LCI as an endpoint in COPD: Comparison with healthy controls, using SF ₆ and nitrogen washout

Background Lung Clearance Index (LCI) is well established in cystic fibrosis (CF), but has not been widely applied in other diseases. Commercial systems are available for SF 6 & nitrogen (N 2 ) multiple breath washout (MBW). Objectives LCI, FRC and test time were evaluated in COPD patients & healthy controls (HC) using 2 different validated washout devices to assess suitability as an outcome measure in COPD. Methods MBW was performed on the same day in triplicate using an indirect N 2 MBW system (Exhalyzer D) & a modified Innocor TM SF 6 analyser, in random order. Results are expressed as mean (SD). Results 11 COPD patients (age 62 (7.4) yrs, FEV 1 80 (4.9) %predicted), and 10 HC (26.9 (6.7)yrs, 101 (13.2) %predicted) were enrolled. 1 COPD subject was unable to perform N 2 MBW due to repeated leakage, and was excluded from comparison. In patients with COPD, LCI was greater than HC values in both systems. FRC-SF 6 =3.6 (0.9)L in COPD vs 3.3 (0.9)L in HC, p=ns. FRC-N 2 =3.9 (1.1)L in COPD vs 3.3 (2.3)L in HC, p=ns. LCI-SF 6 =10.2 (1.5) in COPD vs 6.3 (0.2) in HC, p<0.0001. LCI-N 2 =11.9 (1.7) in COPD vs 7.0 (0.7) in HC, p<0.0001. LCI-N 2 was significantly greater than LCI-SF 6 in COPD (limits of agreement -0.9 to 4.2, p=0.003) & HC (-0.4 to 1.6, p=0.006). Average total COPD test time was 41.2mins for triple N 2 MBW vs 40.9mins for SF 6 (p=ns). Conclusions Similar to observations in CF, LCI measured indirectly using N 2 washout was higher than SF 6 in COPD & controls. Both systems differentiated COPD from healthy subjects but success rates were higher for SF 6 MBW. Long test times were a feature of both systems. Reducing time taken to wash tracer in is a potential method of improving SF 6 MBW.