Airway resistance by the interrupter technique: which algorithm for measuring pressure?

Airway resistance using the interrupter technique (Rint) can be measured using commercial devices which employ different algorithms for estimating pressure change. We aim to describe differences in Rint due to algorithm. We compared Rint and change in Rint after bronchodilator, using four algorithms to estimate pressure change following interruption: 1) two-point back-extrapolation to interruption from points 70 msec and 30 msec from interruption, and similarly 2) to 15 msec from interruption, 3) at two-thirds from interruption, and 4) near end-interruption. Flow was measured immediately before interruption. Our subjects were 39 asymptomatic children 2-5 years old with previous intermittent wheeze. Rint differed significantly with algorithm. Geometric mean Rint (95% confidence interval (CI)) for algorithms 1-4 were 1.21 kPa x l(-1) x sec (1.18-1.24 kPa x l(-1) x sec), 1.31 kPa x l(-1) x sec (1.28-1.34 kPa x l(-1) x sec), 1.57 kPa x l(-1) x sec (1.54-1.61 kPa x l(-1) x sec) and 1.71 kPa x l(-1) x sec (1.67-1.75 kPa x l(-1) x sec), respectively. Measurement of change in R(int) following bronchodilator (BDR) did not differ on average with algorithm. Geometric means (95% CI) for BDR measurements for algorithms 1-4 were 29.9% (26.0-34.0%), 30.4% (26.4-34.5%), 32.9% (28.8-37.1%), and 31.7% (27.6-35.8%), respectively. However, measurement of change in individuals could differ by up to 40%, depending on algorithm. In conclusion, there are significant differences in Rint, depending on algorithm used to estimate pressure change. Measurement of change in Rint is unaffected on average, although in individuals there could be significant differences. Each laboratory should state its method and use the same algorithm for longitudinal and group data.