title: Evolution of inspiratory and expiratory muscle pressures during endurance exercise.
abstract: We investigated the relationship between minute ventilation (VE) and net respiratory muscle pressure (Pmus) throughout the breathing cycle [Total Pmus = mean Pmus, I (inspiratory) + mean Pmus, E (expiratory)] in six normal subjects performing constant-work heavy exercise (CWHE, at approximately 80% maximum) to exhaustion on a cycle ergometer. Pmus was calculated as the sum of chest wall pressure (elastic + resistive) and pleural pressure, and all mean Pmus variables were averaged over the total breath duration. Pmus, I was also expressed as a fraction of volume-matched, flow-corrected dynamic capacity of the inspiratory muscles (P(cap, I)). VE increased significantly from 3 min to the end of CWHE and was the result of a significantly linear increase in Total Pmus (Delta = 43 +/- 9% from 3 min to end exercise, P < 0.005) in all subjects (r = 0. 81-0.99). Although mean Pmus, I during inspiratory flow increased significantly (Delta = 35 +/- 10%), postinspiratory Pmus, I fell (Delta = -54 +/- 10%) and postexpiratory expiratory activity was negligible or absent throughout CWHE. There was a greater increase in mean Pmus, E (Delta = 168 +/- 48%), which served to increase VE throughout CWHE. In five of six subjects, there were significant linear relationships between VE and mean Pmus, I (r = 0.50-0.97) and mean Pmus, E (r = 0.82-0.93) during CWHE. The subjects generated a wide range of Pmus, I/P(cap, I) values (25-80%), and mean Pmus, I/P(cap, I) increased significantly (Delta = 42 +/- 16%) and in a linear fashion (r = 0.69-0.99) with VE throughout CWHE. The progressive increase in VE during CWHE is due to 1) a linear increase in Total Pmus, 2) a linear increase in inspiratory muscle load, and 3) a progressive fall in postinspiratory inspiratory activity. We conclude that the relationship between respiratory muscle pressure and VE during exercise is linear and not curvilinear.
authors.affiliation: Division of Respiratory Medicine, Department of Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0W8. bharath.krishnan@uregina.ca
authors.forename: B S
authors.initials: BS
authors.lastname: Krishnan
authors.forename: T
authors.initials: T
authors.lastname: Zintel
authors.forename: C
authors.initials: C
authors.lastname: McParland
authors.forename: C G
authors.initials: CG
authors.lastname: Gallagher
mesh_terms: D000328:Adult; D015444:Exercise / Q000502:physiology*; D006801:Humans; D016014:Linear Models; D008176:Lung Volume Measurements; D008297:Male; D010807:Physical Endurance / Q000502:physiology*; D011312:Pressure; D012123:Pulmonary Ventilation / Q000502:physiology*; D012119:Respiration; D012132:Respiratory Muscles / Q000502:physiology*; D013990:Tidal Volume; D013997:Time Factors