In an article published in the JBP, Fernandez et al.(1) evaluated 187 patients referred for pulmonary function testing and concluded that a difference between slow VC (SVC) and FVC (∆SVC−FVC) > 0.20 L is useful for defining airflow limitation (AFL) in patients with normal test results and for reducing the number of cases designated as nonspecific (i.e., cases in which there is a proportional reduction in FVC and FEV1). In 82 of those patients, AFL had already been characterized by forced spirometry.(1)

The value of 0.20 L was suggested in the Brazilian Thoracic Association 2002 Guidelines for Pulmonary Function Testing.(2) In 2019, Saint-Pierre et al.(3) evaluated functional test results of 13,893 individuals and reported that a preserved FEV1/FVC ratio with a reduced FEV1/VC ratio was observed in 20.4% of cases. The low predicted value used in that study to characterize the lower limit for the FEV1/FVC ratio greatly decreases the sensitivity of this parameter for detecting AFL.

Several considerations should be made regarding ∆SVC−FVC. The higher degree of alveolar gas compression during a forced maneuver results from several factors, including obstructive disease, greater muscle effort, and higher thoracic gas volume to be compressed. Since gas compression is based on muscle effort during a maximal expiratory maneuver, some degree of compression can be found in all normal individuals. Soares et al.(4) measured lung volumes in a sample of 244 normal individuals in Brazil. In that sample, 10% had a ∆SVC−FVC > 0.20 L. In comparison with the remaining individuals in the sample, those 10% were found to be more frequently male, to be taller, and to have higher FVC, a characteristic profile of what is called the normal variant, given the increased expiratory effort generated in men with larger lungs.

Similarly to Fernandez et al.,(1) most authors use percentages of the predicted values for SVC, assuming that these values are equal to those derived for FVC. They are not. Kubota et al.(5) evaluated pulmonary function in 20,341 normal individuals in Japan and found that, due to the expected loss of elastic recoil, the ∆SVC−FVC increases with age, although a greater difference was also found in certain younger individuals. It is not surprising that Saint-Pierre et al.(3) acknowledge that, in the elderly, in whom the prevalence of COPD is higher, the FEV1/SVC ratio should not be considered.

In the study by Fernandez et al.,(1) half of the sample consisted of obese individuals. A ∆SVC−FVC > 0.20 L was significantly associated with a body mass index > 30 kg/m². Likewise, Saint-Pierre et al.(3) observed lower values for the FEV1/SVC ratio versus the FEV1/FVC ratio in obese individuals. Reference values do not generally include obese individuals, and, therefore, ∆SVC−FVC data in obese individuals without cardiopulmonary disease are not available for large samples. Campos et al.(6) evaluated 24 individuals before and after bariatric surgery and showed that ∆SVC−FVC dropped from 0.21 L to 0.080 L after the intervention. Obese individuals have lower FVC relative to SVC. Many obese individuals have the so-called nonspecific pattern, and caution should be exercised in assuming that the measurement of SVC alone solves the problem, without those of TLC and RV. An SVC within the predicted range does not exclude the possibility of a reduced TLC if an appropriate percent prediction equation is used.

In the study by Fernandez et al.,(1) the values for specific airway conductance and for RV did not differ between the groups with and without a ∆SVC−FVC > 0.20 L. These data are surprising, since consistency with other data indicative of obstruction should have been observed. Finally, mid and end-expiratory flows, which could detect obstruction in the presence of an FEV1/FVC ratio within the predicted range, were not reported.

Patients with obstructive disease more frequently have a ∆SVC−FVC > 0.20 L when compared with normal individuals-20% of 190 cases evaluated at the Centro Diagnóstico Brasil compared with 10% of normal individuals (unpublished data)-however, that difference in patients with mild airflow limitation was similar to that observed in normal individuals.

We consider it unwise, in patients whose forced spirometry parameters, including end-expiratory flows, are within the normal range, to characterize the presence of AFL solely on the basis of a ∆SVC−FVC > 0.20 L, or even > 0.25 L, as suggested by Saint-Pierre et al.(3) Obtaining acceptable and reproducible values for measurement of SVC is time consuming. This time-consuming factor does not justify the routine use of measuring SVC in high-volume laboratories, given the uncertain meaning of this measurement compared with that of forced spirometry.

REFERENCES

1. Fernandez JJ, Castellano MVCO, Vianna FAF, Nacif SR, Rodrigues Junior R, Rodrigues SCS. Clinical and functional correlations of the difference between slow vital capacity and FVC. J Bras Pneumol. 2019;46(1):e20180328. https://doi.org/10.1590/1806-3713/e20180328
2. Pereira CA. Espirometria. In: Sociedade Brasileira de Pneumologia e Tisiologia. Diretrizes para Testes de Função Pulmonar. J Pneumol. 2002;28(Suppl 3):S1-S82.
3. Saint-Pierre M, Ladha J, Berton DC, Reimao G, Castelli G, Marillier M, et al. Is the Slow Vital Capacity Clinically Useful to Uncover Airflow Limitation in Subjects With Preserved FEV1/FVC Ratio?. Chest. 2019;156(3):497-506. https://doi.org/10.1016/j.chest.2019.02.001
4. Soares MR, Pereira CAC, Lessa T, Guimarães VP, Matos RL, Rassi RH. Diferença entre CV lenta e forçada e VEF1/CV e VEF1/CVF em adultos brancos normais em uma amostra da população brasileira. In: Proceedings of the 18th Congresso Paulista de Pneumologia e Tisiologia; 2019 Nov 20-23; São Paulo. Pneumol Paulista. Supplementary edition: P35.
5. Kubota M, Kobayashi H, Quanjer PH, Omori H, Tatsumi K, Kanazawa M, et al. Reference values for spirometry, including vital capacity, in Japanese adults calculated with the LMS method and compared with previous values. Respir Investig. 2014;52(4):242-250. https://doi.org/10.1016/j.resinv.2014.03.003
6. Campos EC, Peixoto-Souza FS, Alves VC, Basso-Vanelli R, Barbalho-Moulim M, Lauri-no-Neto RM. Improvement in lung function and functional capacity in morbidly obese women subjected to bariatric surgery. Clinics (Sao Paulo). 2018;73:e20. https://doi.org/10.6061/clinics/2018/e20