The Slow Approach to Airway Obstruction

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Stephen Biehl
Respiratory therapist specializing in lung health investigations. If not I’m the unit, find me in the PFT lab, bronch suite, rehab, or taking the stairs.

The Pre-brief

Patients present for Pulmonary Function Testing every day. The first test they must complete is spirometry, and one of the most important results from this test is the FEV1/FVC. This measurement helps identify if a person has an obstructive lung disorder, restrictive lung disorder, or is normal. The result is calculated by using the amount of air that one can forcefully exhale in the first second and divide that by the total volume of air that one can forcefully exhale. Unfortunately, due to the forced nature of this test, patients with obstructive lung disease can be under-diagnosed. But what if there was a better way?

Flow-Volume Loop during FVC

What Exactly is the Difference?

To understand better, we must first look at what classifies as obstructive lung disease. This would include airway inflammation and bronchospasms limiting the expiratory flow of air. A COPD diagnosis would result in a patient that has an FEV1/FVC below 70%. To help identify an airway obstruction, the patient needs to perform spirometry. As the patient forcefully exhales, airway collapse leads to air trapping, limiting the air that can be expelled. Performing a slow vital capacity rather than a forced vital capacity allows for complete exhalation, preventing airway collapse, leading to a larger volume being expelled. 

While the SVC and FVC both measure vital capacity, the forced test versus the slow test lead to differences. The differences observed in healthy and restrictive patients are not nearly as evident as those in obstructive patients. In healthy and restrictive patients, the difference can be minimal. In patients that have an airway obstruction, the difference in SVC as compared to FVC can be significant, increasing with the severity of the disease. This is one of the reasons that the ATS/ERS allows for the calculation using the largest VC. Using the FEV1/SVC can lead to improving the accuracy of diagnostic testing. 

Normal Spirometry Values

Several studies have looked at this with diagnosing patients with airway obstruction. In a study by Huprikar et al., they found that the number of patients diagnosed with obstruction increased from 26% to 45% when they switched from FEV1/FVC to FEV1/SVC. In a similar study by Hyatt et al., this number increased 50%. Another study by Barros showed this number increased from 44% to 52%. While a final study by Rasheed et al. found an increase from 13% to 22%. 

Not using the SVC to calculate the FEV1/VC in patients with suspected obstruction can lead to misdiagnosis. This is especially true in symptomatic patients who have a normal FEV1/FVC ratio. This leads to more reliable data in detection of obstructive pulmonary disease using the FEV1/SVC method. This method can also decrease the chance of interpretation errors of PFTs, leading to inappropriate treatment methods. 

It is essential to identify airway obstruction in patients who are symptomatic but have a normal lung function. As the disease progresses, they are at a greater risk of increased exacerbations and hospitalizations. Using the SVC instead of the FVC can help to identify these individuals to ensure they are appropriately treated earlier on in their disease in hopes of slowing the progression of the disease. 

Volume versus time with FEV1 noted

The Debrief

  • FEV1/VC is used to determine the severity of obstruction in patients
  • The FVC can cause air trapping, preventing the complete exhalation of the VC in order to calculate the FEV1/FVC properly


  1. Fortis, S., Comellas, A. P., Bhatt, S. P., Hoffman, E. A., Han, M. K., Bhakta, N. R., Ronish, B., Dransfield, M. T., Hoesterey, D., Barr, R., Buhr, R. G., Peters, S. P., Couper, D., Arjomandi, M., Drummond, M., Kaner, R. J., Kanner, R. E., Kim, V., Criner, G. J., . . . Barjaktarevic, I. (2020, May). Ratio of FEV1/Slow Vital Capacity of < 0.7 Is Associated With Clinical, Functional, and Radiologic Features of Obstructive Lung Disease in Smokers With Preserved Lung Function. ATS Journals.

    Barros, A. R., Pires, M. B., & Raposo, N. M. (2013). Importance of slow vital capacity in the detection of airway obstruction. Jornal brasileiro de pneumologia : publicacao oficial da Sociedade Brasileira de Pneumologia e Tisilogia, 39(3), 317–322.

  3. Huprikar, N. A., Skabelund, A. J., Bedsole, V. G., Sjulin, T. J., Karandikar, A. V., Aden, J. K., & Morris, M. J. (2019, July 1). Comparison of Forced and Slow Vital Capacity Maneuvers in Defining Airway Obstruction. American Association for Respiratory Care.


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