Pulmonary diseases can be very sneaky, developing subtly over time with very little outward signs or symptoms. When these diseases are underrecognized and undertreated it can lead to increased damage to the lungs, faster progression of the disease and worsening quality of life. Luckily we can catch them early with Pulmonary Function Tests.
Pulmonary function tests are a series of noninvasive tests that tell you how well a patient’s lungs are working. They measure the lungs’ volumes, capacities, rates of flow and how well they exchange gases. Respiratory Therapists will coach patient’s through the performance of these tests either in a PFT lab or at the bedside. Coaching the patient, while vitally important, seems to come easily to RT students. Interpretation, on the other hand, can seem confusing. This article aims to help demystify PFT interpretation with a 4-step approach to understanding test results.
It helps to have an understanding of what each of the tests within a PFT is actually measuring and how those results are valuable. Below we will look at the ABCs of PFTs. What do all those acronyms mean?
- FVC – Forced Vital Capacity – The maximal amount of air that can be exhaled as fast as possible after a maximal inhalation. The result of this test is then compared to a predicted normal based on age, height, and weight, ethnic origin & smoking history.. A decreased FVC can signal both an obstructive lung disease or a restrictive lung disease. To determine which is present the first second of the test is evaluated and reported as FEV1.
- FEV1 – Forced Expiratory Volume in 1 second – the volume of air that was exhaled in the first second of the FVC. This volume is reported as a percentage of the total volume exhaled. When the FVC is greater than 80% predicted and the FEV1 is less than 80% of the total volume exhaled, an obstructive lung disease is present. (FEV1/FVC < 80% predicted). If BOTH the FVC and the FEV1 are less than 80% predicted there is a restrictive disease present. FEV1 is also used to determine the severity of disease – more on that later.
- MVV – the largest amount of air that a person can inhale and then exhale during a 12- to 15-s interval with maximal voluntary effort. The test is not easy to do and is heavily dependent on the patient’s effort and understanding of the instructions
so the value of it is questionable. Like the other tests, there is a “normal” value predicted as 30-40 times the FEV1. It can be decreased when respiratory muscle weakness is present and in obstructive lung disease. Looked at in conjunction with the other tests it does seem to correlate with exercise capacity and subjective dyspnea.
- DLCO2 – Diffusing Capacity of Lungs for Carbon Monoxide – is a measurement of the ease of transfer for CO molecules from alveolar gas to the hemoglobin of the red blood cells in the pulmonary circulation. Lung diseases can affect the ability of gases to diffuse due to damage to the alveolar-capillary membrane or alveolar damage which has led to less surface area accessible for diffusion. The severity of the diffusion disorder is directly related to the results compared to the predicted percentage.
Now that you have a basic understanding of all the tests involved:
Four Quick Steps to PFT Interpretation
Step 1: Determining Obstructive Disease
Look at the FEV1/FVC result. If it is <80% of predicted yo
u can determine your patient is likely to haveOBSTRUCTIVE Disease. The % FEV1 will help to determine the Severity:
- 70 – 79% – Mild
- 60 – 69% – Moderate
- 50 – 59% – Moderately Severe
- 35 – 49% – Severe
- <35% – Very Severe
Because Asthma is identified as both Obstructive and Restrictive it is important that the patient take a bronchodilator treatment and repeat the test. If the FEV1 increases more than 12-15% it indicates that the obstruction is reversible and Asthma is present. If the patient has COPD there will be no change post treatment.
Step 2: Determining Restrictive Disease
If the FEV1/FVC result is > 80% predicted look at each measurement individually. If the FVC and the FEV1 results are both <80% predicted you can determine your patient is likely to have RESTRICTIVE Disease. The % FEV1 (seen above) will help to determine the Severity.
Step 3: Identifying a Mixed Defect
If the FEV1/FVC and the FVC are both DECREASED below 80% predicted there is a MIXED defect.
Step 4: Identifying the severity of GAS EXCHANGE DISORDER
Look at the DLCO2 result as a percentage compared to predicted to determine the severity of diffusion disorder:
- 60 – 79% – Mild
- 40 – 60% – Moderate
- < 40% – Severe
The goal of PFTs is to answer the following questions:
Is lung disease present?
Is it Obstructive or Restrictive lung disease – or both?
Is it Mild, Moderate, Severe, or Very Severe?
Is it Reversible?
Is there a gas diffusion defect?
Does it require bronchodilators? Airway Clearance interventions? Pulmonary Rehab?
Using this 4-step approach to basic PFT interpretation along with reviewing the patient’s respiratory history and physical, chest x-rays and arterial blood gas results will get you on your way to identifying disease processes and making recommendations for therapeutic interventions.
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