The pathophysiology of inspiratory muscle fatigue.

The critical value of the rate of energy consumption of the inspiratory muscles above which fatigue occurs appears in some instances to be predictable from the relationship between energy demands and energy supplies rather than from the percentage of fatigue-resistant fibres in the inspiratory muscles. When this is the case the critical value of the external power produced by the inspiratory muscles is given by the product of muscular efficiency and the rate at which energy is supplied. Efficiency is reduced by hyperinflation and recruitment of the intercostal and accessory muscles of inspiration. The rate at which energy is supplied is decreased in states characterized by low cardiac output. The condition of low cardiac output, combined with the high oxygen cost of breathing against fatiguing loads, may be lethal in cardiogenic shock. Although the immediate cause of fatigue may not be related to reduced energy supplies, clinically useful predictions of conditions predisposing to fatigue result from an understanding of factors determining the balance between the energy demands and supplies of the inspiratory muscles. These predictions aid in the diagnosis of inspiratory muscle fatigue and have important therapeutic implications.