Pulmonary Function Testing has been a major step forward in assessing the functional status of the lungs as it relates to :
- How much air volume can be moved in and out of the lungs
- How fast the air in the lungs can be moved in and out
- How stiff are the lungs and chest wall - a question about compliance
- The diffusion characteristics of the membrane through which the gas moves (determined by special tests)
- How the lungs respond to chest physical therapy procedures
Pulmonary Function Tests are used for the following reasons :
- Screening for the presence of obstructive and restrictive diseases
- 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
Note : A vital capacity is an important preoperative assessment tool. Significant reductions in vital capacity (less than 20 cc/Kg of ideal body weight) indicates that the patient is at a higher risk for postoperative respiratory complications. This is because vital capacity reflects the patient's ability to take a deep breath, to cough, and to clear the airways of excess secretions.
- 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.
- Documenting the progression of pulmonary disease - restrictive or obstructive
- 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. Spirometers with electronic signal outputs (pneumotachs) also measure flow (volume per unit of time). 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 like the unit used in this laboratory. 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 maybe 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"
Over the last several decades much research has been undertaken to determine what are the normal values for lung volumes and lung capacities. This has made spirometry very useful since now we know that we can 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. Normal tables account for this variable by giving predicted PFT data for males or females of a certain age and height. Sometimes as people age they begin to increase their body mass by increasing their body fat to lean body mass ratio. If they become too obese, the abdominal mass prevents the diaphragm from descending as far as it could and the PFT results will demonstrate a smaller measured PFT outcome then expected - i.e. the observed (measured) values are actually smaller than the predicted values (predicted values from the normal tables).
- 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.
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