The respiratory musculature: impairments and limitations in health and disease
School of Kinesiology, The University of British Columbia, Vancouver, BC, Canada
In healthy humans, breathing in most physiological states (i.e. wakefulness, sleep, and exercise) is remarkably well-controlled. Although the partial pressure of oxygen and carbon dioxide within the alveoli are regulated precisely within a few millimetres of mercury, we are able to speak, cough, chew, swallow, and breathe with minimal effort, all of which occurs through the same series of airways and using many of the same skeletal muscles. The system that allows us to simultaneously accomplish this wide range of tasks with such precision and efficiency consists of three highly integrative, overlapping levels of control including: (1) the central controller (or driver) of respiratory rhythm and pattern, (2) distribution and synchronization of respiratory motor output to the appropriate respiratory muscles and, (3) sensory inputs to the central pattern generator. With this framework in mind, the presentation will first focus on the respiratory muscles and their mechanisms of action. This will be followed by a presentation of two specific clinical examples of respiratory muscle impairment: Duchenne muscular dystrophy and spinal cord injury. Emphasis will be placed on therapeutic interventions designed to improve respiratory health. Finally, the presentation will conclude with an examination of the respiratory musculature in health, under conditions of strenuous whole-body exercise. The high levels of ventilation that accompany exercise requires increases in force output by the diaphragm and many obligatory and accessory inspiratory muscles that can reach approximately 60-90% of their dynamic capacity for force generation. Emphasis will be placed on the effects of a high work of breathing and diaphragmatic fatigue on neurovascular control.