Respiratory muscle function.

The functional anatomy of the respiratory muscles has been reviewed. The diaphragm has been emphasized, since this is the most important inspiratory muscle, but the view has been presented that the intercostal, scaleni, and other accessory inspiratory muscles become increasingly important as airflow obstruction leads to hyperinflation. As work increases, the demand for energy and hence blood flow to those muscles has to increase. In spite of a large reserve there are situations in which demands may outstrip supply. This leads to local metabolic changes that result in muscle fatigue. We are now capable of detecting this change as alterations in EMG or in the ability to generate pressures. The latter leads to a decrease in the capacity of the respiratory pump to exchange gas, ultimately resulting in hypercapnia and hypoxemia. The true importance of respiratory muscle fatigue and its differentiation from weakness in patients with severe CAO requires more analysis. It is intuitively appropriate to address the overall decrement in the ability to maintain adequate ventilatory work at low energy cost with the different therapeutic modalities thought to be beneficial. A combination of a decrease in the load imposed on the respiratory muscles, an improvement in the contractility of those muscles, and, when there is absolute need, the resting of the fatigued muscles should result in a better chance to lead a meaningful life and perhaps to improve survival in these patients.