Clinical intervention in chronic respiratory failure.

D espite the obvious partnership of the lungs and the respiratory muscles in ventilation and gas exchange, only within the past 10 years has the possible contribution ofthe respiratory muscles to chronic respiratory failure been investigated in any detail. The opinion has been challenged that the respiratory muscles, albeit a vital pump, are not immune to fatigue.' Increase in the elastic and resistive impedance to breathing in chronic obstructive pulmonary disease (COPD) requires greater force generation by the respiratory muscles. Changes in the elastic properties of the lungs and the prolonged time constant for emptying cause hyperinflation, which puts the inspiratory muscles at a disadvantage. The problems of force generation by the inspiratory muscles are compounded by malnutrition, which can adversely affect ventilatory muscle performance #{149}2,3 Knowing these things, respiratory physicians have attempted with mixed success to improve the ventilatory muscles by training, resting, and feeding, where appropriate, of patients with COPD. Implicit in these interventions is the notion that ventilation may be improved at rest or on exertion, or that the intensity of dyspnea may be modified by altered inspiratory muscle function. I will review the basis for such therapeutic approaches and explore some of the possible reasons why we have not witnessed a more signfficant impact on the functioning ofpatients with COPD. INSPIRATORY MUSCLE TRAINING The performance of certain ventilatory tasks by normal subjects can be improved by training of the inspiratory muscles.. '' Because the exercise capacity of patients with COPD is ventilatory-limited, it is reasonable to postulate that their exercise performance may be improved by appropriate training of the ventilatory muscles. A number of investigators have confirmed that inspiratory muscle training (IMT) can improve inspiratory muscle performance in patients with COPD,,-'3 which in some cases is also accompanied by improvement in exercise performance... An important question concerning the relationship between IMT and improved exercise performance that has not been satisfactorily answered is the mechanism of the improvement. Is the improved exercise performance a direct consequence of IMT? Certain arguments can be offered in favor of the link between improvements in exercise performance and inspir-atory muscle function. In all of the studies, subjects after training were capable of either tolerating higher resistive loads to inspiration or sustaining higher levels of voluntary ventilation for longer periods, indicating that a specffic training effect had been achieved. Even if objective tests of inspiratory muscle performance were not used, part of …