APL Valve Adjustment Does Not Convert a Closed System to an Open System
Adjusting the APL (Adjustable Pressure Limiting) valve does not change the fundamental classification of an anesthesia breathing system from closed to open—it only regulates pressure within the existing circuit configuration. 1
Understanding System Classification vs. APL Valve Function
The classification of anesthesia systems (closed, semiclosed, semiopen, open) is determined by the technical construction and fresh gas flow rates, not by APL valve position. 2 The APL valve is simply a pressure-regulating safety device within whatever system type you're using.
Key Functional Points:
The APL valve regulates circuit pressure during manual ventilation by allowing excess gas to exit to the scavenging system when pressure exceeds the set threshold 3, 4
System classification depends on: the breathing circuit design (rebreathing vs. non-rebreathing), presence of CO2 absorption, and fresh gas flow relative to minute ventilation 2, 5
Opening the APL valve fully allows gas to vent more freely to scavenging but doesn't eliminate the circuit's fundamental characteristics (reservoir bag, unidirectional valves, CO2 absorber remain functional) 1
Clinical Implications of APL Valve Position
During Manual Ventilation:
- The APL valve must be appropriately adjusted (partially closed) to allow adequate pressure generation for effective bag-mask ventilation 3, 6
- Test APL function during the two-bag test by squeezing both bags to confirm proper pressure regulation 1, 3
During Mechanical Ventilation:
- The APL valve must be fully closed or the circuit switched to ventilator mode, otherwise inadequate tidal volumes and hypoventilation will occur 3
- Failure to close the APL valve is a common critical incident resulting in patient harm 3
Common Pitfall to Avoid
The most dangerous error is leaving the APL valve open during mechanical ventilation, which causes circuit pressure to vent inappropriately, preventing adequate ventilation. 3 This represents operator error in valve positioning, not a change in system classification.
Pre-Use Safety Checks:
- Verify the APL valve can be fully opened and closed before each case 1, 3
- Perform occlusion testing on co-axial systems to confirm APL valve integrity 1
- Ensure scavenging tubing is properly connected to the APL valve exhaust port to prevent operating room pollution 3
Equipment-Specific Considerations
- Some APL valves demonstrate non-linear pressure behavior (particularly GE Aisys CS2), where airway pressures can exceed set values by up to 27 cmH2O, making safe manual ventilation difficult 6
- Physical damage to APL valves (cracks in housing) can cause complete loss of circuit pressure and ineffective ventilation 4
- Closed scavenging systems without active suction may inadvertently add PEEP (up to 8.62 cmH2O in GE Avance CS2), which can be mitigated by manufacturer-recommended modifications 7