Background and Objectives: Interatrial block (IAB) is characterized by a delay of inter-atrial conduction and is defined electrocardiographically by a P-wave duration (PWD) equal to or more than 120 ms. Several studies have already implicated IAB in the development of new onset atrial fibrillation (AF). The possible connection between postoperative IAB and incidence of postoperative atrial fibrillation (POAF) in a large cohort of patients undergoing open-heart surgery has not been investigated so far. 

Methods: 1316 patients without prior history of AF who underwent open-heart surgery at Paracelsus Medical University Nuremberg Heart Centre in 2013–2020 were included in a single-centre University Hospital retrospective observational case-control study. Patients underwent AVR (n = 433) with partial (n = 386) or median (n = 47) sternotomy, CABG (n = 454), combination of AVR and CABG (n = 353), or a combination of MVR and CABG (n = 76). Paper pre- and postoperative ECG tracings were collected and read by three independent readers blinded to outcome. Logistic regression analysis was used to study incidence as well as recurrence of POAF, comparing patients with IAB or its subtypes in the postoperative ECGs to those without IAB. IAB was defined as P-wave duration ≥ 120 ms without (partial IAB; P-IAB) or with (advanced IAB; A-IAB) biphasic (+/-) P-wave morphology in the inferior ECG leads (II, III, aVF). Intermittent IAB (I-IAB) was defined with a changing P-wave morphology/duration in the same ECG recording. 

Results: POAF was detected in 35.9% of patients, which is highly comparable to other studies. 17.6% of patients had two or more episode of POAF during the duration of the hospitalisation. The overall prevalence of postoperative IAB was 27.1%.

Patients with any type of IAB exhibited higher rates of POAF than patients without IAB (48.7% vs. 31,1%; p < 0.001) in the combined patient collective for IAB of all subtypes.  The ratio of patients developing POAF was 85/173 (49.1%), 45/95 (47.4%) and 56/113 (49.6%) for the patients with P-IAB, I-IAB and A-IAB respectively and was found to be significant in all groups (p < 0.002). The incidence of recurrent POAF was significantly higher (p < 0.001) in the combined collective for IAB (94/357 vs. 138/959; 26.3% vs. 14.4%) as well as for P-IAB, I-IAB and A-IAB (28.3%, 25.3%, 26.5% respectively; p < 0.01). 

Conclusion: Postoperative IAB is associated with increased incidence as well as recurrence of POAF accross all subtypes in patients after any type of open-heart surgery. Further studies are waranted to define the role of IAB as potential indicator for POAF recurrence and more agressive anticoagulation strategy in those patients after open-heart surgery.