What is the proper use and setting of the Adjustable Pressure Limiting (APL) valve in an anesthetic machine?

APL Valve in Anesthetic Machine

The APL (Adjustable Pressure Limiting) valve must be checked before each case to ensure it can be fully opened and closed, and its function should be tested during the two-bag test by squeezing both bags to confirm proper pressure regulation during manual ventilation. 1

Function and Purpose

The APL valve is a critical safety component that regulates pressure within the anesthesia circuit during manual bag-valve ventilation. 2 It prevents excessive airway pressures that could cause barotrauma while allowing controlled positive pressure ventilation.

Pre-Use Checking Protocol

According to the Association of Anaesthetists of Great Britain and Ireland guidelines, the APL valve requires systematic checking: 1

For Alternative Breathing Systems

• Perform an occlusion test on the inner tube for Bain-type and circle co-axial systems 1
• Verify the APL valve can be fully opened and closed 1

Two-Bag Test (Mandatory)

This test must be performed after checking the breathing system, vaporizers, and ventilator individually: 1

1. Attach the patient-end of the breathing system (including angle piece and filter) to a test lung or bag 1
2. Set fresh gas flow to 5 L/min and ventilate manually 1
3. Check APL valve function by squeezing both bags - this confirms the valve opens appropriately to release excess pressure 1
4. Turn on the ventilator, then turn off or minimize fresh gas flow 1
5. Open and close each vaporizer in turn - there should be no loss of volume in the system 1

Clinical Use Settings

During Manual Ventilation

• The APL valve must be adjusted to allow adequate ventilation while preventing excessive airway pressures 2
• Typical settings range from 1-20 cmH₂O depending on patient size and lung compliance 3

During Mechanical Ventilation

• The APL valve should be fully closed or the circuit switched to ventilator mode (general anesthesia knowledge)
• Failure to close the APL valve during mechanical ventilation will result in inadequate tidal volumes and hypoventilation

Critical Safety Considerations

Machine-Specific Performance Variations

Not all APL valves perform identically. Research has demonstrated significant differences between manufacturers: 3

• The GE Aisys CS2 APL valve shows non-linear behavior with sharp pressure increases at 8-10 cmH₂O settings 3
• GE Aisys CS2 valves can generate airway pressures up to 27 cmH₂O higher than the set pressure 3
• The Draeger Fabius APL valve demonstrates more predictable near-linear pressure control 3

Common Failure Modes

Physical damage to the APL valve can cause complete ventilation failure. A reported case involved a crack in the APL valve that allowed pressure release from the circuit, resulting in ineffective bag-valve-mask ventilation of an infant. 2 This emphasizes why the pre-use check is non-negotiable.

APL valve failure during induction of anesthesia requires immediate recognition and troubleshooting. 4 If manual ventilation becomes ineffective:

• Check that the APL valve is not fully open (which would prevent pressure buildup) 2
• Inspect for visible cracks or damage to the valve housing 2
• Verify the valve is properly seated and not obstructed 2
• Have backup ventilation equipment immediately available

Algorithm for Intraoperative APL Valve Problems

If inadequate manual ventilation occurs: 2

1. Verify APL valve position (should be partially closed, typically 10-20 cmH₂O)
2. Check for circuit disconnections or leaks
3. Inspect APL valve for visible damage
4. Switch to backup ventilation method (Ambu bag, different machine)
5. Replace or repair the APL valve before continuing

Integration with Scavenging

The APL valve exhaust must be connected to the scavenging system to prevent operating room pollution. 1 Verify that scavenging tubing is attached to the appropriate exhaust port of the breathing system before use. 1