The diaphragm consists of two separate muscles, the costal and crural parts, with different segmental innervation and different action on the rib cage. Diaphragmatic endurance is determined by the balance between energy supply and demand. Both an increase in demand and a decrease in supply of energy lead to diaphragmatic fatigue. Under conditions of reduced O2 transport to the body, the inspiratory muscles may command a disproportionate amount of the total body O2 consumption, particularly if the work of breathing is increased. Their energy demands may thus contribute significantly to the pathogenesis of cardiogenic shock and lactic acidosis. Inspiratory muscle fatigue can be diagnosed by observing and palpating: (1) abdominal paradox, a fall in abdominal pressure and an inward displacement of the abdomen during inspiration, and (2) respiratory alternans, a variation in abdominal pressure changes from breath to breath, reflected in variations in the inspiratory movements of rib cage and abdomen. Treatment consists in decreasing energy demands and increasing supplies. Artificial ventilation should be considered to accomplish the former. Methyl xanthines improve diaphragmatic contractility and specific diaphragmatic training programs improve diaphragmatic endurance.