Overview of Pathophysiologic Mechanisms in Chronic Ventilatory Insufficiency Direct Consequences of Respiratory Muscle Weakness

the ventilatory pump upon which the act of breathing depends. In many neuromuscular disorders, muscle weakness involves the respiratory muscles to an equal or even greater extent than other skeletal muscles. The degree of limb muscle weakness cannot be used as a reliable guide to the presence of respiratory muscle impairment since the correlation between the two may be quite poor.1,2 Respiratory symptoms are often initially minimal because of the inherently large reserve of the respiratory system. Respiratory muscle involvement may also be masked because patients with weak limb muscles spontaneously decrease their overall activity level, thereby reducing the daily physiologic challenge faced by the respiratory system. For all of these reasons, it is not unusual for respiratory muscle weakness to go undetected until overt respiratory failure is precipitated by an acute episode of pulmonary aspiration or infection. Accordingly, the clinician must be vigilant with regard to the possible presence of respiratory muscle weakness in any patient with a known neuromuscular disorder or unexplained exertional dyspnea. Other common symptoms found in patients with neuromuscular disease involving the respiratory muscles include orthopnea, cough during swallowing, weak cough, fatigue, hypersomnolence, morning headaches, insomnia, nightmares, and decreased intellectual performance. Respiratory muscle weakness may be caused by a large and diverse number of diseases affecting the central nervous system, the spinal cord, the nerves, the neuromuscular junction, or the muscle itself. A list of some of the more common or prototypical disorders is shown in Table 26-1, but a comprehensive review of specific disease entities is beyond the scope of this chapter. Instead, our purpose is to discuss general pathophysiologic mechanisms as well as principles of evaluation and treatment that are broadly applicable to most neuromuscular diseases affecting the respiratory system. OVERVIEW OF PATHOPHYSIOLOGIC MECHANISMS IN CHRONIC VENTILATORY INSUFFICIENCY