Evaluation of Methods Used in Detecting Changes of Airway Resistance in Man1,2,3

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Abstract
The methods examined were the body plethysmograph, Clements'' rotary air-flow interrupter, the Wright peak flow meter, maximal breathing capacity, the expiratory flow-volume loop, and the forced expiratory vital capacity. The maximal midexpiratory flow rate and percentages of the vital capacity exhaled in one and in three seconds were calculated from the latter chart record. In a series with 10 normal subjects in whom bronchospasm and then relaxation were induced, corresponding changes in airway resistance as measured by the plethysmograph were found always to be present and to be of considerable magnitude. The maximal breathing capacity, maximal midexpiratory flow, and the flow at mid-volume of a forced expiratory vital capacity displayed by flow-volume loop showed significant changes in about 70 per cent of those instances recognized by the plethysmograph. The peak flow and one-second per cent of vital capacity were minimally affected, while two essentially unchanged values were the measurements made with the rotary interrupter and the three-second per cent of vital capacity. In addition to the comparison, several of the methods have been discussed from a more basic standpoint of physiology, and secondary experiments have been included to implement this discussion.