This study is investigating the "second gas effect", a phenomenon produced by the uptake of nitrous oxide (N2O) by the lungs, during the course of a typical anaesthetic. The effect is to increase the concentration of other breathed gases in the lung. These include oxygen and volatile anaesthetic agents such as sevoflurane, which are also normally administered along with N2O. We wish to i) measure the magnitude of the second gas effects on both blood and expired concentrations of sevoflurane (Part 1), and ii) see if a demonstrable difference exists between the effects on blood and expired concentrations.

This study is investigating the "second gas effect", a phenomenon produced by the uptake of nitrous oxide (N2O) by the lungs, during the course of a typical anaesthetic. The effect is to increase the concentration of other breathed gases in the lung. These include oxygen and volatile anaesthetic agents such as sevoflurane, which are also normally administered along with N2O.

While the second gas effect has been demonstrated previously, by measuring the concentration of volatile anaesthetic in the expired breath, no study has yet shown that it has a significant effect on the concentrations in the blood. The blood concentration is in fact more important, as it directly determines the concentration of anaesthetic reaching the brain, and therefore the effect on the depth of anaesthesia. The second gas effect on blood concentrations may be more powerful than that on expired concentrations, due to the detrimental effect of anaesthesia on the evenness of distribution of ventilation and blood flow in the lung.

The proposed study will have two parallel components or Parts. We wish to i) measure the magnitude of the second gas effects on both blood and expired concentrations of sevoflurane (Part 1), and ii) see if a demonstrable difference exists between the effects on blood and expired concentrations.

Inclusion Criteria:

• Patients scheduled for elective surgery requiring relaxant general anaesthesia with arterial blood pressure monitoring via an arterial line.

Exclusion criteria:

• Patients with moderately or severely impaired respiratory disease (FEV1 < 1.5L, or FVC < 2.0L).
• Patients under 18 years of age.
• Morbidly obese patients: BMI > 30
• Patients unable to give informed consent.