Comparison of Methods of Eliciting Vital Capacity: Forced Versus Slow Vital Capacity.

Background: Methods to elicit the vital capacity (VC) include forced vital capacity (FVC) and slow vital capacity (SVC). Because the FVC maneuver can be affected by air trapping or inefficiencies in lung emptying vs. the SVC, the SVC-FVC difference may be substantial and diagnostically meaningful in elderly individuals and patients with respiratory obstruction. However, the degree of SVC-FVC difference in non-respiratory disordered individuals is unclear - some studies indicate a negligible difference, and other studies/guidelines suggest a mean difference in this target group up to 200 mL. This study examined the relationship between FVC vs. SVCs in a group of typical young adult speakers with three forms of measurement device (pneumotach-based; turbine flow meter; vortex whistle).

Methods: FVC and SVCs were collected from 66 non-respiratory disordered subjects (18-30 years) using three methods: a pneumotach system (KoKo Sx1000); a turbine flow spirometer (Micro Spirometer); and 3-D printed vortex whistles and analysis software. FVC and SVC measures were compared and correlated.

Results: The upper limit of the 95% CIs for the mean SVC-FVC difference were observed to be < 60-115 mL, depending upon the measurement system used. However, mean SVC-FVC differences were nonsignificant in all methods. Strong correlations between FVC vs. SVC were observed within each device (r's > 0.95), and both FVC and SVC measurements were highly correlated between devices (r's > 0.90).

Conclusion: Mean SVC tends to be slightly greater than mean FVC, though this difference is nonsignificant in typical young adult subjects, making VC measures from SVC vs. FVC essentially interchangeable in this target group. Any SVC vs. FVC differences are detectable via both pneumotach-based spirometry and lower-cost technologies. The SVC method has several benefits vs. FVC for speech-language pathologists who may be conducting VC testing with a large variety of patient groups in diverse (often non-medical) settings.