This study aims to investigate the effect of tracheal intubation on regional ventilation of the lungs by comparing measurements before and after the patient is anaesthetised and intubated. We aim to show whether altered patterns of ventilation are caused by patients simply being asleep and ventilated, or whether these changes are due to the use of a tracheal tube itself. The exact effect of tube position will also be studied by measuring ventilation as the tube is deliberately advanced until it enters one of the lungs. This will give us information about the ideal position for a tube within the trachea to promote optimal ventilation patterns within the lungs