General anesthesia impairs breathing by two mechanisms, it decreases the impetus to breath (central respiratory depression), and it leads to upper airway obstruction. Additionally, neuromuscular blockers, which are often administered during general anesthesia, paralyze the muscles of respiration. For these reasons, breathing may be supported or controlled during anesthesia to ensure adequate minute ventilation. The anesthesia provider can create intermittent positive pressure in the breathing circuit by rhythmically squeezing the reservoir bag.

Ventilatory support is often provided in this way for short periods of time, especially during the induction of anesthesia. During mechanical ventilation, a selector switch is toggled to disconnect the reservoir bag and APL valve from the breathing circuit and connect an anesthesia ventilator instead. Anesthesia ventilators provide a means tomechanically control ventilation, delivering consistent respiratory support for extended periods of time and freeing the anesthesia provider’s hands and attention for other tasks. Most surgical patients have normal pulmonary mechanics and can be adequately ventilated with an unsophisticated ventilator designed for ease of use. But, high performance anesthesia ventilators allow safe and effective ventilation of a wide variety of patients, including neonates and the critically ill.

Most anesthesia ventilators are pneumatically powered, electronically controlled, and time cycled. All can be set to deliver a constant tidal volume at a constant rate (volume control). Many can also be set to deliver a constant inspiratory pressure at a constant rate (pressure control). All anesthesia ventilators allow spontaneous patient breaths between ventilator breaths (intermittent mandatory ventilation, IMV), and all can provide PEEP during positive pressure ventilation (note that in some older systems PEEP is set using a PEEP-valve integrated into the expiratory limb of the breathing circuit, and is not actively controlled by the ventilator). In general, anesthesia ventilators do not sense patient effort, and thus do not provide synchronized modes of ventilation, pressure support, or continuous positive airway pressure (CPAP).

As explained above, the anesthesia delivery system conserves anesthetic gases by having the patient rebreathe previously exhaled gas. Unlike intensive care ventilators, which deliver new gas to the patient during every breath, anesthesia ventilators function as a component of the anesthesia delivery system and maintain rebreathing during mechanical ventilation. In most anesthesia ventilators, this is achieved by incorporating a bellows assembly (see Fig. 4). The bellows assembly consists of a distensible bellows that is housed in a clear rigid chamber. The bellows is functionally equivalent to the reservoir bag; it is attached to, and filled with gas from, the breathing circuit.

During inspiration, the ventilator injects drive gas into the rigid chamber; this squeezes the bellows and directs gas from the bellows to the patient via the inspiratory limb of the breathing circuit. The drive gas, usually oxygen or air, remains outside of the bellows and never enters the breathing circuit. During exhalation, the drive gas within the rigid chamber is vented to the atmosphere, and the patient exhales into the bellows through the expiratory limb of the breathing circuit. The bellows assembly also contains an exhaust valve that vents gas from the breathing circuit to the scavenger system. This ventilator exhaust valve serves the same function during mechanical ventilation that the APL valve serves during manual or spontaneous ventilation. However, unlike the APL valve, it is held closed during inspiration to ensure that the set tidal volume dispensed from the ventilator bellows is delivered to the patient. Excess gas then escapes from the breathing circuit through this valve during exhalation.

The tidal volume set on an anesthesia ventilator is not accurately delivered to the patient; it is augmented by fresh gas flow from the anesthesia machine, and reduced due to compression-loss within the breathing circuit. Fresh gas, flowing into the breathing circuit from the anesthesia machine, augments the tidal volume delivered from the ventilator because the ventilator exhaust valve, which is the only route for gas to escape from the breathing circuit, is held closed during inspiration. For example, at a fresh gas flow rate of 3 L_min_1 (50mL_s_1), and ventilator settings of 10 breaths min_1 and an I/E ratio of 1:2, the delivered tidal volume is augmented by 100mL per breath (2 s per breath _ 50mL_s_1). Conversely, the delivered tidal volume is reduced due to compression loss within the breathing circuit. The magnitude of this loss depends on the compliance of the breathing circuit and the peak airway pressure. Circle breathing circuits typically have a compliance of 7–9 mL_cm_1 H2O (70–90 mL_kPa_1), which is significantly higher than the typical 1–3 mL_cm_1 H2O (10–30 mL_kPa_1) circuit compliance of intensive care ventilators, because of their large internal volume. For example, when ventilating a patient with a peak airway pressure of 20 cm H2O (2 kPa) using an anesthesia ventilator with a breathing circuit compliance of 10 mL_cm H2O, delivered tidal volume is reduced by 200 mL per breath.