Ventilatory Muscle Function

Normal ventilation is accomplished by the respiratory muscles overcoming forces that oppose volume movement: lung and chest wall compliance, and airway resistance. In general, ven-tilatory muscle dysfunction is a result of an imbalance between energy demands and energy availability. Energy demands are defined by the interrelationship among ventilatory muscle strength, work of breathing, and ventilatory muscle efficiency. Energy availability is determined by the relationship between energy supply and the body's energy stores. In mechanically ventilated patients, dyssynchrony between the patient and ventilator is commonly considered a result of the patient's in-ability to work with the mechanical ventilator. However, dys-synchrony can be a result of problems with the artificial air-way, the mechanical ventilator, or the patient. The cause of dyssynchrony is frequently not a result of a change in the pa-tient's clinical status. A lthough having large reserves, ventilatoryAlamuscles are as susceptible to dysfunction as are other voluntary muscles. ' Dysfunction is com-monly referred to as fatigue, although fatigue is the extreme of dysfunction and difficult to identi-fy clinically. In controlled laboratory settings, fa-tigue is easily defined, the factors causing it easi-ly outlined and its presence easily identified.2 However, at the bedside, numerous factors affect ventilatory muscle function, making diagnosis of fatigue questionable. This article discusses venti-latory muscle function and dysfunction and those factors that create or worsen dysfunction during mechanical ventilation. The Ventilatory Muscles Inspiration is accomplished under normal resting conditions by contraction of the diaphragm, sca