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 e quipment in clinical use. The aims of this study were to examine its r esistance characteristics and to determine the effect of the filter on clinical measurements of lung function. Measurements: Twenty-one rand omly selected patients and four normal subjects had lung function meas ured with and without the filter between the mouth and measuring equip ment. Measurements of ventilatory function were made with a pneumotach ograph (Lilly; Hoechberg, Germany), total lung capacity and airway res istance by constant volume plethysmography, and diffusing capacity for carbon monoxide by the single breath method. Resistance was determine d in five unused filters over the flow range 1 to 12 L/s and at a sing le now rate (12 L/s) just after a normal subject expired 20 forced vit al capacity (FVC) breaths through each of them. Results: The resistanc e (mean+/-SD) of unused filters was 0.19+/-0.02 cm H2O/L/s at 1 L/s an d 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 fil ter to the breathing circuit caused statistically significant decrease s in forced expiratory volume in 1 s (FEV(1)) (0.044+/-0.08 L, p=0.014 ) and peak expiratory now 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 FEV(1) and PEF R; however, this difference was believed not to affect the clinical ut ility of routine lung function testing.