In the article by Fernandez et al., a diagnosis of obstructive lung disease (OLD) was made if there was a reduced FEV1/slow VC (SVC) ratio and/or a reduced FEV1/FVC ratio.(1) The finding of a ∆SVC−FVC ≥ 200 mL alone did not define OLD. As the correspondence points out, in 82 of 187 cases, the FEV1/FVC ratio had already revealed OLD; however, in 46 (25%) of those 187 cases, there was disagreement between diagnoses, a finding that is similar to that of Saint-Pierre et al.(1,2) In 21 of 73 cases with normal spirometry and in 15 of 32 cases considered nonspecific based on the analysis of forced expiratory maneuver parameters, obstruction was revealed only by a reduced FEV1/SVC ratio.(1)

Kubota et al.(3) evaluated normal individuals and found a greater ∆SVC−FVC in the elderly, probably because of air trapping or heterogeneous lung emptying in the forced expiratory maneuver because of the loss of elastic recoil; that difference was less pronounced in young individuals. Therefore, the authors suggested that "reference values for SVC would be preferable for the interpretation of pulmonary function in the elderly". (3) Pistelli et al.(4) also calculated predicted values for SVC, finding a difference of only 50 mL between the mean values for SVC and FVC; however, the age group studied (8-64) was younger than that in the study by Kubota et al. (17-95 years).(3,4) There are no spirometry predicted values for SVC in Brazilians.

In our study, the values for specific airway conductance did not differ between those with and those without a ∆SVC−FVC ≥ 200 mL, a finding that may be attributable to the characteristics of the sample, which also included individuals with interstitial lung diseases (such as sarcoidosis, hypersensitivity pneumonitis, and fibrosis with emphysema), in whom changes in volume and flow can be masked by the balance of interstitial and airway involvement. In addition, reductions in FEV1% and FEV1/(F)VC, OLD, increased functional residual capacity, and reduced inspiratory capacity/TLC (i.e., findings of airflow limitation and air trapping) were predictors of a ∆SVC−FVC ≥ 200 mL.

∆SVC−FVC correlates positively with body mass index, and analysis of FEV1/SVC may increase the prevalence of the diagnosis of OLD. In general, functional residual capacity and expiratory reserve volume are the volumes most affected in obese individuals, and impairment of TLC is less pronounced. In individual cases, especially if there is dissonance with the clinical findings, plethysmography is essential for assessing the mechanisms underlying the reduction in (F)VC and FEV1.

The finding of reduced end-expiratory flows alone (similarly to that of ∆SVC−FVC ≥ 200 mL alone) should be supported by other functional test results in order to confirm OLD. In the study by Saint-Pierre et al.,(2) discordant cases (i.e., normal FEV1/FVC, but reduced FEV1/SVC) had lower FEF25-75% values.

Determination of the ∆SVC−FVC provides an additional piece of information, since, although that difference can occur in healthy individuals due to dynamic compression of the airways (young individuals) or loss of elastic recoil (elderly individuals), it can also be due to airflow limitation. Recommendations by the American Thoracic Society continue to support the use of the highest VC value as the denominator of the FEV1/(F)VC ratio.(5)

In the Pulmonary Function Laboratory of the Instituto de Assistência ao Servidor Público Estadual de São Paulo (São Paulo Institute for the Medical Care of State Civil Servants), we perform approximately 800 tests/month. We serve a wide age group with a wide variety and great complexity of diseases. The SVC maneuver is performed without disrupting the laboratory's routine, and the analysis of its parameters additionally provides information about bronchodilator response.

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. 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
3. 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
4. Pistelli F, Bottai M, Viegi G, Di Pede F, Carrozzi L, Baldacci S, et al. Smooth reference equations for slow vital capacity and flow-volume curve indexes. Am J Respir Crit Care Med. 2000;161(3 Pt 1):899-905. https://doi.org/10.1164/ajrccm.161.3.9906006
5. Culver BH, Graham BL, Coates AL, Wanger J, Berry CE, Clarke PK, et al. Recommendations for a Standardized Pulmonary Function Report. An Official American Thoracic Society Technical Statement. Am J Respir Crit Care Med. 2017;196(11):1463-1472. doi:10.1164/rccm.201710-1981ST