The anaesthetic machine: basic overview

What the anaesthetic machine is (and what it is not)

• Purpose: to deliver oxygen and anaesthetic gases/vapours safely, and to ventilate the patient via a breathing system.
• It is part of a wider setup: patient monitoring, suction, airway equipment, drugs, and a plan for emergencies are equally important.
• Key idea: the machine mixes gases, adds vapour (if used), and delivers them through a breathing circuit with safety features to reduce the chance of hypoxia (low oxygen).

Main building blocks (big picture)

• Gas supply: pipeline (wall) and cylinders (backup). Usually oxygen (O2), air, and nitrous oxide (N2O).
• Pressure regulation and safety: reduces high cylinder pressure to a safe working pressure; includes alarms and safety valves.
• Flow control: flowmeters/controls set how much of each gas is delivered (fresh gas flow).
• Vaporiser: adds volatile agent (e.g., sevoflurane) to the fresh gas flow (only if you are using a volatile).
• Breathing system: connects machine to patient; includes reservoir bag, APL valve, and CO2 absorber (if circle system).
• Ventilator: provides mechanical breaths; can switch between manual/spontaneous and ventilator modes.
• Scavenging: removes waste anaesthetic gases from the breathing system to protect staff.

Gas supplies: pipeline vs cylinders (what to know on day 1)

• Pipeline supply is the usual source; cylinders are the backup if pipeline fails.
• Know where the oxygen cylinder is on your machine and how to open it (and how to close it afterwards).
• Oxygen failure is time-critical: always know how to rapidly provide oxygen (e.g., cylinder oxygen, self-inflating bag, or alternative source).
• If pipeline pressure is lost, many machines automatically switch to cylinder supply if the cylinder is open (check local machine behaviour).
• Practical tip: keep the oxygen cylinder present and sufficiently full; do not rely on an empty or missing cylinder.

Fresh gas flow (FGF): simple definition and why it matters

• Fresh gas flow = the flow of gas coming from the machine into the breathing system (e.g., O2 + air ± N2O).
• Higher FGF: faster wash-in/wash-out, less rebreathing, but more gas use and more theatre pollution if leaks.
• Lower FGF (especially with circle system): more rebreathing and greater dependence on CO2 absorber and circuit integrity.
• New starter rule: when in doubt (e.g., uncertain circuit leak), increase FGF temporarily and troubleshoot.

Vaporisers: basics and safe handling

• A vaporiser adds a controlled amount of volatile anaesthetic to the gas flow (e.g., sevoflurane).
• Only one vaporiser should be ON at a time; ensure it is seated/locked correctly on the back bar.
• Fill with the correct agent only; use the correct keyed filling system and avoid overfilling.
• Turn vaporiser OFF when not in use (especially during transfer, emergence, or if switching to TIVA).
• If you smell agent strongly or see unexpected depth of anaesthesia, consider a leak, incorrect setting, or vaporiser left on.

Breathing systems: what you will actually touch

• Common in theatre: circle system (allows rebreathing via CO2 absorber) and Mapleson circuits (often used for short cases/transfer).
• Circle system key parts: inspiratory limb, expiratory limb, one-way valves, reservoir bag, APL valve, CO2 absorber, and patient connection (HMEF/filter).
• APL valve (adjustable pressure limiting valve): in manual/spontaneous mode it controls pressure and allows excess gas to escape to scavenging.
• Reservoir bag: lets you feel compliance, assist ventilation, and acts as a buffer for gas flow.
• CO2 absorber: removes CO2 from exhaled gas to allow safe rebreathing; exhausted absorber can cause rising inspired CO2.

Ventilator vs manual: switching safely

• Manual/spontaneous mode: you ventilate with the bag; APL valve matters; ventilator is not delivering breaths.
• Ventilator mode: the ventilator delivers breaths; APL valve may be bypassed depending on machine design (know your local machine).
• Before switching to ventilator: confirm circuit connected, ventilator settings appropriate (mode, tidal volume/pressure, rate, PEEP), and alarms set.
• When switching back to manual: open/adjust APL appropriately and ensure you can ventilate with the bag immediately.
• Always confirm ventilation with capnography (ETCO2 waveform) and chest movement, not just the ventilator screen.

Scavenging and suction: staff safety and patient safety

• Scavenging removes waste anaesthetic gas; ensure it is connected and not kinked or disconnected.
• If scavenging is blocked or misconnected, circuit pressures can behave unexpectedly (depending on system).
• Suction should be checked and ready before induction (especially for airway soiling, regurgitation, ENT/dental cases).

The minimum safe pre-use approach (new starter friendly)

• Use the local pre-use checklist every time (do not rely on memory).
• Confirm oxygen supply: pipeline connected and pressure present; oxygen cylinder present and checked (and know how to use it).
• Check breathing system integrity: correct circuit, tight connections, filter/HMEF in place, no obvious cracks, reservoir bag attached.
• Leak test (as per machine/circuit type): ensure the circuit holds pressure appropriately and the APL valve functions.
• Check ventilator: power on, correct mode, alarms enabled, test breath delivered, bellows/piston moving as expected.
• Check vaporiser(s): correct agent, adequate fill, seated/locked, only intended vaporiser ON.
• Check monitors: ECG, NIBP, SpO2, capnography, agent monitoring (if available), oxygen analyser (if present).
• Have backups ready: self-inflating bag-valve-mask (BVM), airway equipment, and emergency drugs accessible.

Common first-time scenarios: what to do

• No capnography trace after intubation: immediately check patient (chest rise, auscultation), confirm tube position, check circuit connection, check CO2 sampling line, and consider oesophageal intubation; ventilate with 100% oxygen while troubleshooting.
• High airway pressure alarm: check patient (bronchospasm, coughing, obstruction), then circuit (kinked tube, closed APL in manual, blocked filter, water in circuit), then ventilator settings; switch to manual ventilation to assess compliance.
• Low pressure/leak alarm: check disconnections at patient end first, then circuit connections, cuff leak, APL position, and sampling line; increase FGF temporarily and ventilate with 100% oxygen while fixing.
• Unexpectedly light anaesthesia: check vaporiser ON and filled, check fresh gas flows, check circuit leak, check IV access/drug delivery; treat patient signs promptly and call for help if needed.
• Pipeline oxygen failure: call for help, switch to cylinder oxygen (or ensure it is supplying), turn off nitrous oxide, ventilate with 100% oxygen, and consider using a self-inflating bag if machine function is uncertain.