Adaptation of respiratory muscles to acute and chronic stress. Considerations on energy and fuels.
Although the majority of patients with acute or chronic pulmonary disease and respiratory failure do not have respiratory muscle disease or dysfunction at the outset of their clinical condition, most if not all do not have a normal respiratory musculature at the time of respiratory failure, especially those who have had a chronic course. Indeed, a large number of alterations in the muscle structure and metabolism occur as a result of a chronic load. Most of these changes are compensatory, but some may be maladaptive and deleterious to function. In addition, the compensatory mechanisms can be limited, and it is now believed that respiratory failure is at least in part exaggerated, or initiated, by the failure of the respiratory musculature. Therefore, the failure of these muscles can lead to hypoventilation, apnea, poor gas exchange, and clinically serious cardiovascular consequences. Although research efforts have increased in the past several years, there is still a great deal to learn about the function of the respiratory musculature under stress. Some of the notable questions are: What is the relation between inspiratory muscles (for example, the diaphragm and the intercostals) and the muscles of the airways during loaded breathing and in the presence of respiratory failure? How does the differential respiratory output to the various motor neuron pools change and evolve during the chronic loaded breathing? What kind of compensatory mechanisms (for example, humoral, biochemical, neural, or mechanical) can the respiratory muscles use to preserve function? What is the relation between the nutritional status of the individual and its effect on the function of these muscles? and What effect(s) do the changes associated with chronic pulmonary disease (for example, hypoxia, hypercapnea) have on respiratory muscle metabolism and function? This is an exciting area of research that has enormous potential for clinical applicability, and we believe that we are just at the very beginning.