The turbine spirometer is a popular laboratory device used to measure respiratory volumes and flows during exercise, and while performing gross, voluntary respiratory manoeuvres (i.e., inspiratory capacity efforts). Indeed, these devices have been incorporated into many commercially-available pulmonary function and metabolic systems. Yet, while the turbine spirometer may provide accurate/reliable measurements of respiratory volumes at modest-to-high flows, these devices perform poorly at low flow rates.

PURPOSE: To improve the accuracy of a turbine spirometer over an extended range of low flows using the “weighted averaging technique” described by Yeh et al. (J Appl Physiol, 53(1): p280, 1982).

METHODS: A commercially-available turbine spirometer was interfaced with a custom-designed microcontroller unit (MCU). The MCU recorded discrete rotations of the turbine rotor, and the corresponding rotational frequency (frot). Repeated 5-fold cross-validation was used to determine the optimal number of bins in frot and iterations used in the Weighted Averaging algorithm. This method yielded a discrete array of calibration constants (K) across a relevant range of frot. (<1-1800 Hz). The accuracy of this “nonlinear” calibration curve was compared to that obtained by assuming a constant K across all frequencies (i.e., flows). Over 200 calibrations strokes were recorded using a 3 L syringe.

RESULTS: By assuming a constant K (15.6 mL[BULLET OPERATOR]pulse-1), the turbine spirometer exhibited an average volume error of +94 mL (+3.1%) over a 95% confidence interval (CI95%) of -856 to +375 mL (-28.5 to 12.5%). Conversely, applying the nonlinear K curve resulted in an average volume error of <0.001 mL (<0.001%) and a CI95% ranging from -60 to +60 mL (-2.0 to 2.0%). Importantly, the nonlinear K curve provided accurate (within ±3%) volume measurements down to 0.33 Hz (~7 mL[BULLET OPERATOR]s-1).

CONCLUSIONS: The “weighted averaging technique” improved the accuracy/reliability the turbine spirometer to within ±3% across an interval of flows ranging between ~0.01 to 20 L[BULLET OPERATOR]s-1.