Attacking the disease spiral in chronic obstructive pulmonary disease: an update

Abstract

-In chronic obstructive pulmonary disease (COPD) a pathophysiological cycle occurs such that locomotor muscle weakness and fatiguabilty exist, which in turn limit exercise performance both because of leg discomfort and also because anaerobic metabolism leads to lactic acid production. Since the lactic acid is buffered by bicarbonate there is consequent carbon dioxide (CO 2 ) production. Patients with advanced COPD are flow limited and cannot excrete the CO 2 by raising ventilation and thus these patients experience breathlessness which discourages exercise and, in turn, prompts further deconditioning. Structured exercise, termed pulmonary rehabilitation is at the core of reversing the cycle but novel strategies should be employed for patients with advanced disease and alternative therapeutic opportunities may soon be available to improve pulmonary mechanics.

An Official American Thoracic Society/European Respiratory Society Statement: Update on Limb Muscle Dysfunction in Chronic Obstructive Pulmonary Disease

“…On the other hand, several observations are in favor of the existence of a myopathy in patients with COPD: (1) muscle function (strength or endurance) is not (or is poorly) related to the degree of physical activity in COPD, as assessed by questionnaires or actual measurements (302), suggesting that mechanisms other than inactivity may exist; (2) when patients with COPD are compared with healthy subjects with consistent and comparable low physical activity, differences in muscle function and/ or structure remain (259,308); (3) although exercise training improves some muscle features in COPD, muscle typology does not improve, (i.e., increase in type I fiber proportion [163,315,316]) to the same extent in COPD in comparison to what is seen in healthy subjects (317-319); (4) quadriceps weakness occurs in patients with COPD across all severity stages (310).…”

“…Whether limb muscle dysfunction and cellular alterations in COPD are further compounded by a myopathy (that is, intrinsic alterations in limb muscle over and above the changes due to inactivity, and which are not fully reversible by exercise training) has been the subject of debate and controversy in last decade and is still not resolved (307)(308)(309)(310)(311). This may be in part because of failure to identify and separately study and discuss findings in different phenotypic (and in the future genotypic) subgroups of COPD.…”

“…Against the hypothesis of a disease-specific myopathy are the following: (1) many functional and cellular findings in COPD are identical to those of disuse; (2) limb muscle is trainable, because training has been shown to improve muscle strength and endurance, oxidative enzyme activities, fiber cross-sectional area, and muscle capillarization, at least in some patients (307,310); (3) in a small study, highintensity aerobic interval training was shown to restore work performance and oxidative capacity of the quadriceps to a level similar to that found in sedentary individuals (312); and (4) the fact that full recovery of muscle function is unusual does not necessarily indicate a myopathy, because the duration of the training program may be too short to normalize alterations that have developed over several decades (313,314), or early ventilatory limitation may compromise the ability to tolerate sufficient muscle stimulus to improve global or cellular function (311).…”

Pulmonary Rehabilitation Exercise Prescription in Chronic Obstructive Pulmonary Disease

“…This downward spiral of decreasing exercise and increasing dyspnea on exertion can result in greater levels of functional impairment and disability. 6 Physical deconditioning due to physical inactivity provides a major rationale to consider exercise training as part of comprehensive pulmonary rehabilitation (PR).…”

Targeting Limb Muscle Dysfunction in COPD

“…First, when people with COPD are compared to people without COPD but matched with equally low physical activity levels, differences in structure and function remain [29]. Second, quadriceps weakness occurs already in the early stages of COPD [30] when the reduced respiratory capacity should be of minor importance. And third, the training response seems blunted on a structural level for people with COPD [31].…”