Chapter 24

In the early years following the first demonstration of anesthesia with an inhalation device, different inhalation anesthetic agents were developed and tested in spontaneously breathing subjects. However, the increasing complexity of surgical procedures, the discovery and use of neuromuscular blocking as well as intravenous anesthetic agents, and the need to protect the airways resulted in the need for mechanical ventilation during general anesthesia. Accordingly, anesthesia ventilators were developed to match the particular needs of general anesthesia, which differ from those in other settings, such as the intensive care unit (ICU). Moreover, modern anesthesia is not restricted to the intraoperative period and mechanical ventilation may be required preoperatively and postoperatively.

This chapter provides a comprehensive review of aspects related to the principles and practice of mechanical ventilation during general anesthesia, as well as during the perioperative period.

General anesthesia has several effects on respiratory function, most notably on control of breathing and the activity of respiratory muscles, which influence the distributions of ventilation and perfusion within the lungs. Moreover, surgery-related positioning of patients, as well as manipulation or displacement of intraabdominal and thoracic organs may further affect ventilation and perfusion, with deterioration in pulmonary gas exchange.

### Control of Breathing and General Anesthesia

#### Objectives of the Control of Breathing

The main objective of control of breathing is the maintenance of blood gases, especially of arterial carbon dioxide (PaCO2), which is kept within a relatively restricted range. Even a small increase in PaCO2 may result in a significant increase in minute ventilation. The output of the respiratory center is less sensitive to reductions in partial pressure of arterial oxygen (PaO2), but hypoxia may enhance the response to PaCO2.1 Another objective of the control of breathing is to maintain the brain pH. Because CO2 is highly diffusible across the blood–brain barrier, this is achieved through modulation of alveolar ventilation, which regulates PaCO2. Last but not least, control of breathing aims at optimizing breathing frequencies and maximizing the work output of the respiratory muscles to maintain adequate gas exchange.2

#### Control of Breathing

The respiratory rhythm originates from complex neuronal network interactions throughout the nervous system.3 This network consists of smaller and larger networks that interact in generating different breathing patterns, including the regular breathing (eupnea), but also sighs and gasping. The group of neurons responsible for the control of breathing is dispersed across a region in the brainstem termed the respiratory centers. These centers are located bilaterally in the reticular formation of the medulla oblongata and pons, beneath the floor of the fourth ventricle, and are grouped into inspiratory, pneumotaxic, and expiratory areas.4

The inspiratory area is located in the dorsal part of the medulla and generates rhythmic cycles that are transmitted to the diaphragm, upon ...

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