How do I perform a manual leak test on an anaesthesia machine?

Manual Leak Test of the Anaesthesia Machine

Perform a pressure leak test on the breathing system by occluding the patient-end and compressing the reservoir bag to achieve 20-60 cmH₂O pressure, then perform a vaporiser leak test only on basic 'Boyle's' machines (not modern workstations), followed by a comprehensive two-bag test to verify the entire system integrity. 1

Breathing System Leak Test

The fundamental leak test involves the following steps 1:

• Visually inspect all breathing systems for correct configuration and assembly
• Secure all connections using 'push and twist' technique to the anaesthetic machine
• Check for obstructions in reservoir bags and breathing system, ensuring no foreign material is present
• Perform pressure leak test: Occlude the patient-end and compress the reservoir bag to generate pressure between 20-60 cmH₂O
• Observe for leaks: The system should maintain pressure without visible or audible leaks

Vaporiser Leak Test - Critical Caveat

Important warning: Manual leak testing of vaporisers should only be performed on basic 'Boyle's' machines and may be harmful to many modern anaesthetic workstations. Always refer to manufacturer's recommendations before performing this test 1.

For appropriate machines, the vaporiser leak test procedure is 1:

1. Set oxygen flow to 5 L/min with vaporiser turned OFF
2. Temporarily occlude the common gas outlet
3. Observe: There should be no leak from any part of the vaporiser and the flowmeter bobbin (if present) should dip
4. If multiple vaporisers present: Turn each vaporiser ON in turn and repeat this test
5. After testing: Ensure vaporisers and flowmeters are turned off

Common Pitfall

Failure to repeat the leak test when changing vaporisers during use is a common cause of critical incidents 1. Some modern anaesthetic workstations automatically test vaporiser integrity, making manual testing unnecessary or potentially harmful.

Two-Bag Test - Comprehensive System Check

After individual component testing, perform the two-bag test to verify the entire system 1:

1. Attach the patient-end of the breathing system (including angle piece and filter) to a test lung or bag
2. Set fresh gas flow to 5 L/min and ventilate manually
3. Verify patency: Check the whole breathing system is patent and unidirectional valves are moving (if present)
4. Test APL valve function by squeezing both bags
5. Turn on ventilator to ventilate the test lung
6. Turn off or minimize fresh gas flow
7. Open and close each vaporiser in turn - there should be no loss of volume in the system

Evidence Quality Considerations

Research comparing leak test methods demonstrates that negative pressure tests have superior sensitivity (100%) compared to positive pressure tests (92% sensitivity) for detecting low-pressure system leaks 2. However, positive pressure tests can detect specific problems like defective oxygen flush devices that negative pressure tests may miss 3. The low-flow leak test shows better performance (97% sensitivity, 84% specificity) than oxygen flush leak test (78% sensitivity, 80% specificity) 4.

Despite this research evidence, the Association of Anaesthetists guideline (2012) recommends the positive pressure approach described above as the standard method [1-1], which remains the accepted clinical practice standard.

Additional Critical Checks

When performing leak tests, always verify 1:

• Breathing system changes: Confirm integrity and correct configuration whenever changing systems during a case or list
• Paediatric lists: Particularly important when breathing systems are changed frequently
• Auxiliary common gas outlets (ACGO): Exercise particular care - incidents of patient harm have resulted from misconnection or misselection
• Gas monitoring lines: Often cause significant leaks; ensure proper attachment and blank off unused sampling ports