Solubility vs MAC: Volatile Agents for EDAIC

Blood–gas solubility describes how an inhaled anaesthetic distributes between blood and gas at equilibrium. Minimum alveolar concentration, or MAC, describes potency against a specified movement response. They answer different questions: the first helps explain uptake and changing partial pressures, while the second relates concentration to an effect.

A low blood–gas partition coefficient does not, by definition, mean high potency. A low MAC does indicate greater potency for the endpoint used to define MAC, when the comparison conditions are appropriate. Separating these two ideas is a central EDAIC pharmacology skill.

Name the endpoint before comparing potency

MAC is the steady-state alveolar concentration that prevents movement in response to a defined surgical stimulus in 50% of a population. Its foundational use as a potency standard is described by Eger and colleagues.

This is a population endpoint for immobility. It should not be translated into “half the patients are conscious” or a guarantee of amnesia in an individual. Awareness, response to command and movement are distinct endpoints, even though they are often discussed in the same anaesthetic.

In an exam, replace the vague phrase “depth of anaesthesia” with the precise effect under discussion. That single editing step often reveals whether a statement is using MAC correctly.

Original example: two fictional agents

Consider fictional agents A and B at the same reference age and pressure. Assign A a MAC of 1% and a blood–gas coefficient of 1.5. Assign B a MAC of 2% and a coefficient of 0.5. These values illustrate concepts and are not prescriptions or the specifications of named products.

A requires the lower alveolar concentration for the defined movement endpoint, so A is more potent by that comparison. B has the lower blood–gas solubility. If other relevant delivery and physiological conditions are held comparable, B's lower uptake into blood favours a more rapid rise in alveolar fraction relative to inspired fraction.

The same table therefore supports two different rankings. “More potent” identifies A; “less soluble in blood” identifies B. Trying to choose one universally stronger or faster agent loses the distinction the exercise was designed to test.

OpenAnesthesia's uptake and distribution review describes blood solubility, pulmonary blood flow and partial-pressure gradients as contributors to uptake. Solubility is one determinant, not a complete prediction of the time course in every patient.

Equilibrium does not mean equal concentrations

A partition coefficient compares concentrations in two phases at equilibrium under specified conditions. Equal partial pressures can coexist with different concentrations because the phases dissolve different amounts of agent.

For an invented partition exercise, suppose the blood–gas coefficient is 0.5 and the gas-phase concentration is expressed as 2 arbitrary amount-per-volume units. At equilibrium, the corresponding blood concentration is 1 unit in the same compatible concentration convention. A coefficient of 1.5 would instead give 3 units.

The equal-pressure condition does not require the numerical concentrations to match. Also, “0.5” does not mean that half of all delivered molecules must be in the blood: total amounts additionally depend on the volumes of the phases.

The experimental sevoflurane partition-coefficient study is a useful primary example of why the medium and temperature are stated when reporting solubility. A blood–gas coefficient cannot simply be substituted for an oil–gas or tissue–blood coefficient.

Calculate a MAC fraction carefully

Return to fictional agent B, whose reference MAC in the exercise is 2%. An equilibrated alveolar concentration of 1% corresponds arithmetically to 1/2 = 0.5 MAC under those reference conditions. A concentration of 3% corresponds to 1.5 MAC.

These ratios compare the supplied concentration with a reference concentration. They do not supply a treatment target or a personalised probability of recall. If the reference changes, the ratio changes even when the measured percentage stays constant.

Age is one reason the reference matters. The sevoflurane product information gives age-related MAC information and separately reports blood solubility. That separation in the label mirrors the separation required in your revision notes.

Avoid three shortcuts

First, inspired concentration and alveolar concentration are different during changing conditions. A vaporiser setting alone is not the same measurement as an equilibrated alveolar concentration.

Second, the phrase “more soluble” is incomplete without naming the phases. Blood–gas solubility and lipid solubility should not share an unlabeled column in a revision table.

Third, rapid wash-in does not establish universal clinical superiority. A pharmacokinetic property does not settle questions about airway effects, equipment, context or patient suitability. Answer the property asked about rather than importing a broader preference.

Self-test

1. In the fictional table, A is more potent for the MAC endpoint.
2. B is more soluble in blood.
3. Equal equilibrium partial pressures require equal blood and gas concentrations.
4. For B, 1% divided by a 2% reference equals 0.5 MAC.

Answers: true, false, false, true. The first and fourth concern potency and a reference ratio. The second and third concern distribution. Being able to label the type of question is as useful as recalling its numerical answer.

For broader context, read the EDAIC pharmacology overview. In AnesCORE practice, keep separate columns for solubility, delivered concentration, measured concentration and effect.

Sources

• OpenAnesthesia: Inhaled agents, uptake and distribution
• Eger et al: Minimum alveolar anesthetic concentration
• DailyMed: Sevoflurane prescribing information
• Strum and Eger: Sevoflurane partition coefficients

Prepared with AI assistance for examination revision; this article is not a patient-care protocol or a substitute for current primary guidance. Sources checked 7 October 2026.