We determined the effect of exercise‐induced bronchoconstriction (EIB) on the shape of the maximal expiratory flow‐volume (MEFV) curve in asthmatic adults. The slope‐ratio index (SR) was used to quantitate the shape of the MEFV curve. We hypothesized that EIB would be accompanied by increases in SR and thus increased curvilinearity of the MEFV curve. Adult asthmatic ( n  = 10) and non‐asthmatic control subjects ( n  = 9) cycled for 6–8 min at 85% of peak power. Following exercise, subjects remained on the ergometer and performed a maximal forced exhalation every 2 min for a total 20 min. In each MEFV curve, the slope‐ratio index (SR) was calculated in 1% volume increments beginning at peak expiratory flow (PEF) and ending at 20% of forced vital capacity (FVC). Baseline spirometry was lower in asthmatics compared to control subjects (FEV1% predicted, 89.1 ± 14.3 vs. 96.5 ± 12.2% [SD] in asthma vs. control; p  < 0.05). In asthmatic subjects, post‐exercise FEV1 decreased by 29.9 ± 13.2% from baseline (3.48 ± 0.74 and 2.24 ± 0.59 [SD] L for baseline and post‐exercise nadir; p  < 0.001). At baseline and at all timepoints after exercise, average SR between 80 and 20% of FVC was larger in asthmatic than control subjects (1.48 ± 0.02 vs. 1.23 ± 0.02 [SD] for asthma vs. control; p < 0.005). This averaged SR did not change after exercise in either subject group. In contrast, post‐exercise SR between PEF and 75% of FVC was increased from baseline in subjects with asthma, suggesting that airway caliber heterogeneity increases with EIB. These findings suggest that the SR‐index might provide useful information on the physiology of acute airway narrowing that complements traditional spirometric measures.

Background Several indices of airflow and volume derived from the maximal expiratory flow volume (MEFV) curve are used to identify and diagnose the severity of respiratory disease. In addition to the objective measures of expiratory airflow and volume, the shape of the MEFV curve is thought to provide information on pulmonary (patho)physiology. In the asthmatic, the MEFV curve is often convex relative to the volume axis (“scooped”). This “scooped” curve is thought to indicate heterogeneous airway emptying during the forced exhalation. The effect of exercise‐induced airway narrowing on the shape of the MEFV curve is not known. The purpose of this study was to determine the effect of exercise‐induced bronchoconstriction (EIB) on the shape of the MEFV curve in asthmatic adults. We hypothesized that the slope‐ratio (SR) index, and thus the overall shape of the MEFV curve would not be affected by EIB. Methods Adult male and female asthmatics (n=9; age, 26.8 ± 8.4 yrs; BMI, 27.4 ± 4.3 kg/m2) and non‐asthmatics (n=9; age, 25.4 ± 7.8 yrs; BMI, 21.9 ± 2.0 kg/m2) completed a high‐intensity cycle ergometry bout for six minutes at 85% of peak power. Maximal forced exhalations (MFE) were performed before exercise. Following exercise cessation, subjects remained on the ergometer and performed a MFE every two‐minutes for a total 20 minutes. In each MEFV curve, the SR index was calculated in 1% volume increments between 20% and 80% of the vital capacity. To calculate SR, the tangent and chord slope were generated at each point of interest, and the tangent slope was divided by the chord slope. Results Pulmonary function at baseline was lower in asthmatics compared to the control group (FEV1% predicted, 84.9 ± 11.0 vs. 96.5 ± 12.2% in asthmatics vs. control, respectively; P<0.05). In asthmatic subjects, post‐exercise nadir FEV1 decreased by 29.1 ± 13.7% from baseline (3.40 ± 0.74 to 2.47 ± 0.7 L for baseline and nadir, respectively; P<0.001), whereas FEV1 did not change after exercise in control subjects (3.84 ± 0.9 to 3.70 ± 0.8 L for baseline and nadir, respectively). At all time points, the average SR between 20 and 80% of vital capacity was significantly larger in asthmatic than control subjects (1.49 ± 0.02 vs. 1.23 ± 0.02 [SD] for asthma vs. control; P<0.005). However, in both control and asthmatic subjects, post‐exercise SR was not different from baseline at any time point. In asthmatic subjects, average SR between 20 and 80% of vital capacity was 1.49 ± 0.13 at baseline and averaged 1.49 ± 0.02 across all post‐exercise time points (range = 1.48 to 1.53). Conclusion In this group of adults with asthma, EIB did not influence the SR index, indicating no change to the overall shape of the MEFV curve. Thus, these findings suggest that inhomogeneities in airway caliber and lung emptying are not magnified during an episode of EIB.