Question
How is reversible airflow obstruction determined in patients using bronchodilator studies?
Answer
Spirometry before and after bronchodilator is the preferred method for diagnosing asthma because it offers higher precision and can be calibrated. Additionally, using predicted values allows for a more accurate assessment. Analyzing the flow volume loops helps evaluate patient effort and bronchospasm reversibility, which is essential in determining the presence of airway obstruction. The ratio of Forced Expiratory Volume in one second (FEV1) to Forced Vital Capacity (FVC) is also examined during this test. Peak expiratory flow monitoring is another method used to assess airflow limitation. However, spirometry is generally considered more reliable due to its precision and ability to provide more comprehensive data. By utilizing these diagnostic methods, healthcare professionals can effectively evaluate and diagnose asthma, enabling the implementation of appropriate treatment plans to manage the condition and improve patients' respiratory health.
In adult patients, it is essential to differentiate pure asthma from asthma and COPD overlap, especially in chronic smokers. For non-smoking adults without a high suspicion of emphysema or chronic bronchitis, we can assess if their airway obstruction is reversible using a bronchodilator. To determine the need for bronchodilator studies, we examine the FEV1 to FVC percent ratio, which measures the amount of air a person can exhale in the first second (FEV1) compared to the total amount of air exhaled (FVC). A normal FEV1/FVC ratio is around 75 to 80%. If this ratio is less than predicted, it indicates possible airflow obstruction, which could result from inflammation, excess mucus, or bronchospasm. To confirm the presence of reversible airflow obstruction, we administer a bronchodilator, such as albuterol, and wait for 15 minutes, the peak onset of action for short-acting beta agonists like albuterol. Waiting for the peak effect ensures precise and valid measurement of lung function.
An anecdotal example highlights the importance of waiting for the peak effect of the bronchodilator. In a case involving my three-and-a-half-year-old son, the nurse practitioner administered a Xopenex nebulizer treatment and assessed his lung sounds immediately after the treatment. However, my son's airways were still constricted at that moment, and the wheezing only became audible 10 minutes later when the bronchodilator's peak effect took effect. In summary, accurately evaluating reversible airflow obstruction using spirometry and bronchodilator studies aids in diagnosing asthma and providing appropriate management for patients, especially those without underlying COPD components or chronic smoking history. I posed the question to my students, "What happened?" The reason for the nurse practitioner's failure to hear anything, not to criticize her profession, but to inquire, was due to her lack of waiting for the peak effect of the short-acting beta agonist. Consequently, my son's airways were so constricted that he could not breathe enough air to produce a wheeze until the bronchodilator took effect.
The 15-minute wait time after administering the bronchodilator is crucial to observe its peak effect. According to the ATS criteria, significant reversibility of airway obstruction is determined by increases greater than 12% and 200 milliliters in either FVC or FEV1. In the provided spirometry report, we focus on the FVC and FEV1 values. The FEV1 shows a change of more than 12%, meeting the criterion. However, the FVC doesn't meet the 12% change requirement. To check for the 200-milliliter increase, we compare the pre-measured value of 1.34 liters with the post-measured value of 1.57 liters. The difference is indeed greater than 200 milliliters, indicating a significant bronchodilator response in this patient. Therefore, the pulmonary doctor would include in the interpretation that the patient exhibited a significant response to the bronchodilator. To back up a minute, how do we know that they have airway obstruction? Well, their predicted ratio of FEV1/FVC is 78, and this patient's measured is 63. In summary, they definitely have airway obstruction.
This Ask the Expert is an edited excerpt from the course, Objective Measures of Asthma, presented by Kevin Collins, PhD, RRT, RPFT, AE-C.
