Safety Features of the Anesthesia Machine
Modern anesthesia machines incorporate multiple built-in safety features designed to prevent hypoxic gas delivery, equipment failure, and patient harm, and these must be systematically checked before each use according to standardized protocols. 1
Core Safety Features
Gas Supply Safety Systems
Oxygen Failure Warning Device
- Automatically sounds an audible alarm (minimum 7 seconds) when oxygen supply pressure drops 1
- Linked to a gas shut-off mechanism that stops flow of other gases when oxygen fails
- Must be checked weekly by disconnecting the oxygen hose while flowmeter is on, with written documentation 1
- Daily checks use a "tug test" to confirm proper pipeline connections without repeatedly disconnecting hoses (which damages equipment) 1
Anti-Hypoxia Device (Oxygen Ratio Monitor Controller)
- Ensures minimum 25% oxygen delivery when nitrous oxide is used 1
- Test by turning on nitrous oxide first—oxygen should automatically flow
- Then turn off oxygen—nitrous oxide flow should stop 1
Emergency Oxygen Bypass (Oxygen Flush)
- Delivers 100% oxygen at 35-75 L/min directly to common gas outlet 1
- Must function without significant pressure drop in pipeline supply 1
- Should cease operation immediately when released 1
Vaporizer Safety Features
Pin-Index Safety System & Keyed Filling Systems
- Prevents incorrect volatile agent filling
- Locking mechanisms must be fully engaged 1
- Critical caveat: Tilting a vaporizer can cause dangerously high vapor concentrations 1
- Vaporizers must remain upright at all times 1
Vaporizer Interlock System
- Prevents simultaneous activation of multiple vaporizers
- Modern workstations may automatically test vaporizer integrity 1
- Important: Manual leak testing should only be performed on basic 'Boyle's' machines—it may harm modern workstations 1
Breathing System Safety
Pressure Relief Valves
- Prevent barotrauma from excessive circuit pressure
- Must be tested at set pressure during pre-use check 1
Unidirectional Valves (in circle systems)
- Prevent rebreathing of exhaled gases
- Checked during two-bag test to ensure proper movement 1
Auxiliary Common Gas Outlet (ACGO) Protection
- Major hazard: Patient harm has resulted from misconnection to ACGO 1
- Requires particular vigilance when changing breathing systems
- Especially critical during pediatric cases with frequent system changes 1
Monitoring and Alarm Systems
Ventilator Alarms
- Pressure alarms (high and low)
- Volume alarms
- Disconnect alarms
- Must be tested and correctly configured before each case 1
Oxygen Analyzer
- Should display approaching 100% when only oxygen flows 1
- Provides continuous FiO₂ monitoring
Systematic Pre-Use Checking Protocol
Start of Session Checks (Comprehensive)
1. Power and Backup Systems
- Verify electrical connections and backup battery charge 1
- Confirm backup generator availability
- Set system clock if present 1
2. Gas Supply Verification
- Pipeline pressures: 400-500 kPa for all connected gases 1
- Adequate oxygen reserve cylinder available
- All cylinders securely seated, turned off after content check 1
- No CO₂ cylinders should be present—use blanking plugs in empty yokes 1
3. Flowmeter Function
- Control valves operate smoothly
- Bobbins move freely without sticking throughout range 1
4. Breathing System Integrity
- Visual inspection for correct configuration
- All connections secured with "push and twist" 1
- Pressure leak test: 20-60 cmH₂O by occluding patient-end and compressing reservoir bag 1
5. Two-Bag Test (Critical comprehensive check) 1
- Attach test lung to patient-end with angle piece and filter
- Set fresh gas flow to 5 L/min, ventilate manually
- Verify system patency and unidirectional valve movement
- Test APL valve function by squeezing both bags
- Switch to ventilator mode, reduce/stop fresh gas flow
- Open and close each vaporizer—no volume loss should occur 1
6. Scavenging System
- Verify system is on and functioning
- Confirm tubing attached to appropriate exhaust port 1
7. Suction
- Test for rapid development of adequate negative pressure
- Verify all connections secure 1
Between-Case Checks (Abbreviated)
When changing breathing systems during a list:
- Confirm integrity and correct configuration
- Repeat leak test if vaporizer changed—failure to do so is a common cause of critical incidents 1
- Particularly important during pediatric lists with frequent changes 1
Common Pitfalls and Critical Incidents
Most Dangerous Errors (based on incident reporting) 1:
- Failure to perform pre-use checks—major contributor to hypoxic brain damage and death 1
- Not retesting after vaporizer changes—common cause of critical incidents 1
- ACGO misconnection—has caused patient harm 1
- Tilted vaporizers—delivers dangerously high vapor concentrations 1
Human Factors vs. Machine Faults
- Human factors contribute to greater complications than actual machine faults 2
- Knowledge of checks may be tested in FRCA examinations 1
- Training and competency documentation required by NHS standards 1
Modern Workstation Considerations
Automated Self-Testing
- Many modern machines perform self-tests on startup 1
- Functions tested by machine need not be retested manually 1
- However: AAGBI checklist still required in conjunction with manufacturer's checks 1
- Manufacturer-specific checklists should supplement, not replace, standardized checks 1
Documentation Requirements
- Written records of weekly oxygen failure alarm checks 1
- Training records for all personnel 1
- Regular servicing documentation 2
Protection When Not in Use
- Breathing systems should be protected with test lung or bag to prevent foreign body intrusion 1
The evidence strongly emphasizes that systematic checking takes only a few minutes but is essential to patient safety 1, and that anaesthetists must not use equipment unless trained and competent 1. The 2012 AAGBI guidelines represent the current standard of care, endorsed by Chief Medical Officers across the UK 1.