Anesthesiology APPLIED Exam › Pharmacology, equipment and physics

Volatile agents and nitrous oxide: when and when not

Pharmacology, equipment and physics·~5 min read· Physician-reviewed✓ Sourced · 4

Stem

A 44-year-old woman, 126 kg, with moderate persistent asthma and severe nausea after her last two anesthetics, is having a laparoscopic sleeve gastrectomy. Intubation was uneventful. Your resident proposes desflurane for a fast wake-up, with 50% nitrous oxide to reduce the volatile dose.

Question

What will you use to maintain her anesthesia, and will you use nitrous oxide?

Ideal answer

HeadlineNo desflurane and no nitrous: her asthma wins over a faster wake-up, so sevoflurane at 0.8–1 MAC in oxygen and air, or propofol TIVA.

Sevoflurane as default because it is non-pungent, hemodynamically predictable and fast enough for almost every case; I accept a slightly slower emergence.
Desflurane for morbid obesity and long cases, not for inhalational induction or reactive airways, because it irritates the airway and causes sympathetic surges with rapid increases.
Propofol TIVA as a strong alternative, because volatiles themselves add to nausea and she has a high PONV risk.
Nitrous off: high PONV risk, and it distends the bowel in a laparoscopic case; multimodal antiemetic prophylaxis instead.

Watch for: the other no-volatile or low-volatile cases: malignant hyperthermia susceptibility, evoked potentials, and a uterus that must contract.

Breaking down the answer

Sevoflurane vs. desflurane — desflurane's blood-gas solubility is the lowest (0.42 vs. 0.65–0.69),¹ hence faster emergence. Pungency causes coughing and laryngospasm; the label contraindicates it for inhalational induction in children, and rapid increases markedly raise heart rate.²
Compound A and fresh gas flow — the sevoflurane label rejects flows below 1 L/min and limits 1 to <2 L/min to 2 MAC-hours, so beyond that I run 2 L/min.³ Calcium hydroxide absorbents form none, so many run lower.
Dry absorbent — desflurane produces carbon monoxide;² sevoflurane can generate extreme heat, smoke and fire in the circuit.³ I change absorbent left with oxygen flowing over a weekend.
Brain and neuromonitoring — above about 1 MAC, volatiles uncouple flow from metabolism and raise intracranial pressure; desflurane's label notes CSF pressure rising with dose.² Tight brain: <1 MAC or propofol. Motor evoked potentials suffer more than SSEPs: TIVA, or 0.5 MAC or less.
Uterine tone — volatiles relax the uterus dose-dependently; useful for retained placenta, harmful after delivery at cesarean, where I keep below 0.5–0.75 MAC once the baby is out.
Nitrous contraindications — about 34 times more soluble in blood than nitrogen, it enters closed spaces faster than nitrogen leaves: pneumothorax, bowel obstruction, pneumocephalus, tympanic membrane grafting, intraocular gas (SF6 or C3F8) within weeks of retinal surgery, pulmonary air cysts, venous air embolism.¹
Nitrous beyond gas spaces — it lowers volatile requirement, but raises pulmonary vascular resistance, limits inspired oxygen, worsens raised intracranial pressure and inactivates vitamin B12, reducing methionine synthase activity,¹ which matters in B12 and MTHFR deficiency.

Follow-up question

While you are on break, your resident turns on 60% nitrous oxide to save sevoflurane. When you return, the surgeon says the bowel is distended, and the airway pressure climbs as the abdomen tightens. What do you do?

Follow-up answer

HeadlineNitrous is expanding gas in the bowel and peritoneal space; I turn it off and tell the surgeon why.

I increase oxygen with air, ask for a lower insufflation pressure and deepen with volatile. I check the airway pressure trace and listen to the chest, because a sudden rise could equally be a capnothorax or, in her, bronchospasm; if the SpO2 falls and one side is silent, the surgeon desufflates. Once the bowel decompresses, the case proceeds without nitrous.

Pitfalls that cost points

Forgetting the eye with intraocular gas; the nitrous can cause blindness through central retinal artery occlusion.
Ignoring climate cost: ASA says avoid disproportionate agents (desflurane about 40–50 times sevoflurane, potency-scaled) and use the lowest possible fresh gas flow.⁴
Treating lightening of anesthesia with nitrous alone; at 1 atmosphere it cannot reach 1 MAC (MAC 104%).

Rapid-fire

Q: Why does nitrous speed the uptake of a second gas? A: Its large-volume uptake concentrates the remaining alveolar gases and draws in more inspired gas, the second-gas and concentration effects.

Q: What is diffusion hypoxia? A: At the end of a nitrous anesthetic, large volumes of nitrous leave blood for the alveoli and dilute oxygen; I give 100% oxygen for several minutes after turning it off.

Q: Which volatile is best for a patient with coronary disease? A: Any of them at stable concentrations; what matters is avoiding tachycardia and hypotension, so I avoid rapid desflurane increases.

Q: Why avoid volatiles in Duchenne dystrophy? A: They can cause rhabdomyolysis and hyperkalemia even without succinylcholine, so I use TIVA.

Numbers to know

Item	Value
MAC sevoflurane / desflurane / isoflurane	2.1% (age 40)³ / ~6% / 1.2%
MAC nitrous oxide	104%
Blood-gas partition: des / sevo / N2O	0.42 / 0.65–0.69 / 0.47¹
Sevoflurane fresh gas flow (FDA label)	Never <1 L/min; at 1 to <2 L/min, ≤2 MAC-h³
MAC reduction per decade over 40	~6%

Sources

1.Inhaled Anesthetic Gases · StatPearls · statpearls.comUsed for blood:gas coefficients and nitrous contraindications only; its isoflurane MAC (1.4%) differs from the usual 1.15–1.2%.
2.Suprane (desflurane) prescribing information · FDA label (via Drugs.com) · drugs.com
3.Sevoflurane prescribing information · FDA label (via Drugs.com) · drugs.com
4.Reduce carbon footprint from inhaled anesthesia with new guidance published · ASA, 2022 · asahq.org

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.