All volatile agents cause a dose-dependent reduction in tidal volume accompanied by an increase in respiratory rate. However, with high concentrations, alveolar ventilation will become inadequate.

Nitrous oxide has little effect on minute ventilation.

Isoflurane is an irritant vapour and may cause coughing and laryngospasm. It is a poor choice for a gaseous induction.

Sevoflurane is a much more pleasant vapour to breathe than isoflurane and is used for gas inductions. It is more potent than isoflurane in reducing minute ventilation and may cause apnoea.

The effects of sevoflurane on tidal volume and respiratory rate depend on the inhaled concentration. For a gaseous induction using sevoflurane overpressure with 8% is commonly used.

Once the patient is almost asleep it is important to reduce the vaporizer setting, as shown in the animation on Fig 1, to maintain adequate alveolar ventilation.

If this reduction is not made and 8% sevoflurane is continued, as shown in Fig 2, then alveolar ventilation is reduced and eventually apnoea will follow.