Maximum flow-volume (M.F.-V.) curves for both inspiration and expiration have been obtained in healthy subjects and in patients with bullous emphysema, exacerbation of asthma, and with severe fibrosis of the lungs. The tracheobronchial collapse pattern on the conventional spirogram or the M.F.-V. curve appeared to be related to the severity of airways obstruction more than to the type of airways obstruction. The pattern was observed in exacerbation of asthma as well as in emphysema and occurred when forced expirations were started from low in the vital capacity in normal subjects. The expiratory M.F.-V. slope was normal or steeper than normal in fibrosis and was much lower than normal in asthma and emphysema. In patients with fibrosis maximum expiratory flow (M.E.F.) and maximum inspiratory flow (M.I.F.) at 50% of vital capacity were both reduced and the ratio between them was similar to that in healthy subjects. In both asthma and emphysema there was a low M.E.F.50%/M.I.F.50% ratio; the only patient with airways obstruction who had a normal M.E.F./M.I.F. ratio was a woman with tracheal stenosis. A theoretical analysis suggests that most forms of airways obstruction would be expected to lead to a greater impairment of M.E.F. than of M.I.F. The M.F.-V. curve did not help in distinguishing a patient with asthma from one with emphysema, but the changes in tracheal obstruction were distinctive.
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