The anaesthetic machine is designed to enable oxygenation, safe delivery of both gases and volatile anaesthetic agents and appropriate ventilation. Modern machines are complex pieces of equipment dependent on electronic hardware and software. Many incorporate electronic monitoring systems for both the patient and the gas delivery. Breathing systems and ventilators may be included to form a ‘workstation’. Familiarity with the anaesthetic machine is fundamental for safe practice. It must be checked before every operating list.

Gas supply is usually via pipelines drawn from a central supply or from cylinders. Pressure and flow of gases to the patient are regulated by valves and flowmeters. The concentration of volatile agents is controlled by individual vaporisers. Anaesthetic gas scavenging systems are designed to safely remove anaesthetic waste.

Anaesthetic breathing systems or circuits consist of several components: fresh gas, inspiratory and expiratory limbs, a pressure limiting valve and a reservoir bag. They may also have a carbon dioxide absorber and unidirectional valves. They are classified as either circle systems or semi-closed rebreathing systems. Ventilators can be considered as constant or non-constant generators of pressure or flow. Both are described in detail.

Anaesthetic breathing systems are used to deliver oxygen and anaesthetic gases to patients and remove carbon dioxide. A breathing system is most commonly attached to an anaesthetic machine, which is designed to deliver the fresh gas flow to the patient via a facemask, a supraglottic device or an endotracheal tube. The breathing system used can affect the composition of the gas and volatile anaesthetic mixture inhaled by the patient, and so it is important to understand the different breathing systems used in anaesthesia. This chapter describes the key components of the different breathing systems and explores the benefits and disadvantages of the circuits in the Mapleson classification.