The role of expiratory muscle activity in ventilation during exercise

Abstract

<p>The purpose of this thesis is to investigate the partitioning of work performed by the expiratory muscles between controlling operating lung volume and increasing expiratory flow, as well as the importance of the expiratory work in determining maximal ventilation during exercise. Healthy subjects were studied at rest and six levels of exercise, including maximum exercise (WRmax). Esophageal and gastric pressure (Pes and Pga) were measured using standard balloon catheter systems. Lung volume was measured using inductance plethysmography, calibrated at each exercise work rate with inert gas dilution and spirometric volumes. The static effects of lung and chest wall recoil on esophageal pressure (Pes-stat and Pel,w respectively) were measured at rest (Pel,w was estimated for some subjects). Work of breathing was measured for both inspiratory and expiratory muscle activity and further partitioned into elastic and resistive work. During the transition from rest to mild exercise (50W), end-expiratory lung volume fell from 53.0 ± 3.7 to 47.6 ± 3.6 (M ± SE) percent of total lung capacity, there was a further significant decrease with 100W exercise to 44.0 ± 3.8 %TLC, at maximal exercise, end-expiratory volume was 40.3 ± 2.5 %TLC. During exercise, expiratory flow was greater than that which could be achieved passively (i.e. due to the combined elastic recoil of the lung and chest wall). At maximal exercise, approximately half of the measured expiratory flow was due to expiratory muscle activity. The extent to which expiratory flow was dependent on expiratory muscle activity was greatest at low lung volumes, where the combined elastic recoil of the lung and chest wall was relatively small. Expiratory esophageal pressure progressively increased during the course of exercise. This increase in pressure was associated with significant increases in expiratory flow at all work rates, including maximal exercise. Expiratory pressure remained effective throughout the majority of expiration (i.e. it did not exceed the point at which starling type resistance prevents further increases in expiratory flow). Transient ineffective pressure toward end expiration may have occurred during maximal exercise. The total work of breathing at a given level of ventilation was in agreement with studies reported in the literature. At rest all work was performed by inspiratory muscles. During mild exercise (50W), expiratory muscle work was significant, accounting for 10.5 ± 2.1 % of the total work of breathing (p < 0.05). During mild to moderate exercise (50-150W) the expiratory work used to lower end-expiratory volume below functional residual capacity exceeded that used for increasing expiratory flow (p < 0.05). At maximal exercise however, the work performed increasing expiratory flow accounted for 71 ± 9.2 % of the total expiratory work and was significantly greater than the work used to lower end-expiratory volume (p < 0.05). Ventilation during maximal exercise could not be achieved without expiratory muscle activity, illustrating the importance of these muscles in lowering end-expiratory lung volume and increasing expiratory flow. (Abstract shortened by UMI.)</p>