While numerous investigations in humans have recognized multiple determinants influencing atrial fibrillation (AF) onset, equivalent data concerning dogs remain scarce. The present systematic review aims to identify the primary factors contributing to AF occurrence in canine populations. In compliance with the PRISMA 2020 framework, an extensive search was carried out across the Web of Science and Scopus databases to locate studies documenting naturally developed AF in dogs. The quality of the gathered evidence was evaluated according to the National Institute of Health’s Evidence Grading System. Out of an initial pool of 1,043 papers, twenty met the inclusion criteria, representing a total of 2,359,275 dogs, of which 4,807 displayed spontaneous AF. Major determinants for AF onset among dogs with cardiac disorders—particularly myxomatous mitral valve disease (MMVD) and dilated cardiomyopathy (DCM)—included hereditary factors in Irish Wolfhounds, elevated body weight, and enlargement of the left atrium. Differences between MMVD and DCM were identified, such as the influence of congestive heart failure and echocardiographic evidence showing elevated left atrial pressure or right atrial dilation. Additionally, comparisons with humans revealed that age and male sex, strong predictors in people, are unreliable indicators for dogs.
