Practical importance of a preceding full inhalation or exhalation upon the measurement of airway resistance.

1. Airway resistance was measured close to functional residual capacity before and after a full inhalation of total lung capacity, as well as before and after a full exhalation to residual volume. 2. The effects of these volume manoeuvres upon airway resistance (and associated lung volume) were determined in four resting normal male subjects and in six normal men during experimentally induced bronchoconstriction after breathing an air/histamine mist from a Wright's nebulizer. 3. In four men the duration of the effect of a full inhalation upon airway resistance after induced bronchoconstriction was assessed separately. 4. Neither a full inhalation nor a full exhalation altered airway resistance under normal conditions. However, a full inhalation reduced airway resistance in the presence of bronchoconstriction and this effect lasted for a period of 45 s. Even with bronchoconstriction, a full exhalation had no effect on airway resistance. 5. Account must therefore be taken of the potential reduction in airway resistance which may result from a full inhalation, particularly when indirect measurements of airway function which involve a full inhalation, such as forced spirometry, are used to assess airway obstruction.