Mechanically supporting the patient's breathing using only one of the two lungs, known as one-lung ventilation, is used in lung surgery to allow the surgeon to gain access to one side of the chest whilst the anaesthetist supports the lung on the other side. One-lung ventilation markedly alters the exchange of gases in the lungs, including anaesthetic gases. Because of this derangement, it is not known if the end-tidal concentrations are related to blood levels of anaesthetic gases during this type of mechanical ventilation. The aims of the study is to determine if end-tidal concentrations of anaesthetics as measured by a standard gas monitor are related to the blood levels of the gases isoflurane and sevoflurane during one-lung ventilation.