Pressure Changes in a Continuous-Flow Anaesthesia Machine

1. High-Pressure Section - Cylinder (~2000 psig)

• E-cylinders (backup supply on the back of the machine) store O2 and air at approximately ~2000 psig, and N2O at ~745 psig.
• These are the only high-pressure elements in the whole machine.
• The cylinder connects to the machine via the hanger yoke assembly, which has:
  • A gas-tight seal
  • Unidirectional (check) valves to prevent backflow
  • The Pin Index Safety System (PISS) to prevent wrong-gas connection (each gas has a unique two-pin pattern on the yoke)
• A cylinder pressure gauge displays the tank pressure so the clinician can estimate remaining supply.
• Immediately downstream, a first-stage pressure regulator drops this variable, high cylinder pressure down to a fixed ~45-60 psig - the same range as the hospital pipeline. This makes cylinder and pipeline inputs compatible and interchangeable.

Key point: During normal operation, the high-pressure section is largely dormant because the hospital pipeline is the primary gas source. The cylinder is a backup.

2. Intermediate-Pressure Section - Pipeline / Post-Regulator (~50-55 psig)

• The hospital central pipeline delivers gas at 50-55 psig directly to the machine via colour-coded, DISS (Diameter Index Safety System) quick-connect wall outlets.
• Each pipeline inlet has:
  • A filter to trap particulates
  • A check valve to prevent backflow into the hospital system
  • A pipeline pressure gauge (visible on the front of the machine)
• Because the cylinder regulator is set to ~45-50 psig (just below pipeline pressure ~50-55 psig), the machine preferentially draws from the pipeline when both are connected. The cylinder only kicks in if pipeline pressure drops.
• From here, intermediate-pressure gas feeds:
  • The flow control valves (the knobs the anaesthetist turns)
  • The O2 flush valve (delivers ~35-75 L/min directly to the circuit, bypassing the flowmeters)
  • Fail-safe / oxygen supply failure protection devices - these monitor O2 pressure and shut off or proportionally reduce other gases (N2O, air) if O2 supply pressure falls
  • The O2:N2O proportioning system (e.g., Link-25) which mechanically prevents delivering a hypoxic mixture
• Some machines also have second-stage regulators here, which further drop pressure from ~50 psig to 14-35 psig to give the flow control valves a more stable input regardless of pipeline pressure fluctuations.

3. Low-Pressure Section - Flowmeters to Fresh Gas Outlet (near atmospheric / a few cmH2O)

• Begins at the flow control valves and ends at the fresh gas outlet (common gas outlet).
• Pressure here is just slightly above atmospheric - only a few cmH2O above ambient.
• Contains:
  • Flowmeters (Thorpe tubes) or electronic flow sensors - these measure and set the flow rate (L/min) of each gas at near-atmospheric pressure
  • Vaporizer - anaesthetic agent is added here; the low pressure and precise gas flow allow accurate agent delivery
  • Check valves / backpressure relief valves to protect flowmeters from pressure surges (e.g., from ventilator cycling)
• The combined gas mixture (O2 + N2O or air + anaesthetic vapour) exits at the fresh gas outlet into the breathing circuit.

Pressure Summary Table

Why Does This Staged Reduction Matter?

1. Safety: A sudden 2000 psig surge reaching the patient would be fatal. Staged regulation prevents this.
2. Accuracy: Flowmeters only work accurately at low, stable pressure. Regulators ensure consistent input.
3. Preferential use of pipeline: Setting the cylinder regulator just below pipeline pressure means the machine automatically switches to the cylinder only when the pipeline fails - a passive fail-safe.
4. Fail-safe systems: The intermediate pressure section houses the O2 failure protection device. If O2 pressure drops, N2O is automatically cut off, preventing a hypoxic mixture from reaching the patient.