Does expiratory muscle activity influence dynamic hyperinflation and exertional dyspnea in COPD?
Introduction
Activity-related dyspnea occurs in the majority of patients with moderate-to-severe chronic obstructive pulmonary disease (COPD) and current treatments are only partially effective in providing relief (Parshall et al., 2012). Previous studies on mechanisms of dyspnea during exercise in COPD have largely focused on the role of the inspiratory muscles and the mechanical constraints imposed by resting and dynamic lung hyperinflation (Guenette et al., 2012, Kyroussis et al., 2000, Laveneziana et al., 2011, Leblanc et al., 1986, O’Donnell et al., 1997, O’Donnell et al., 2012, O’Donnell et al., 2006, Puente-Maestu et al., 2005). The influence of increased expiratory muscle activity on the intensity and quality of dyspnea is unknown and is the primary focus of the current study.
Expiratory muscle activity in more advanced COPD has been shown to be variable and, in the majority of mechanical studies, peak tidal expiratory esophageal pressure rose smoothly to ∼20–25% of maximal expiratory pressure at end-exercise (Kyroussis et al., 2000, Marin et al., 1999, Montes de Oca and Celli, 2000). It has been proposed that increased expiratory muscle activity may convey a mechanical advantage in COPD by optimizing the length-tension characteristics of the diaphragm (Dodd et al., 1984), with possible attendant salutary effects on perceived dyspnea (Younes, 1991). A contrasting view is that excessive abdominal and internal intercostal muscle activation at higher levels of ventilation (V′E) may contribute to the overall sense of dyspnea (Aliverti et al., 2004, Aliverti et al., 2008). Thus, the inability of patients with expiratory flow limitation to increase tidal expiratory flow rates and reduce end-expiratory lung volume (EELV) below the resting value, by increasing expiratory muscle activity, places the diaphragm under a mechanical disadvantage and excessive intra-thoracic expiratory pressures may also have negative cardio-circulatory consequences (Kyroussis et al., 2000, Potter et al., 1971). Moreover, it has recently been proposed based on optoelectronic plethysmography that a subset of patients with COPD may avoid dynamic lung hyperinflation during exercise (“euvolumics”) by increasing expiratory muscle recruitment (Aliverti et al., 2004). However, the paucity of esophageal and gastric pressure measurements in that study precluded any definitive conclusion about the interaction between expiratory muscle activity and dynamic hyperinflation.
The purpose of the current study was therefore to examine the relationships between expiratory muscle activity, dynamic end-inspiratory and end-expiratory lung volumes, diaphragmatic function and perceived dyspnea during exercise in COPD. Accordingly, we compared detailed dynamic respiratory mechanics (including measurements of expiratory and inspiratory muscle function) and the intensity and quality of dyspnea in COPD and age-matched healthy controls during incremental cycle exercise.
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Subjects
Twelve clinically stable patients with COPD (FEV1/FVC < 0.7), a FEV1 ≤ 80%predicted (Rabe et al., 2007), and a smoking history ≥10 pack-years were included. Twelve age- and sex-matched healthy controls with normal spirometry and a smoking history <10 pack-years were included for comparison. Exclusion criteria were as follows: a disease other than COPD that could contribute to dyspnea or exercise limitation; important contraindications to clinical exercise testing; or use of supplemental oxygen or
Results
Subject characteristics and pulmonary function measurements are summarized in Table 1. There were 6 subjects with GOLD stage II COPD (2 men, 4 women) and 6 with GOLD stage III COPD (4 men, 2 women). In the COPD group, 5 subjects (2 men, 3 woman) were current smokers and the remaining 7 subjects (4 men, 3 women) were ex-smokers. In the control group, 9 subjects (4 men, 5 women) had never smoked and 3 ex-smokers (2 men, 1 woman) had less than a 10 pack-year history and had stopped smoking for at
Discussion
The main findings of this study are as follows: (1) patients with moderate-to-severe COPD showed increased expiratory muscle activity at any given during exercise compared with healthy controls; (2) significant dynamic lung hyperinflation occurred despite increased expiratory muscle activity in COPD; (3) dynamic diaphragmatic function was not different in health and in COPD throughout exercise; and (4) qualitative descriptor choices alluding to perceived expiratory difficulty were selected
Contributions
All authors played a role in the content and writing of the manuscript. In addition: DEO was the principal investigator and contributed the original idea for the study; DEO, PL and KAW had input into the study design and conduct of study; PL and KW collected the data; KAW and PL performed data analysis and prepared it for presentation.
Sources of support
William Spear/Richard Start Endowment Fund, Queen's University (2005–2006); Pierantonio Laveneziana received a John Alexander Stuart Fellowship, Department of Medicine, Queen's University (2007–2008).
Acknowledgements
This study was supported by the William Spear/Richard Start Endowment Fund, Queen's University. Pierantonio Laveneziana received a John Alexander Stuart Fellowship, Department of Medicine, Queen's University.
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