[Relationship of respiratory muscle fatigue and respiratory effort sensation: mechanisms and treatment].
We investigated whether fatigue of the expiratory muscle may account for a change in the respiratory effort sensation in normal subjects. We also examined the effects of inspiratory muscle training (IMT) and a beta 2-agonist, fenoterol, on respiratory effort sensation during loaded breathing. Experiment 1: Expiratory threshold loading was performed until exhaustion or 30 min, to induce expiratory muscle fatigue. The respiratory effort sensation was scored using a modified Borg scale. The maximum static expiratory pressure during loading (PEmaxload) decreased with time, and the Borg score rose with time. The relationship between the sensory score and PEmaxload was virtually linear. Experiment 2: Twice daily, IMT was performed using a pressure threshold device for 15 min in each session. The inspiratory threshold was set to 30% of each subject's maximum inspiratory pressure (Plmax) and IMT was continued for 4 weeks. Respiratory sensation (Borg score) was evaluated during a progressive exercise test. IMT increased inspiratory muscle pressure by 30%. However, there was no change in Borg score during the exercise test. Experiment 3: Fenoterol (5 mg) was given 2 hours before the inspiratory threshold loading, and then threshold loaded breathing was continued until exhaustion or 60 min. Endurance time increased by 50% in fenoterol group compared to placebo group. The increase in Borg score with time was less in fenoterol group than in placebo group. The decrease in respiratory muscle strength after loading was comparable in both groups. We conclude that the increased respiratory effort sensation during expiratory threshold loading is closely related to expiratory muscle fatigue as indicated by the decrease in expiratory muscle force.(ABSTRACT TRUNCATED AT 250 WORDS)
Suzuki
S
S
First Department of Internal Medicine, Yokohama City University School of Medicine, Kanagawa.
D004417:Dyspnea / Q000188:drug therapy / Q000209:etiology* / Q000503:physiopathology; D005221:Fatigue; D005280:Fenoterol / Q000494:pharmacology / Q000627:therapeutic use; D006801:Humans; D015656:Respiratory Mechanics / Q000502:physiology; D012132:Respiratory Muscles / Q000187:drug effects / Q000503:physiopathology*
D005280:Fenoterol