What are the recommended practices for delivering inhalation anesthesia with a draw‑over vaporiser?

Recommendations for Inhalation Anaesthesia Practice with Draw-Over Vaporisers

Draw-over vaporisers require meticulous pre-use safety checks, strict upright positioning at all times, and when linked to oxygen concentrators must incorporate an oxygen economizer tube to ensure consistent oxygen delivery regardless of ventilation pattern.

Essential Pre-Use Safety Checks

All vaporisers must undergo comprehensive verification before each case to prevent critical incidents:

• Verify the vaporiser is correctly fitted to the anaesthetic machine with locking mechanisms fully engaged 1
• Confirm control knobs rotate fully through their complete range to guarantee intended concentration settings 1, 2
• Check adequate filling without overfilling, and ensure filling ports are tightly closed 1, 2
• Vaporisers must always remain upright—tilting can result in delivery of dangerously high concentrations of vapour 1, 2
• Perform leak testing when changing vaporisers during use whenever possible, as failure to do so is a common cause of critical incidents 1, 2

Critical Distinction for Modern vs. Basic Equipment

• Manual leak testing should only be performed on basic 'Boyle's' machines, as it may damage modern anaesthetic workstations 2
• Modern workstations often perform automatic vaporiser integrity testing, eliminating the need for manual checks 1, 2
• Always consult manufacturer recommendations before performing any manual leak testing 2

Draw-Over Vaporiser-Specific Considerations

When using draw-over systems, oxygen delivery configuration is critical:

• An oxygen economizer tube (OET)—a 900-mL corrugated tube—is essential when linking a draw-over vaporiser to an oxygen concentrator 3
• With an OET in place, fractional oxygen concentration depends only on minute volume and remains independent of ventilation pattern (inspiratory/expiratory ratios and pauses) 3
• Without an OET, system performance is considerably impaired, with oxygen concentration becoming unpredictably dependent on both minute volume and ventilation pattern 3
• Draw-over systems can be safely adapted for modern operating room use when equipped with appropriate safety monitors required by accreditation standards 4, 5

System Integrity Verification

The two-bag test confirms complete system integrity after individual component checks:

• Attach the patient-end of the breathing system (including angle piece and filter) to a test lung 1
• Set fresh gas flow to 5 L·min⁻¹ and ventilate manually, checking system patency and unidirectional valve movement 1
• Turn on the ventilator, then turn off or minimize fresh gas flow 1
• Open and close each vaporiser in turn—there should be no loss of volume in the system 1, 2

Critical Safety Pitfalls to Avoid

Several common errors pose immediate danger to patients:

• Never operate multiple vaporisers simultaneously on non-interlocked systems—this can result in uncontrolled and potentially lethal anaesthetic concentrations 2, 6
• Do not tilt vaporisers during filling, transport, or use 1, 2
• Avoid overfilling vaporisers or leaving filling ports open, as this compromises output accuracy 2
• Do not skip leak testing after changing vaporisers intra-operatively 1, 2

Special Considerations for Malignant Hyperthermia-Susceptible Patients

Preparing trigger-free anaesthesia requires complete vaporiser removal:

• All vaporisers must be physically removed from the machine before flushing to eliminate the large internal agent reservoir 2
• Replace breathing circuits, soda-lime canisters, and fresh-gas hoses with uncontaminated components 2
• Perform high-flow flush at >10 L/min for the manufacturer-specified duration 2
• When available, insert activated-charcoal filters to reduce residual volatile agent to <5 ppm within 2–3 minutes 2
• Modern workstations with extensive plastic/rubber components require longer washout periods than older machines 2

Monitoring and Scavenging

Essential monitoring and waste gas management:

• Verify the anaesthetic gas scavenging system is switched on and functioning before use 1
• Ensure tubing is attached to the appropriate exhaust port 1
• In resource-limited settings without end-tidal anaesthetic gas monitoring, agent-vaporiser mismatch should be avoided unless absolutely necessary, as it poses significant safety risks 7