Background:Postoperative atrial fibrillation (POAF) is a common complication of cardiac surgery and may result in stroke, heart failure and poor prognosis. This study evaluated a novel index of total atrial conduction time derived from the P-wave onset (lead II) to the peak A’ wave on tissue Doppler imaging (PA-TDI duration) in patients undergoing mitral valve surgery (MVS) for mitral valve regurgitation.Methods and Results:Seventy-three patients undergoing MVS had transthoracic echocardiography with tissue Doppler imaging preoperatively and were monitored postoperatively with continuous electrocardiographic telemetry for 14 days. Preoperative characteristics, echocardiographic data, operative data and postoperative findings were compared between patients with (n=44) and without (n=29) POAF. Postoperative cardiac events were higher in patients with than without POAF (12/44, 27% vs. 3/29, 10%; P=0.0798) and cerebral events occurred in only 2 POAF patients. On multivariate analysis the independent predictors of POAF were degenerative disease etiology (OR, 4.61; 95% CI: 1.41–15.0; P=0.0112) and PA-TDI duration (OR, 1.04; 95% CI: 1.01–1.07; P=0.0048). On ROC curve analysis a PA-TDI cut-off of 159.4 ms was optimal for predicting POAF.Conclusions:PA-TDI duration was an independent predictor of POAF after MVS. Patients with PA-TDI duration >159.4 ms should be considered high risk and treated appropriately to improve outcome. (Circ J 2016; 80: 101–109)