The aim of this study is compare the surgical convenience and early and long-term outcomes of cryomaze procedure using nitrous oxide with using Argon gas.

The investigators will analyze the pathologic findings of atrial tissue after cryoablation using two probes and early and long-term outcomes with Holter monitoring. Atrial fibrillation burden 3 months and 1 year postoperatively and atrial activity will be checked.

The investigators have used the Medtronic ATS cryoprobe (Argon based) since 2009 for cryomaze procedure for treatment of atrial fibrillation combined with valvular heart disease.

However, sometimes, it was inconvenient due to excess flexibility, wide lesion related to excess low temperature and time-consuming detachment from atrium after cryoablation. The investigators introduced Atricure cryoprobe (Nitreos Oxide based) in hope of overcoming these shortcomings of ATS probes last year.

Up to date, the investigators have been satisfactory in terms of the surgical convenience. On the other hand, we have the questionability of transmural lesion of Atricure probe because of the higher temperature of Atricure probe compared with ATS probe.

The aim of this study is compare the surgical convenience and early and long-term outcomes of cryomaze procedure using nitrous oxide with using Argon gas.

The investigators will analyze the pathologic findings of atrial tissue after cryoablation using two probes and early and long-term outcomes with Holter monitoring. Atrial fibrillation burden 3 months and 1 year postoperatively and atrial activity will be checked.

Inclusion Criteria:

• requiring concomitant maze operation with valve surgery (persistent atrial fibrillation)
• between 18 and 75 years old

Exclusion Criteria:

• previous cardiac surgery
• Behcet disease
• Takayasu's arteritis
• Infective endocarditis
• Congenital heart disease
• left atrial size over 80mm
• moderate or greater functional tricuspid regurgitation