Effect of a microaerosol barrier filter on the measurement of lung function.

STUDY OBJECTIVE A disposable barrier filter (Pall Biomedical, United Kingdom) was developed to prevent the contamination of lung function equipment in clinical use. The aims of this study were to examine its resistance characteristics and to determine the effect of the filter on clinical measurements of lung function. MEASUREMENTS Twenty-one randomly selected patients and four normal subjects had lung function measured with and without the filter between the mouth and measuring equipment. Measurements of ventilatory function were made with a pneumotachograph (Lilly; Hoechberg, Germany), total lung capacity and airway resistance by constant volume plethysmography, and diffusing capacity for carbon monoxide by the single breath method. Resistance was determined in five unused filters over the flow range 1 to 12 L/s and at a single flow rate (12 L/s) just after a normal subject expired 20 forced vital capacity (FVC) breaths through each of them. RESULTS The resistance (mean +/- SD) of unused filters was 0.19 +/- 0.02 cm H2O/L/s at 1 L/s and increased linearly to 0.56 +/- 0.02 cm H2O/L/s at 12 L/s. There was no significant increase in resistance after use. The addition of the filter to the breathing circuit caused statistically significant decreases in forced expiratory volume in 1 s (FEV1) (0.044 +/- 0.08 L, p = 0.014) and peak expiratory flow rate (PEFR) (0.47 +/- 0.073 L/s, p = 0.004). The filter did not affect other indices of lung function. CONCLUSION The filter caused a statistically significant reduction in FEV1 and PEFR; however, this difference was believed not to affect the clinical utility of routine lung function testing.