Atrial premature complexes are often the result of atrial dilation (left or right); however, additional considerations would be mechanical irritation (perihilar lymph node enlargement, atrial/auricular mass, heart base tumor, pericardial disease, etc.), infectious/tick-borne disease, metabolic disturbances, or potentially altered autonomic tone (sympathetic/parasympathetic imbalance).  

This rhythm is fortunately not considered fatal in itself.  

If atrial dilation is the underlying etiology, your pet may go into atrial fibrillation in the future.  Antiarrhythmic therapy may be indicated based on the frequency and grade of atrial arrhythmias noted on the ECG.