Inspiratory Muscle Training: Integrative Review

This article provides a critical review of inspiratory muscle training (IMT) in chronic obstructive pulmonary disease (COPD). Although extensive research on IMT has accumulated, its benefits have been debated, primarily because of methodological limitations of studies. Using relevant key words, multiple databases were searchedfrom 1966. Selected studies used PI^sub max^ (maximal inspiratory pressure) as an outcome variable. Overall, research demonstrated that a standard protocol of 30% or higher for a duration of 20 to 30 minutes per day for 10 to 12 weeks improves dyspnea and inspiratory strength and endurance with either inspiratory resistive or inspiratory threshold training. Regardless of method, IMT protocols for people with COPD and inspiratory muscle weakness and dyspnea are generally safe, feasible, and effective. Patient selectivity and study of subgroups are recommended. Keywords: inspiratory muscle training; respiratory muscle dysfunction; respiratory muscle training Inspiratory muscle training (IMT), though not always by that name, has been evident in the literature since the mid-l970s. Various terms have been used to describe this intervention, including ventilatory, respiratory, and IMT, though the latter is currently predominant. Regardless of technique, which has also evolved over time, the purpose of IMT is to improve the performance of the inspiratory muscles, which may be limited in a variety of conditions. Interest in IMT as a means to treat respiratory muscle dysfunction has grown over the past two decades; recently, IMT was included in the Cochrane Database of Systematic Reviews (Bradley, Moran, & Greenstone, 2002; Ram, Picot, Barnes, & Polkey, 2002). IMT has been used in an increasingly wide range of clinical situations, but the benefits continue to be debated. One of the predominant reasons is related to methodological concerns of the studies conducted to date. The purpose of this article is to provide a comprehensive, critical review of IMT. No attempt will be made to address expiratory muscle training since the overwhelming majority of the research has been conducted in the area of IMT. Scientific knowledge is most extensive in the area of chronic obstructive pulmonary disease (COPD), yet many questions remain. This analysis will focus on the efficacy of IMT in relationship to specific outcomes, including inspiratory muscle strength and endurance, dyspnea, functional performance outcomes, and health-related quality of life (HRQOL). This paper will include a review of the pathophysiologic basis and clinical significance of respiratory muscle dysfunction, a rationale for and principles of training respiratory skeletal muscles, a mechanism of action and types of IMT, and a critical review of the research related to IMT in COPD. PATHOPHYSIOLOGIC BASIS OF RESPIRATORY MUSCLE DYSFUNCTION The muscles primarily responsible for inspiration include the diaphragm, the intercostal muscles, and the scalene, with accessory muscles recruited during increased ventilation levels (Reid, Clarke, & Wallace, 2001); exhalation is generally passive (Breslin, 1996). The respiratory muscles uniquely perform other nonventilatory functions as well. Reduced function of the respiratory muscles, which may be acute or chronic (Breslin, 1996), has been attributed to their weakness (Rochester, 1991) or fatigue (Roussos, Moxham,B fatigue occurs whenever energy demand exceeds supply (Roussos, 1985) and results in the inability to generate or maintain the forces needed for respiration (Larson & Kim, 1987a). This decline in force is related to both reduced central motor drive as well as peripheral muscle contractile failure (Roussos&Zakynthinos, 1996) that may be interdependent (Enoka&Stuart, 1992). …