Measurement of the anaesthetic gases such as isoflurane and sevoflurane allow anaesthetists to gauge their blood levels and in turn their brain levels and so help assess how deep the patient is under anaesthesia. These measurements, made as the patient breathes out, are known as end-tidal concentrations of anesthetic gases and are routinely made with gas monitors. Measurement of blood levels of anaesthetic gases is possible but infrequently done as it is time-consuming, expensive, and requires a scientist in a laboratory with specialized equipment. Depth of anaesthesia is assessed using clinical signs along with end-tidal concentrations though more recently it can be assessed by monitoring the brain's electrical waves. This technique remains controversial.~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.~Patients over 18 years old undergoing planned lung surgery with one-lung ventilation and anaesthesia with isoflurane or sevoflurane, and who have a routine tube inserted into an artery in their wrist to monitor blood pressure will be recruited for the study. During the study, two blood samples the size of two teaspoons each will be taken from the tube in the artery and two end-tidal concentration recordings.
The aims of the study is to determine if end-tidal concentrations of the anaesthetic gases isoflurane and sevoflurane as measured by a standard gas monitor are related to the blood levels during one-lung ventilation.