Blood:gas partition coefficient
Blood:gas partition coefficient (no units), also known as Ostwald coefficient for blood–gas, is a pharmocology term to describe the solubility of inhaled anesthetics in blood. It is defined as the ratio of the concentration, in the blood, to the concentration in the gas phase (alveoli), that is in contact with the blood, when the partial pressure in both compartments has reached equilibrium.

The blood:gas partition coefficient describes how the gas will partition itself between the two phases (blood and gas), after equilibrium (partial pressure) has been reached.

The concentration of the anaesthetic, in blood, includes the portion that is undissolved in plasma, and the portion that is dissolved (bound to plasma proteins). The more soluble the inhaled anesthetic is in blood, compared with in the air, the more it binds to plasma proteins in the blood, and the higher the blood:gas coefficient.

For example, if the coefficient is 1.4, then the concentration in the blood will be 1.4 times that of the concentration in the alveoli (at equilibrium).

A higher blood:gas partition coefficient produces a higher uptake of the gas into the blood and, consequently, a slower induction (and reversal) time, because it takes longer for the partial pressure to reach equilibrium with the brain partial pressure. Newer anaesthetics (such as Desflurane) typically have lower blood:gas coefficients than older ones; these newer agents produce faster onset and emergence from anaesthesia, at reversal. If an anesthetic has a high coefficient, then a large amount of it will have to be taken up in the blood, before the undissolved fraction reaches the lipid tissues of the brain, where it exerts its effect.

The potency of an anaesthetic is associated with its lipid solubility, which is measured by its oil:gas partition coefficient.