01How a vapor reaches the patient
Volatile anesthetics are one of the few drugs used only in anesthesia. They start as a liquid in an agent-specific vaporizer. A carrier gas (oxygen, air, or nitrous) flows from the pipeline/cylinder → flow meters → vaporizer, where it picks up a precise % of vapor → down the inspiratory limb → to the patient.
Once inhaled, the vapor moves lung → blood → brain to produce anesthesia, then leaves the same way. A little is metabolized, but most is simply "blown off" (exhaled unchanged) — which is what lets the patient wake up as the level falls.
02Medical gases & hose colors
Know the gases and their US color codes at a glance:
| Gas | Color | Notes |
|---|---|---|
| Oxygen (O₂) | Green | Medical-grade pure O₂ |
| Air | Yellow | Medical-grade air |
| Nitrous oxide (N₂O) | Blue | "Laughing gas"; common for dental sedation |
| Waste / scavenging | White | Carries waste gas away |
03Choosing the volatile: solubility & pungency
Two properties drive agent choice:
Solubility — the more soluble in blood, the slower the patient goes to sleep and wakes up. From most to least soluble: Iso > Sevo > Des. So iso is slowest to emerge (handy when you want the patient to stay deep/intubated; historically used in cardiac cases for stability), and des wakes fastest (good for the obese and long cases).
Pungency — Sevo is the least pungent, so it's the agent for inhalational (mask) inductions (the standard for kids). Des is the most pungent and an airway irritant — be cautious in smokers and asthmatics.
| Agent | ~1 MAC | Min. fresh-gas flow | Best for | Watch out |
|---|---|---|---|---|
| Isoflurane | ~1.2% | 1 L/min | Hemodynamic stability, staying intubated | Slow wake-up |
| Sevoflurane | ~1.8–2% | 2 L/min | Mask induction; least pungent | Compound A at very low flows |
| Desflurane | ~6.6% | 1 L/min | Obese / long cases; fastest emergence | Pungent — airway irritant |
Changing depth faster: turn up the fresh-gas flow, increase tidal volume, or dial up the vaporizer %. (The "second gas effect" of nitrous is textbook trivia and rarely clinically meaningful.)
- Nitrous can be added (e.g., 50%) to cut volatile requirement, but avoid it in closed air spaces and when PONV risk is high.