ABA Anesthesiology BASIC Exam Sample Questions

The correct answer is A. At equilibrium, the concentration of Desflurane in the blood is 42 percent of alveoli.

Explanation: Partition coefficient is an important concept in understanding induction, maintenance and emergence from inhalational anesthesia. Partition coefficient is a measure of hydrophobicity (or lipophilicity) of a compound and is defined as the ratio of its concentration in one solvent to the concentration in another solvent at equilibrium. If the blood –gas partition coefficient of desflurane is 0.42, it indicates that blood concentration is 42 percent of what’s is in alveoli. Similarly, one can derive percentage concentrations of other agents by their partition coefficients (blood gas) which are respectively 0.47, 0.65, 1.4,1.8 and 2.3 for nitrous oxide, sevoflurane, isoflurane, enflurane, and halothane. Generally, higher lipophilicity indicates greater potency and higher solubility. However, greater solubility also signifies slower induction, more time needed to change the brain concentrations by altering alveolar concentrations and longer time needed to washout the inhalational anesthetics, for example during emergence. As a result, induction with halothane is slower in comparison to sevoflurane, so also emergence. Solubility reduces the rate of rise of partial pressures of inhalational anesthetics. It is the agent that is present in the aqueous phase that contributes to the partial pressure and the attainment of anesthetic state is determined by achieving the appropriate partial pressure. The rise in the partial pressure in blood (and brain) is slower with highly soluble anesthetics. Due to greater solubility, the rate of rise of alveolar concentration (compared to inspired) is slower as well. (B) Although the partial pressure is determined by the desflurane present in the aqueous form, partition coefficient is determined by the concentration of all forms of inhalational anesthetic present in a given phase (blood, oil etc). (C) Solubility is directly related to the potency and opposite is true regarding the speed of induction. However, the ratio of partition coefficients of individual anesthetics does not determine the ratio of their uptake from the lungs. (D) In fact, the blood gas solubility of nitrous oxide is higher than desflurane. However, the rate of rise of FA/FI (fraction of alveolar to inspired concentration of an anesthetic) is faster with nitrous oxide.