Triplets of premature narrow complexes during exercise were observed in 8 workouts (9 arrhythmia instances). Five workouts were in one horse in the CARDIAC group (4 during gallops and 1 during jumping), and 3 workouts (1 each in 3 horses) were in the NON‐CARDIAC group (one workout had 2 instances). The horse in the cardiac group had hyperechoic areas in the myocardium. Triplets of premature narrow complexes in the fast heart rate deceleration period were observed after 2 workouts (Gallops) in 2 horses (one each) in the NON‐CARDIAC group (Figure 2). A triplet of premature wide complexes during fast heart rate deceleration was observed after 1 workout (Gallop) in a horse in the NON‐CARDIAC group 1 (Figure 3). Two episodes of paroxysmal atrial fibrillation were observed after 2 workouts (Gallops) from the same horse in the NON‐CARDIAC group (Figure 4). One run of 6 premature narrow complexes during exercise was observed in one workout (Gallop) in a horse in the CARDIAC group. This horse had a triplet of premature narrow complexes during the same workout. The horse in the cardiac group had hyperechoic areas in the myocardium.