Smallest detectable change in volume differs between mass flow sensor and pneumotachograph

To assess a pulmonary function change over time the mass flow sensor and the pneumotachograph are widely used in commercially available instruments. However, the smallest detectable change for both devices has never been compared. Therefore, the aim of this study is to determine the smallest detectable change in vital capacity (VC) and single-breath diffusion parameters measured by mass flow sensor and or pneumotachograph.
Method
In 28 healthy pulmonary function technicians VC, transfer factor for carbon monoxide (DLCO) and alveolar volume (VA) was repeatedly (10×) measured. The smallest detectable change was calculated by 1.96 x Standard Error of Measurement ×√2.
Findings
The mean (range) of the smallest detectable change measured by mass flow sensor and pneumotachograph respectively, were for VC (in Liter): 0.53 (0.46-0.65); 0.25 (0.17-0.36) (p = 0.04), DLCO (in mmol*kPa-1*min-1): 1.53 (1.26-1.7); 1.18 (0.84-1.39) (p = 0.07), VA (in Liter): 0.66. (0.53-0.82); 0.43 (0.34-0.53) (p = 0.04) and DLCO/VA (in mmol*kPa-1*min-1*L-1): 0.22 (0.19-0.28); 0.19 (0.14-0.22) (p = 0.79).
Conclusions
Smallest detectable significant change in VC and VA as measured by pneumotachograph are smaller than by mass flow sensor. Therefore, the pneumotachograph is the preferred instrument to estimate lung volume change over time in individual patients.
Background
To measure pulmonary function changes over time the mass flow sensor and the pneumotachograph are widely used instruments. Due to international equipment requirements, calibration, validation and measurement procedures both measurement devices are assumed to have identical reliability. However the smallest detectable change, which is the smallest significant change that can be detected between individual measurements, has in neither device, been determined. The smallest detectable change is a very useful parameter for clinical practice because it shows which changes in a single patient can be considered a 'real' change. Hence, pulmonary function instruments with the smallest detectable change are best suited for evaluating changes as a result of disease progress or applied therapy.
The aim of this study is to determine the smallest detectable change of vital capacity (VC) and single-breath diffusion parameters measured by mass flow sensor and pneumotachograph.

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