Vaporizers

Quick Answer

Vaporizers convert liquid volatile anaesthetic agents into vapor for delivery to patients, requiring precise concentration control under varying conditions. Types: Variable bypass (most common, Tec type), measured flow (desflurane specific), injection (obsolete). Variable bypass principle: Fresh gas flow splits into bypass channel (90-95%) and vaporizing chamber (5-10%) containing liquid agent; concentration dial controls split ratio; saturated vapor (SVP dependent on temperature and agent) mixes with bypass gas to deliver set concentration. Temperature compensation: Bimetallic strip or bellows automatically increases bypass flow as temperature drops (maintains constant vapor output); copper construction for thermal capacity and stability. Agent-specific: Keyed filler systems prevent wrong agent; color-coded (sevoflurane yellow, isoflurane purple, desflurane blue). Saturated vapor pressure (SVP): Sevoflurane 157 mmHg at 20°C, isoflurane 238 mmHg, desflurane 669 mmHg (boiling point 22.8°C). Potency (MAC): Inverse relationship with SVP — high SVP agents have high MAC (desflurane MAC 6-7%), low SVP agents have low MAC (sevoflurane MAC 2-2.5%). Desflurane vaporizer (Tec 6/7): Heated to 39°C (SVP ~1500 mmHg), dual-circuit, pressurized, electronic control; delivers precise concentration independent of flow, temperature, duration; very different from variable bypass design. Safety features: Keyed fillers, agent-specific color coding, interlocks (only one vaporizer on at time), low agent alarm, overfilling prevention, tipping safety (if tipped >45°, drain liquid, flush 20-30 minutes before use). Physical principles: Saturated vapor concentration % = (SVP / (760 - SVP)) × 100; at sea level (760 mmHg), agent vapor concentration limited by SVP. [1-10]