General anesthesia can have profound effects on pulmonary function. Some of these are direct effects of anesthetic drugs on the respiratory system; others result from associated aspects of general anesthesia such as altered body position, intubation, mechanical ventilation, and the use of neuromuscular blocking agents and other drugs. Alterations in the cardiovascular system caused by general anesthesia can also affect pulmonary function by changing the amount and distribution of pulmonary blood flow.
It is difficult to generalize the effects of anesthesia on pulmonary function because of differences between anesthetics and the routes of administration employed, differences in the body positions and ventilatory modes used in various surgical procedures, the unavailability of some kinds of data from normal conscious and anesthetized human subjects, and differences in the age and cardiopulmonary status of the patients included in clinical studies. Nonetheless, there is agreement on a number of pulmonary consequences of general anesthesia, although the mechanisms responsible for these alterations frequently remain controversial because of difficulty obtaining data from human subjects and because conclusions drawn from animal experiments may not correspond exactly to human physiology.
General anesthesia may have major effects on the mechanics of the lung and the chest wall, the volume and distribution of alveolar ventilation, the cardiac output and the distribution of pulmonary blood flow, the matching of ventilation and perfusion, and the control of breathing. General anesthesia may also affect the airways, and the transport and diffusion of oxygen and carbon dioxide.
EFFECTS OF ANESTHESIA ON RESPIRATORY MECHANICS
General anesthesia has significant effects on the mechanics of the respiratory system. It alters the functional residual capacity (FRC), respiratory muscle function, the shape and motion of the lungs and chest wall, and it may affect the diameter of the airways.
As noted in Chapter 26, the FRC is determined by the balance between the inward recoil of the lungs and the outward recoil of the chest wall when the respiratory muscles are relaxed. When a normal conscious person changes from the upright to the supine position, the FRC decreases by about one-third. For a typical 70-kg person that would represent a decrease of about 1 l, from an FRC of 3 l in the upright position to an FRC of 2 l in the supine position. As discussed in Chapter 26, this decrease in FRC that occurs when a person changes from a standing or upright position to the supine position is apparently a result of a decreased effect of gravity pulling downward on the diaphragm in the supine position. This decreases the outward elastic recoil of the chest wall and decreases the lung volume at which the outward recoil of the chest wall is equal and opposite to the inward recoil of the lungs.
A number of studies have demonstrated a significant ...