Brief Summary: Post-operative atrial fibrillation (POAF) is one of the common complicaiton following cardiac surgery, occurring in approximately 20%-40% patients. Although POAF is associated with longer hospital stays, most POAF is short lived and resolves in the 2-4 days afer cardiac surgery. However, a recent meta-analysis showed significant increases in 1,5, and 10 year mortality in POAF patients (odds ratio:1.60, 2.60, 1.51;95% confidence intervals: 1.52 to 1.68, 2.00 to 3.38, 1.43 to 1.60;P <0.0001), the combined adjusted risk of death (16 studies, n =84,295) was also significantly increased in patients with POAF (hazard ratio: 1.25;95% confidence interval: 1.2-1.3;P < 0.0001). These data highlight the need to understand better the underlying mechanism of POAF. A latest research in Nature reported levels of circulating calcitonin (CT), which is the main risk factor for atrial fibrillation (AF). Global disruption of CT receptor signalling in mice causes atrial fibrosis and increases susceptibility to AF. Here we will explore the relationship between CT and POAF.