Patients undergoing hybrid AF ablation second stage catheter ablation have the posterior left atrium mapped to see it if is electrically isolated. This is done via a standard electrophysiogical study in accordance with routine clinical practice.

Investigators propose to check left atrium posterior wall isolation via oesophageal pacing and compare this to findings from invasive study

Oesophageal temporary pacing of the heart is an established treatment in heart block and several catheters are licenced for this purpose with minimal procedural risks. Due to the position of the oesophagus, pacing from the oesophagus could sense and pace the posterior left atrium. If the posterior wall could be demonstrated to be electrically isolated with oesophageal pacing then invasive electrophysiological study such as the second stage of a hybrid AF ablation would be unnecessary.

This would save patients from undergoing invasive left atrial mapping and exposure to the consequent risks of the procedure.

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.