Quantitative analysis of the alveolar plateau in the diagnosis of early airway obstruction.

A modified single-breath nitrogen test was used in an emphysema screening center to assess the value of the analysis of the alveolar plateau in the early diagnosis of airway obstruction. Predicted values for the slope of the alveolar plateau (the increase in nitrogen concentration per 1,000 ml) were first constructed using a population of 134 nonsmoking males and 203 nonsmoking females. Data obtained from 530 cigarette smokers attending the screening center were then compared with these predicted values and with predicted values for closing volume, expressed as a percentage of vital capacity, and closing capacity, expressed as a percentage of total lung capacity, derived from virtually the same population of nonsmoking adults. Of the cigarette smokers, 47 per cent had abnormal slope of the alveolar plateau, 44 per cent had an abnormal ratio of closing capacity to total lung capacity, 35 per cent had an abnormal ratio of closing volume to vital capacity, and 11 per cent had an abnormal forced expiratory vo...