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The published figures
These come from the veterinary product labels rather than from secondary sources.
| MAC, dog | Isoflurane 1.28% · Sevoflurane 2.36% |
|---|---|
| MAC, horse | Isoflurane 1.31% · Sevoflurane not stated on label |
| Blood:gas partition coefficient at 37 °C | Isoflurane 1.43 · Sevoflurane 0.63–0.69 |
| Vapour pressure at 20 °C | Isoflurane 238 mmHg · Sevoflurane 157 mmHg |
| Boiling point | Isoflurane 48.5 °C at 760 mmHg |
| Stabiliser | Isoflurane contains none |
Two of these numbers do most of the work.
Blood:gas partition coefficient governs how fast the agent equilibrates. Sevoflurane's 0.65 against isoflurane's 1.43 means induction, depth changes and recovery all happen faster with sevoflurane. The sevoflurane label words this as a caution rather than a selling point: low blood solubility "can cause faster changes in anesthetic depth and blood pressure than other volatile anesthetics." Faster is not automatically safer — it cuts both ways.
MAC governs how much agent you use. Sevoflurane's MAC is roughly 1.8 times isoflurane's in dogs, so you dial a higher percentage to achieve the same depth. Combined with a higher price per bottle, this is why the cost gap between the two agents in practice is wider than the price gap on the invoice.
Why the vaporizer is agent-specific
A precision vaporizer is calibrated around the vapour pressure of one agent. Isoflurane at 238 mmHg and sevoflurane at 157 mmHg saturate the carrier gas to different degrees at the same temperature, so a given splitting ratio produces a different output concentration for each.
Filling an isoflurane vaporizer with sevoflurane therefore does not simply shift the calibration — it produces a delivered concentration that does not correspond to the dial, in a direction that depends on temperature and flow. This is why vaporizers carry agent-specific filling systems, and why "which agent is this vaporizer calibrated for" is the first question to ask about any vaporizer, new or second-hand.
| Agent | Named explicitly. One vaporizer, one agent. |
|---|---|
| Temperature compensation | Output should hold at the dial setting across the working temperature range, not just at 20 °C. |
| Flow compensation | Output should hold across the fresh gas flow range the machine will actually use — including low flows on a circle system. |
| Output range | Must comfortably exceed MAC for the species you treat. A 0–5% isoflurane vaporizer is ample; a sevoflurane vaporizer needs to reach roughly 8%. |
| Calibration interval | The manufacturer's stated interval, and who can perform recalibration in your country. |
| Filling system | Keyed or funnel-fill. Keyed systems prevent cross-filling with the wrong agent. |
| Mounting | Whether it fits the machine's manifold, and whether the machine supports a second vaporizer position. |
The absorbent interaction, and the fire risk
This is the part most comparisons leave out, and it is the one with a safety consequence.
Both agents react badly with desiccated absorbent. Dried-out soda lime can produce carbon monoxide with either agent, raising carboxyhaemoglobin in the patient. Both labels instruct that a possibly desiccated absorbent be replaced before use.
Sevoflurane carries additional hazards. It reacts with soda lime and barium hydroxide lime to form Compound A, with production rising at higher agent concentrations and lower fresh gas flows — exactly the conditions of low-flow anesthesia. The reaction is exothermic, markedly more so when the absorbent is dry. The label reports that spontaneous fire in the breathing circuit has occurred in extremely rare cases with potassium-hydroxide-containing absorbents, and states that these (for example Baralyme) are not recommended with sevoflurane.
The sevoflurane label also makes a point that applies to any practice running a circle system: a lack of colour change does not confirm that absorbent is adequately hydrated, so absorbent should be replaced routinely regardless of the indicator. This matches the practical advice elsewhere in this guide — track hours of use and watch inspired CO₂ on the capnograph rather than trusting colour.
Isoflurane, by contrast, does not decompose in soda lime at normal operating temperatures, according to its label. The carbon monoxide risk with desiccated absorbent still applies.
Practical consequence: a practice running sevoflurane needs a stricter absorbent replacement discipline than one running isoflurane, and must check which absorbent chemistry it is buying. This is a recurring operational cost and a training requirement, not a one-off purchase decision.
When each agent makes sense
| Isoflurane suits | Routine small-animal and equine practice, high case volume with cost pressure, markets where sevoflurane supply is unreliable or expensive, settings where recovery is supervised in a kennel rather than a staffed recovery area. |
|---|---|
| Sevoflurane suits | Brachycephalic patients, where a fast clean recovery with early return of airway control matters; mask induction, where the less pungent agent is better tolerated; high-turnover facilities where recovery time is the bottleneck; some exotic and avian work; cases where rapid control of depth is clinically valuable. |
| Running both | Possible on a machine with two vaporizer positions and an interlock preventing both being open at once. Doubles the vaporizer capital cost and the calibration burden, and requires keyed filling to prevent cross-contamination. |
Three questions buyers forget
Is the agent available and affordable in my country? Agent supply is a national-market question, not a global one. A machine is useless without a reliable source of the agent its vaporizer is calibrated for, and the price differential between the two agents varies widely by market.
Who recalibrates the vaporizer, and where? A vaporizer that must be shipped abroad for recalibration means weeks without a machine. Establish the in-country service route before purchase, not after the first drift.
What absorbent chemistry can I actually buy locally? If only potassium-hydroxide-containing absorbent is available in your market, that constrains the sevoflurane decision directly.
Frequently asked questions
Can I put sevoflurane in an isoflurane vaporizer?
No. Precision vaporizers are calibrated to the vapour pressure of a single agent — 238 mmHg for isoflurane, 157 mmHg for sevoflurane at 20 °C. Using the wrong agent produces a delivered concentration that does not match the dial, varying with temperature and flow. If an agent has been put in the wrong vaporizer, the unit must be drained and serviced before use.
Which agent is cheaper to run?
Isoflurane, usually by a wide margin. The gap is larger than the per-bottle price difference because sevoflurane's MAC in dogs is roughly 1.8 times isoflurane's, so more agent is consumed to reach the same depth. The exception is a practice that can shorten recovery enough to turn over more cases per day — there the time saving can offset the agent cost.
Is sevoflurane safer than isoflurane?
Not straightforwardly. Sevoflurane's faster equilibration gives finer control of depth and a quicker recovery, which helps in brachycephalic and compromised patients. Against that, its own label cautions that low blood solubility can cause faster changes in depth and blood pressure, and it carries Compound A formation, a stronger exothermic reaction with dry absorbent, and a documented if extremely rare fire risk with potassium-hydroxide absorbents. Each agent is safer in the right hands for the right patient.
What is Compound A and does it matter in veterinary practice?
Compound A is a degradation product formed when sevoflurane reacts with carbon dioxide absorbent. Production rises with higher sevoflurane concentration and lower fresh gas flow. The practical implication is that very low-flow sevoflurane anesthesia on a circle system warrants more caution and more frequent absorbent replacement than the equivalent isoflurane technique.
How do I know when to change soda lime?
Track hours of use — roughly eight hours is the common guidance — and watch inspired CO₂ on the capnograph, which should sit at or near zero. Do not rely on the colour indicator: the sevoflurane label states explicitly that absence of colour change does not confirm adequate hydration, and indicators can revert after the canister rests.
Does the vaporizer need to be inside or outside the breathing circuit?
Modern precision vaporizers are designed for use outside the circle (VOC), sitting between the flowmeter and the common gas outlet. Draw-over vaporizers placed inside the circle are a different and now uncommon design with different behaviour at varying ventilation. Confirm which configuration a vaporizer is intended for before mounting it.