Respiratory muscles mechanical load

The mechanical plant is propelled by the respiratory muscles which serve as a mechanical pump. Normally, Pmus is sustained by the inspiratory muscles (principally the diaphragm and, to a lesser extent, the external intercostal and parasternal muscles). During hyperpnea or with increased expiratory load the expiratory muscles maybe recruited [Marroquin, 1991], as are accessory muscles which may contribute significantly to the generation of Pmus in paraplegics. 

Hypercapnic respiratory failure is due to failure of the ventilatory pump caused by acute (drug overdose, acute neuromuscular diseases) or chronic (chest wall abnormalities, chronic neuromuscular diseases) disorders. It is characterized by alveolar hypoventilation, which leads to hypercapnia with coexistent, usually mild, hypoxemia. The central drive may be globally reduced with the fall in Pa02 resulting from the increase in alveolar CO2. More commonly, the drive remains high, but the mechanical load on the respiratory systan is too great or the capacity of the muscles too low to ensure efficient CO2 elimination (Fig. 1). 

Figure 5 Respiratory muscle resistive training with threshold loading in a patient weaning from mechanical ventilation.

Exercise limitation and functional disability in COPD have a complex, multifactorial basis. Ventilatory limitation is caused by increased airways resistance, static and dynamic hyperinflation, increased elastic load to breathing, gas exchange disturbances, and mechanical disadvantage and/or weakness of the respiratory muscles (4-6). Car-diocirculatory disturbances (7,8), nutritional factors (9), and psychological factors, such as anxiety and fear, also contribute commonly to exercise intolerance. Skeletal muscle dysfunction is characterized by reductions in muscle mass (10,11), atrophy of type I (slow twitch, oxidative, endurance) (12,13) and type Ila (fast twitch) muscle fibers (14), altered myosin heavy chain expression (15), as well as reductions in fiber capillarization (16) and oxidative enzyme capacity (17,18). Such a dysfunction is another key factor that contributes... 

---