[A New Method of Analyzing the Closing Volume (CV) Curve: "N2 First Derivative Wave Method"]

Evaluation of the lung function involves the measurement of many factors. The closing volume (CV) curve is clinically important as an index of uneven alveolar ventilation and airway closure. Although conventional methods for CV measurement are usually based on the pattern of the exhaled nitrogen (N2) concentration curve with respect to the lung volume, it is often difficult to measure the steep pattern of patients with chronic obstructive pulmonary disease (COPD). In this paper, we proposed a new method called the "N2 first derivative (fdN) wave method" for measuring CV. The N2 concentration of the CV curve was transformed to a derivative with respect to the lung volume, which revealed the existence of cardiogenic oscillations. Discrimination between phases III and IV was straightforward based on the difference in the slope or in the amplitude of oscillations of the fdN wave. Our new method was able to distinguish phase IV from phase III using the difference in amplitude of the oscillation of the fdN wave even in the presence of COPD with steep patterns of the CV curve. Close relationships were seen among normal subjects including COPD patients in both the slope of the alveolar plateau (ΔN2) and the CV values measured with the conventional and new methods. In conclusion, the new method we propose in this paper was able to provide measurements of CV for all subjects including those with COPD. [Original]