The main function of the respiratory system is to maintain oxygen and carbon dioxide homostasis. Hypercapnic respiratory failure may, in the absence of failure of respiratory control, be viewed as an imbalance between the load placed on the respiratory muscle pump and its capacity. Hypercapnia, both acute1 and chronic,2 carries a poor prognosis in COPD. Whilst load is increased in COPD (for the reasons covered elsewhere in this volume), theory predicts that the capacity of the respiratory muscle pump to develop inspiratory pressure and, in turn, volume change might also be diminished in COPD. An overview of the influences acting on the respiratory muscle pump is shown in Figure 15.1.