Uptake of inhaled anesthetics: V/Q mismatch

The effect of V/Q mismatching on uptake of volatile anesthetics is dependent on the solubility of the anesthetic agent. Using endobronchial intubation as an example – for relatively insoluble agents (ex. desflurane [blood gas solubility coefficient 0.42, MAC 6-7.3%]), the rate of rise of arterial partial pressure of agent is slower for a given inspired concentration (despite the fact that the alveolar partial pressure of desflurane in the intubated lung rises more rapidly). For relatively soluble agents, for which rate of uptake is dependent on minute ventilation (ex. isoflurane [blood gas solubility coefficient 1.46, MAC 1.15%]), the rate of rise of arterial partial pressure of agent in the ventilated lung increases for a given inspired concentration, because the ventilated lung receives an effectively doubled minute ventilation – this higher partial pressure allows more anesthetic agent to transfer to the blood stream, and partially attenuates the effect of the essentially unventilated lung.

Uptake of Inhaled Anesthetics: V/Q mismatch

Insoluble agents (desflurane): markedly decreased rise in arterial partial pressure of anesthetic agent

Soluble agents (isoflurane): decreased rise in arterial partial pressure of anesthetic agent is partially attenuated by relatively higher alveolar partial pressure in ventilated areas