The Significance of Volume-Adjusting the Maximal Midexpiratory Flow in Assessing the Response to a Bronchodilator Drug

doi:10.1378/chest.73.5.568

Chest

Volume 73, Issue 5, May 1978, Pages 568-571

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Forced expiratory spirograms were obtained before and for six hours after 25 subjects ingested ephedrine and placebo in a double-blind crossover study. Significant changes in the forced vital capacity (FVC) and the forced expiratory volume in one second (FEV_1.0) were noted on days when ephedrine was administered, while the mean forced expiratory flow during the middle half of the FVC (FEF25-75%) failed to indicate significant bronchodilation. When FVC increases after therapy with a bronchodilator drug, one is no longer measuring flow during the same volume segment and driving pressure (static transpulmonary pressure [Pst]) as before administration of the bronchodilator drug. Volume-adjusting the FEF25-75% after therapy to the same volume and Pst over which flow is being measured in the tracings before bronchodilator therapy yielded highly significant increases in flow after administration of the bronchodilator drug.

Section snippets

MATERIALS AND METHODS

Twenty-five patients with reversible obstruction of the airways were selected from the outpatient population of our clinic. The purpose of the study was explained, and informed consents were obtained. Each patient demonstrated greater than 25 percent improvement in their FEV_1.0 after inhaling 2 ml of a 1:800 solution of isoproterenol nebulized over a five-minute period by a modified nebulizer (Deutraband D-31). The mean age of the patients was 49 years, with a range of 20 to 73 years.

RESULTS

The mean baseline values for the population under study, expressed as the percentage of predicted values (±1 SE), are as follows: TLC, 136 ± 4 percent; FVC, 70 ± 4 percent; FEV_1.0, 49 ± 4 percent; and FEF25-75%, 26 ± 4 percent. There was less than a 5 percent difference between mean baseline values on the days of study.

Figure 2 is a plot of FVC (percent of change from baseline) against time after ingestion of ephedrine and placebo. Compared with placebo, ingestion of ephedrine produced

DISCUSSION

It seemed paradoxical to us that while we were studying a relatively weak bronchodilator drug, the FVC and FEV_1.0 should change significantly while the FEF25-75% changed little. At different lung volumes, values for elastic recoil pressure (and thus airway driving pressure) are different, and this difference may obscure or enhance a true change in resistance to flow. In all subjects, there were no significant differences in measured TLC before and 30 minutes after ingestion of ephedrine. In the

ACKNOWLEDGMENTS

Statistical assistance was provided by Mr. Emmanual Lerner, Eastern Research Support Center, Veterans Administration, West Haven, Conn. We wish to thank Mr. John O. Ponder for his technical assistance and Gordon L. Snider, M.D., F.C.C.P., for his help with the manuscript.

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: Manuscript received March 15; revision accepted August 3.

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Chest

Clinical InvestigationsThe Significance of Volume-Adjusting the Maximal Midexpiratory Flow in Assessing the Response to a Bronchodilator Drug

Section snippets

MATERIALS AND METHODS

RESULTS

DISCUSSION

ACKNOWLEDGMENTS

Am J Med

Am J Med

Maximal midexpiratory flow

Am Rev Tuberc Pul Dis

Mead J: Resistance of central and peripheral airways measured by a retrograde catheter

J Appl Physiol

Clinical Investigations
The Significance of Volume-Adjusting the Maximal Midexpiratory Flow in Assessing the Response to a Bronchodilator Drug