Expiratory Flow Limitation, Dynamic Hyperinflation, and Inspiratory Muscle Fatigue during Exercise in Asthmatic Subjects

Abstract

PURPOSE: Expiratory flow limitation (EFL) and consequent dynamic hyperinflation (DH) contribute to dyspnea and exercise intolerance in patients with asthma. The effect of DH upon dyspnea is due to the increased elastic load to breathing that it induces. We wished to characterize the inter-relationship of exercise intensity, DH, breathing pattern, and inspiratory muscle fatigue in otherwise healthy subjects with asthma. METHODS: Fifteen clinically diagnosed asthmatic individuals (6M, 9F) gave informed written consent. Peak power (PP) output was determined during a continuous incremental ramp test on a cycle ergometer. On a separate occasion, graded submaximal exercise was undertaken at 20, 30, 40, and 50% of PP (3 min stages). The final test was at 70% of PP to the limit of tolerance (Tlim). Spirometry and maximum inspiratory pressure (MIP) were measured pre- and post-exercise. Metabolic and ventilatory variables were determined at rest and breath-by-breath during exercise using an online system. Tidal volume (TV) was recorded (10-20 breaths) at the end of each min and used to generate a mean TV loop. End-expiratory and end-inspiratory lung volumes (EELV, EILV) were obtained by reference to the maximal inspiratory capacity. Measurements taken during the final minute of the test were termed isoend. RESULTS: All participants demonstrated airflow limitation during Tlim (11.29 ± 10.34 min). At the end of the first 5 min of the Tlim test, TV increased from 0.73±0.24L at rest, to 2.48±0.51L, end inspiratory lung volume increased (EILV) (2.06±0.66L to 3.51±0.73L), and EELV decreased (1.33±0.50L to 1.13±0.58L). During the continuation of the test from minute 5 to isoend, there was a significant decrease in TV (2.21±0.51L) [P<0.01], an increase in EELV (1.38±0.48L) [P<0.01], and no change in EILV (3.55±0.75). MIP decreased significantly from pre- to post-exercise (111.2±26.49to 100.8±25.17cmH20)[P<0.01]. CONCLUSION: Flow limitation and altered breathing mechanics, along with inspiratory muscle fatigue, are present in subjects with asthma during intense exercise.