Neuromuscular disease and hypoventilation.
Alveolar hypoventilation associated with neuromuscular disease can occur in acute and chronic forms. In the acute form, progressive weakness of respiratory muscles leads to rapid reduction in vital capacity followed by respiratory failure with hypoxemia and hypercarbia. Symptoms are those of acute respiratory failure, including dyspnea, tachypnea, and tachycardia. In the chronic form, impairment of the respiratory muscles affects mechanical properties of the lungs and chest wall, decreases the ability to clear secretions, and eventually may alter the function of the central respiratory centers. Symptoms include orthopnea, fatigue, disturbed sleep, and hypersomnolence. Treatment and outcome of the disease's chronic form are dependent on the underlying clinical cause of the alveolar hypoventilation. For chronic but stable diseases such as old polio, quadriplegia, or kyposcoliosis, mechanical support of minute ventilation can reverse symptoms. For chronic and progressive disease such as muscular dystrophy and amyotrophic lateral sclerosis, mechanical support of minute ventilation provides only symptomatic relief and is usually associated with deterioration to the point of complete ventilator dependency for survival. For the chronic progressive forms of alveolar hypoventilation, there is currently a need for quality randomized controlled clinical trials to define physiologic indicators and appropriate timing for mechanical support of minute ventilation.