Vaporisers are agent-specific. Never overfill or tilt during use. Desflurane needs heated vaporiser; sudden increases cause sympathetic surge.
MAC concepts
- MAC: alveolar concentration preventing movement in 50% at incision
- MAC awake ~0.3 to 0.4 of MAC
- Decreased by age, opioids, hypothermia, pregnancy
Approximate MAC (adults, 1 atm)
- Sevoflurane ~2%
- Isoflurane ~1.15%
- Desflurane ~6%
- N2O alone insufficient for surgical anaesthesia
MAC awake (0.3 to 0.4)
- Return of consciousness
- Emergence target
- Awareness risk if too light
About 1 MAC
- Prevents movement in 50% at incision
- Adjust for age and opioids
- Monitor end-tidal agent
High MAC / overdose
- Hypotension, myocardial depression
- Desflurane surge if raised fast
- Reduce agent, support BP
Clinical use
1
1. Induction
Often IV induction then sevoflurane or desflurane after intubation. Avoid desflurane at high inspired concentration in awake patient (irritant).
Often IV induction then sevoflurane or desflurane after intubation. Avoid desflurane at high inspired concentration in awake patient (irritant).
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2
2. Maintain
Adjust vaporiser with age and adjuvants. End-tidal agent monitoring standard.
Adjust vaporiser with age and adjuvants. End-tidal agent monitoring standard.
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3
3. Low-flow
Acceptable with appropriate monitors; remember agent uptake and rebreathing.
Acceptable with appropriate monitors; remember agent uptake and rebreathing.
↓
4
4. Emergence
Reduce agent early; consider remifentanil for smooth wake. MH susceptibility: avoid triggers (see MH sheet).
Reduce agent early; consider remifentanil for smooth wake. MH susceptibility: avoid triggers (see MH sheet).
Fresh gas flow
- Higher flow speeds induction/emergence
- Low flow saves agent and humidity
Pitfalls
- Wrong vaporiser filled with wrong agent
- Relying on dial without end-tidal monitor
MedPrep - www.medprep.uk - Educational revision aid only. Not clinical guidance. Doses are adult examples; check the BNF, current guidelines and local policy, and seek senior advice. Nov 2026