OBSTRUCTION & RESTRICTION
Pulmonary function abnormalities can be grouped into two main categories : obstructive and restrictive defects. This grouping of defects is based on the fact that the routine spirogram measures two basic components - air flow and volume of air out of the lungs. Generally the idea is that if flow is impeded, the defect is obstructive and if volume is reduced, a restrictive defect may be the reason for the pulmonary disorder.
OBSTRUCTED AIRFLOW
The patency (dilatation or openness) is estimated by measuring the flow of air as the patient exhales as hard and as fast as possible. Flow through the tubular passageways of the lung can be reduced for a number of reasons:
• narrowing of the airways due to bronchial smooth muscle contraction as is the case in asthma
• narrowing of the airways due to inflammation and swelling of bronchial mucosa and the hypertrophy and hyperplasia of bronchial glands as is the case in bronchitis
• material inside the bronchial passageways physically obstructing the flow of air as is the case in excessive mucus plugging, inhalation of foreign objects or the presence of pushing and invasive tumors • destruction of lung tissue with the loss of elasticity and hence the loss of the external support of the airways as is the case in emphysema
• external compression of the airways by tumors and trauma
RESTRICTED AIRFLOW
"Restriction" in lung disorders always means a decrease in lung volumes. This term can be applied with confidence to patients whose total lung capacity has been measured and found to be significantly reduced. Total lung capacity is the volume of air in the lungs when the patient has taken a full inspiration. You cannot measure TLC by spirometry because air remains in the lungs at the end of a maximal exhalation - i.e. the residual volume or RV. The TLC is therefore the summation of FVC + RV. There are a variety of restrictive disorders which are as follow:
A. Intrinsic Restrictive Lung Disorders
1. Sarcoidosis
2. Tuberculosis
3. Pneumonectomy (loss of lung)
4. Pneumonia
B. Extrinsic Restrictive Lung Disorders
1. Scoliosis, Kyphosis
2. Ankylosing Spondylitis
3. Pleural Effusion (fluid in the pleural cavity)
4. Pregnancy
5. Gross Obesity
6. Tumors
7. Ascites
8. Pain on inspiration - pleurisy, rib fractures
C. Neuromuscular Restrictive Lung Disorders
1. Generalized Weakness - malnutrition
2. Paralysis of the diaphragm
3. Myasthenia Gravis - lack of acetylcholine or too much cholinesterase at the myoneural junction in which the nerve impulses fail to induce normal muscular contraction. These patients suffer from fatigability and muscular weakness.
4. Muscular Dystrophy
5. Poliomyelitis
6. Amyotrophic Lateral Sclerosis - Lou Greig’s Disease
INTERPRETING THE RESULTS OF A PFT
There are a number of systems which physicians use to determine the severity of disease. Here is just one way that is very commonly used:
• Normal PFT Outcomes - > 85 % of predicted values
• Mild Disease - > 65 % but < 85 % of predicted values
• Moderate Disease - > 50 % but < 65 % of predicted values
• Severe Disease - < 50 % of predicted values
In most good spirometers on the market today, there is a set of normal tables (sometimes multiple sets of tables) which can be chosen as you perform the PFT. Also, there are interpretive microchips in the PFT machines which will tell you what the diagnosis is for a particular patient. These two features make it easy for the clinician to immediately see what the predicted values (normal table values) are for a specific patient and whether or not the PFT has a normal observed outcome. The PFT data are examined by the computerized spirometer and a diagnosis of obstructive or restrictive disease is made.
THE PFT BEFORE AND AFTER AEROSOL BRONCHODILATORS
Patients are almost always tested twice - once before bronchodilators are given and once after one is administered. This is a nice way to evaluate the amount of bronchoconstriction that was present and how responsive the patient was to a bronchodilator medication. This assesses the degree of reversibility of the airway obstruction. The drug that is nearly always used is a Beta-2 selective sympathomimetic because it is a drug that causes bronchodilation but which does not stimulate the heart to any great degree. After the drug has been administered, the PFT is repeated. If two out of three measurements (FVC, FEV1 and FEF25% - 75%) improve, then it can be said that the patient has a reversible airway obstruction that is responsive to medication. The amount of improvement is variable between clinics but some standards are presented below:
1. FVC : an increase of 10% or more
2. FEV1 : an increase of 200 ml or 15% of the baseline FEV1
3. FEF25%-75% : an increase of 20% or more
PULMONARY FUNCTION TESTS - A SYSTEMATIC WAY TO DIAGNOSIS
There is a systematic way to read the PFT and be able to evaluate it for the presence of obstructive or restrictive disease. The following steps will be helpful.
1. Step 1. Look at the forced vital capacity (FVC) to see if it is within normal limits.
2. Step 2. Look at the forced expiratory volume in one second (FEV1) and determine if it is within normal limits.
3. Step 3. If both FVC and FEV1 are normal, then you do not have to go any further - the patient has a normal PFT test.
4. Step 4. If FVC and/or FEV1 are low, then the presence of disease is highly likely.
5. Step 5. If Step 4 indicates that there is disease, then you need to go to the %predicted for FEV1/FVC. If the %predicted for FEV1/FVC is 88%-90% or higher, then the patient has a restricted lung disease. If the %predicted for FEV1/FVC is 69% or lower, then the patient has an obstructed lung disease.
An article prepared by 👇👇👇
Locika Karunarajah
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