The effect of the IV agent lasts only a few minutes. To maintain anaesthesia it is important to introduce a volatile agent as soon as possible. Although the anaesthetic effects of induction agents and volatile agents are synergistic, it takes longer for the alveolar concentration of the volatile agent to reach a maximum than it does for the effect of the induction agent to wear off. The overall action in the cross-over phase must be sufficient to prevent the patient becoming aware.

Fig 1 shows the take-off phase of anaesthesia.

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