Influence of the previous deep inspiration on the spirometric measurement of provoked bronchoconstriction in asthma.

The measurement of the forced expiratory volume in one second (FEV1) after a maximal inspiration is often used to assess the effect of inhaled bronchoconstrictor agents. In persons with asthma, a deep inspiration transiently alters the bronchial tone in a variable way. This may result in a variable decrease of FEV1 despite a similar degree of pharmacologically induced bronchospasm. In 71 patients with asthma we induced a comparable increase (approximating 200%) in airway resistance (Raw) with aerosolized carbachol. We then measured the change in Raw induced by one deep inspiration; when Raw had returned to the value observed before deep inspiration, the FEV1 was measured. A significant correlation (r = 0.74; p less than 0.001) was found between the transient changes in Raw induced by a deep inspiration (ranging individually from a 33% increase to a 70% decrease) and the decrease in FEV1 (from 0 to 70%). The decrease in FEV1 was greatest in patients in whom a deep inspiration increased Raw and least in those in whom Raw decreased. The initial and post-carbachol Raw values of the 17 patients exhibiting a less than 20% decrease in FEV1 were similar to those of the 54 patients with a decrease in FEV1 of more than 20%. Because a prior deep inspiration prevents changes in FEV1 in some patients with asthma, the use of the FEV1 for bronchial provocation tests can be misleading.