A new method for analyzing multiple-breath nitrogen washout curves

Multiple-breath nitrogen washout (MBNW) curves from 39 normal and 45 patients with varying degrees of airway obstruction were analyzed with the following equation: log log [(F0N2 - F infinity N2)/(FN2 - F infinity N2)] = B log (V/FRC) +A, where F0N2 is the initial, F infinity N2 is the equilibrium, and FN2 is the end-tidal fractional nitrogen concentration after a volume (V) of air in liters have been exchanged, and FRC is the measured functional residual capacity of the individual. All 84 curves were well described with the equation when all points corresponding to FN2 below 0.40 were used. The coefficient of determination (r2) exceeded 0.990 in 56/84 (67%) of the transformed curves and the lowest coefficient of determination was 0.946. This suggests that the MBNW can be described by two numbers, B called the index of uniformity, and A the index of efficiency. The ability of these two indices and the lung clearance index (LCI) to identify individuals as abnormal and to separate groups with varying degrees of airway obstruction was compared. B identified more individuals (33/45) as abnormal than did A(15/45) or LCI (19/45). B was also superior to A or LCI in separating groups with varying degrees of airway obstruction. This analysis allows the complex MBNW curve to be described by two numbers that vary depending on the degree of airway obstruction.