Inclusion Criteria: • subjects capable of giving informed consent,  Male or female Aged between 18 and 80 years Undergoing ablation under general anaesthetic with planned isolation of the left atrial posterior wall or who have previously undergone either catheter or surgical ablation for AF with LA posterior wall isolation  Exclusion Criteria:  Unable or willing to give fully informed written consent Turned down for general anaesthetic Pregnancy Terminal illness Any other significant disease or disorder which, in the opinion of the investigator, may either put the participant at risk because of participation in the study, or may influence the result of the study, or the participant's ability to participate.