Methods to Improve Respiratory Muscle Function

The respiratory system can be divided into an organ of gas exchange (formed by the lungs, in­ cluding airways, parenchyma, and vasculature) and an organ that acts as a pump (formed by the chest wall, including the rib cage, respiratory muscles, and the respiratory center) that ventilates the lungs. The lungs are actively inflated when the res­ piratory muscles contract, and thus the pump por­ tion of the respiratory system performs work. The diaphragm and other respiratory muscles behave as other skeletal muscles and their dysfunction can lead to ventilatory failure. This may occur in various clinical conditions, including atrophy of respiratory muscles after prolonged mechanical ventilation, neuromuscular disease and associated weakness of the muscles (such as in spinal cord injury and amyotrophic lateral sclerosis), cardiac disease, which increases inspiratory loads (particu­ larly pulmonary edema), and chronic obstructive pulmonary disease (COPD). In COPD, increases in the elastic and resistive impedances to breathing require that the respiratory muscles generate a greater force. Associated with changes in elastic properties of the lung and prolongation of their time constant for emptying in COPD, there is also hyperinflation, which places the inspiratory mus­ cles at a mechanical disadvantage. Malnutrition, which is often present in these patients with chronic disease, may further affect the perfor­ mance of the respiratory muscles. ' After several years of gathering data through experimental and clinical research, there is still no simple test to make a diagnosis of respiratory mus­ cle fatigue at the bedside or to assess its prevalence in patients. It is generally accepted that acute load­ ing of respiratory muscles can produce ventilatory failure through respiratory muscle fatigue, such as might occur in patients who develop acute severe asthma, in patients who fail weaning from a me­ chanical ventilator, during exercise, during mod­ ification of breathing pattern, and with added inspiratory loads. A decrease in muscle force and increase in respiratory loads may be chronic and develop slowly and progressively, as occurs in neu­ romuscular disease, interstitial lung disease (ILD), COPD, and in malnourished patients. The devel­ opment of respiratory muscle fatigue of a chronic nature has not been definitely proven, and al­ though these conditions can progress to chronic respiratory failure, chronic fatigue of the respira­ tory muscles is not only difficult to demonstrate, but may not be a significant cause of chronic respi­ ratory failure. Since respiratory muscle dysfunction can lead to ventilatory failure, its recognition has important therapeutic implications. The therapeutic strate­ gies for respiratory muscle dysfunction will be dif­ ferent, depending on the pathophysiologic events that produce the hypercapnic respiratory failure (Table 1). In some cases it will be important to reduce the load placed on the inspiratory muscles. The use of bronchodilators, steroids, maneuvers to clear airway secretions, control of bronchopulmo­ nary infections, and reduction of ventilatory and oxygen needs (control of fever, lowered carbohy­ drate intake) seek to accomplish this benefit. It might be important to improve respiratory muscle strength and endurance, which can be achieved by reducing acidosis, correcting electrolyte imbalance and hypophosphatemia, increasing cardiac output and blood flow to the respiratory muscles, adminis­ tering supplemental oxygen therapy, and institut­ ing pharmacologic treatment of respiratory insuffi­ ciency, including theophylline and β-adrenergics. Inspiratory muscle training might be an important therapeutic approach in some patients. Training inspiratory muscles to increase their strength will allow them to generate increased force to overcome