Structure and function of the respiratory muscles in patients with COPD: impairment or adaptation?

Abstract

Structure and function of the respiratory muscles in patients with COPD: impairment or adaptation?. M. Orozco-Levi. #ERS Journals Ltd 2003. ABSTRACT: Respiratory muscles are essential to alveolar ventilation. These muscles work against increased mechanical loads due to airflow limitation and geometrical changes of the thorax derived from pulmonary hyperinflation. Respiratory muscle fibres show several degrees of impairment in cellular and subcellular structures which, in many cases, are proportional to the severity of the disease and accompanying conditions (ageing, deconditioning, starvation, comorbidity). This structural impairment translates, from the functional point of view, to a loss of strength (capacity to generate tension) and an increased susceptibility to failure in the face of a particular load (early onset of fatigue).On the other hand, there is accumulating evidence that the diaphragm and other respiratory muscles are also able to express adaptive changes in response to the chronic mechanical load imposed by the disease. In most cases, impairment and adaptation of the respiratory muscles reaches a balance that permits enough ventilation for patients9 survival. However, this balance can be altered for additional increments of the mechanical or metabolic load on the muscles (e.g. abdominal or thoracic surgeries, pneumonia, pulmonary embolism, etc.). Moreover, loss of balance is not always associated with extreme situations. Many patients develop ventilatory failure and require hospital admission even if the cause of the exacerbation is less dramatic (bronchial infections, pain of any nature, electrolyte disturbances, etc.).Although the physiopathology of chronic obstructive pulmonary disease exacerbations is multifactorial, the above-mentioned fragility suggests the existence of a "fragile balance" between respiratory muscle overload and respiratory muscle adaptations. Assessment of respiratory muscle function through specific tests evaluating the strength and endurance could offer valuable information about this particular susceptibility to muscle imbalance. Identification of patients possessing a fragile respiratory muscle balance could have important implications for the application of specific strategies such as respiratory muscle training, nutrition, or anabolic treatment. Dyspnoea, decrease of exercise capacity and impairment of quality of life are common in patients with chronic obstructive pulmonary disease (COPD). Recently, it has been demonstrated that the decrease in exercise capacity is associated with mortality of these patients. Depending on the individual, the imbalance between the functional capacity of the respiratory muscles and the load they are chronically facing plays an important role in the genesis of dyspnoea [1] and hypercapnia [2,3]. It has also been described that respiratory muscle dysfunction could be an important determinant of increased use of health resources and survival in hospitalised patients with severe COPD [4][5][6][7][8][9].Respiratory muscles are skeletal musc...

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