The speed of onset of anaesthesia when using a volatile agent for induction of anaesthesia depends on how rapidly the alveolar concentration reaches the inspired concentration of the agent.

Fig 1 shows how the alveolar concentration increases with time if we use a constant inspired concentration of isoflurane, sevoflurane and nitrous oxide. These are all examples of exponential wash-in curves. Select play to watch the animation.

The curves for each agent all reach the same end-point, but at different rates, dependent on their blood:gas solubility coefficient. The more insoluble the agent is in blood, the faster its F_A/F_I ratio approaches 1.

Notice that the scale of the y-axis is 0 to 1, as it is a fraction of alveolar to inhaled anaesthetic concentration (F_A/F_I). It gives no information about the actual inspired concentration.