General anesthesia and respiratory mechanics

The study of respiratory mechanics concerns the motion of structures that control the movement of air into and out of the lungs. One set of structures controls intrathoracic pressure, which drives gas flow; these structures comprise the chest wall. Another system maintains the patency of the extrathoracic airway proximal to the glottis; the components of this system comprise the upper airway. This chapter will describe the functional consequences of anesthetic-induced alterations in the activation of skeletal muscles that control the chest wall and upper airway (often referred to as respiratory muscles). Anesthetics also have important effects on smooth muscle lining the airways, which also regulates gas flow within the lungs. This topic is reviewed elsewhere [1] and is beyond the scope of this chapter. Historically, simple observation of external respiratory system motion provided a powerful tool to guide the proper administration of anesthesia [2]. More recently, alterations in the control of respiratory muscles that lead to changes in respiratory mechanics have been invoked to explain much of the impairment of gas exchange observed during and after clinical anesthesia. Thus, anesthetic-induced changes in respiratory muscle activation and respiratory mechanics can have profound clinical consequences.