The unweanable patient.

Inspiratory muscles can be exerted to their maximal limits during situations of: 1) high ventilatory demands, such as in exercise; and 2) during cases of high force demands, as in obstructive or restrictive diseases. In either circumstance, the level of sustainable activity (many hours) seems to be about half of the subject's maximal ventilatory capacity (MVC) or their maximal inspiratory pressure (MIP), respectively. The natural history of chronic hypercapnia in chronic obstructive pulmonary disease (COPD) or in neuromuscular disease suggests that spontaneous ventilation is set at a level below that which will trigger muscle fatigue, even if this lower level results in "chronic ventilatory failure". When this type of patient suffers a pathology that further decreases their global respiratory muscle function or increases their load, we have the makings of an unweanable patient; the mechanical ventilator ultimately replaces the lost inspiratory muscle function. Given time for the muscle to recover force and a reduction of the loads should, thus, be the therapeutic focus.