Using the interrupter technique to evaluate airway resistance in cystic fibrosis patients.

OBJECTIVE To measure airway resistance with the interrupter resistance (Rint) technique in patients with cystic fibrosis (CF) and to determine whether Rint values correlate with spirometric parameters, as well as to evaluate the accuracy of the Rint technique in determining the airway response to a bronchodilator. METHODS This was a cross-sectional study involving 38 children and adolescents with CF followed at the Cystic Fibrosis Outpatient Clinic of the São Lucas Hospital, located in the city of Porto Alegre, Brazil. After Rint had been measured, the subjects underwent spirometry. To evaluate bronchodilator response, we repeated the measurements after the use of inhaled albuterol. RESULTS There was a strong correlation between inverse Rint and FEV1 (r = 0.8; p < 0.001), and there was a moderate correlation between inverse Rint and FEF25-75% (r = 0.74; p < 0.001), as well as between inverse Rint and body mass index (r = 0.62; p < 0.001). We used a ROC curve to compare the bronchodilator response, as determined by Rint, with spirometric values. For a Rint cut-off point of -28%, the area under the curve was 0.75, with a sensitivity of 66% and a specificity of 82%. CONCLUSIONS Our findings suggest that Rint correlates well with spirometric parameters, although the Rint technique was not sufficiently accurate to replace spirometry in the evaluation of bronchodilator responses.