How to convert a closed system to a semi-closed system in an anesthesia machine?

Converting a Closed System to a Semi-Closed System in an Anesthesia Machine

To convert from a closed to a semi-closed system, simply increase the fresh gas flow (FGF) rate above the patient's metabolic consumption—typically setting FGF to 1-3 L/min—and ensure the adjustable pressure limiting (APL) valve is partially open to allow excess gas to escape through the scavenging system. 1

Understanding the Key Difference

The fundamental distinction between closed and semi-closed systems is the fresh gas flow rate relative to patient uptake:

• Closed system: FGF equals exactly what the patient consumes metabolically (typically 200-300 mL/min O₂), with no excess gas vented 2
• Semi-closed system: FGF exceeds patient uptake, with surplus gas eliminated via the APL valve into the scavenging system 2, 3

Step-by-Step Conversion Process

1. Adjust Fresh Gas Flow Rate

• Increase FGF to 1-3 L/min (or higher) from the minimal closed-circuit flow 3
• At FGF of 1 L/min, you achieve "minimal flow anesthesia," which is effectively semi-closed 2
• At FGF of 3 L/min or greater, you have a standard semi-closed system 3
• Higher FGF rates (5-10 L/min) provide faster control of anesthetic depth but increase costs and waste 4, 3

2. Configure the APL Valve

• Ensure the APL valve can be fully opened to allow excess gas to escape 1
• Verify the valve is functioning properly by squeezing the reservoir bag during manual ventilation 1
• The APL valve must be connected to the scavenging system to prevent operating room pollution 1

3. Verify System Integrity

• Perform a two-bag test to confirm proper circuit function 1:
  • Attach a test lung to the patient end
  • Set FGF to 5 L/min and ventilate manually
  • Check that unidirectional valves are moving (if present)
  • Test APL valve function by squeezing both bags
  • Verify no volume loss when vaporizers are opened/closed with ventilator running 1

4. Confirm Scavenging System Function

• Verify the scavenging system is switched on and functioning 1
• Ensure tubing is properly attached to the exhaust port of the breathing system 1
• Check for leaks in gas sampling lines, as these are common sources of system leaks 1

Clinical Considerations

Advantages of Semi-Closed vs. Closed Systems

• Easier control of anesthetic depth: Higher FGF allows faster changes in inspired anesthetic concentration 4, 2
• No need for continuous adjustment: Unlike closed systems that require constant manual titration of gas delivery 2
• More forgiving: Less risk of hypoxia or awareness from calculation errors 2
• Standard practice: Closed systems are not routine due to technical complexity 2

Flow Rate Selection Strategy

• 0.5-1 L/min (minimal flow): Maximizes economy and humidity, requires careful monitoring 2, 3
• 1-3 L/min: Balances cost savings with ease of control 3, 5
• 3-5 L/min: Standard semi-closed approach, easier anesthetic control 3
• >5 L/min: Fastest control but highest cost and waste 3

Common Pitfalls to Avoid

• Forgetting to open the APL valve: This can cause dangerous pressure buildup in a closed system when FGF is increased 1
• Inadequate scavenging: Increased FGF without proper scavenging leads to operating room pollution 1
• Ignoring CO₂ absorbent: Semi-closed systems still require functional CO₂ absorption; check absorbent color and supply 1, 6
• Assuming immediate concentration changes: Even with higher FGF, time to target concentration varies by machine type and internal volume 4