Optimizing Perioperative Ventilation Support with Adequate Settings of Positive End-Expiratory Pressure

1.1 Mechanical ventilation Mechanical ventilation is often employed to replace spontaneous breathing of patients under general anesthesia. Even after operation, the patient still needs ventilation support until the respiratory muscles regain full function. A ventilator delivers a certain amount of air flow through a facial mask or tracheal tube to the patient whose respiratory system fails to function properly due to the effects of anesthetics or diseases. Based on the difference in breath initiation, mechanical ventilation can be divided into two categories: controlled ventilation and assisted ventilation. In this chapter, we focus on controlled mechanical ventilation, under which the patient is not able to trigger a valid breath and the ventilator overtakes all the workload of respiratory muscles. Respiratory parameters such as respiratory rate (RR), inspiratory–to-expiratory time ratio (I:E), tidal volume (Vt) (or minute volume) are controlled by the ventilator. Traditionally, controlled mechanical ventilation can either be volume controlled (VCV) or pressure controlled (PCV). Ideal respiratory signals obtained in a healthy human during VCV and PCV are shown in Fig. 1. In the VCV mode, a patient receives constant flow from the ventilator until a preset Vt is reached. A severe drawback of VCV is missing control of the peak airway pressure. Airway pressure (Paw) depends on respiratory system compliance and resistance. In patients with certain lung diseases, such as acute lung injury (ALI), the same setting of Vt as in patients with healthy lungs may lead to a higher peak Paw with the potential to further injure the lung. Therefore, VCV is often applied with a pressure limitation. Once the peak Paw rises above this limit, the ventilator will stop delivering gas even if the preset Vt is not yet reached. In the PCV mode, a maximum airway pressure (Pmax) is defined. Inspiration ends when Pmax is reached i.e. the flow driven by the pressure difference decreases to zero. PCV may be superior to VCV in patients requiring one-lung