Blood gas coefficient

The Blood Gas Partition Coefficient is also known as Ostwald coefficient for blood–gas. A partition coefficient is the ratio of the concentrations of a compound in one solvent to the concentration in another solvent at equilibrium. The blood/gas partition coefficient describes how the gas will partition itself between the two phases after equilibrium has been reached. For example: Enflurane has a blood/gas partition coefficient of 1.7. Therefore, if the gas is in equilibrium the concentration in blood will be 1.7 times higher than the concentration in the alveoli. Thus, it makes sense that a gas with a higher blood gas coefficient will require higher uptake of gas into the blood and induction will be slower.

Key points:

Higher partition coefficient = higher lipophilicity = higher potency = higher solubility

High solubility = more anesthetic needs to be dissolved = slower onset
MAC decreases as blood gas partition coefficient increases, generally speaking

Updated definition 2020:

Blood gas partition coefficient is the ratio of the concentrations of a gas in each of two phases of matter at steady state(in this case blood vs alveolar gas). The lower the coefficient the less soluble the anesthetic is in blood relative to alveolar gas, resulting in a faster achievement of steady state and faster induction. If the coefficient is less than 1, this means that alveolar gas has a greater capacity to hold the anesthetic than blood, thus blood will absorb less of the anesthetic and will reach steady state more quickly. For example, Desflurane has a blood gas partition coefficient of 0.45. Therefore, blood has 45% of the capacity for desflurane as does alveolar gas. Halothane has a blood gas partition coefficient of 2.4. Therefore, desflurane is five times less soluble in blood than halothane. Much less desflurane will be absorbed by theblood and a steady state between alveolar gas and the blood will be achieved quicker.