Updates on cardiac arrhythmias in sport horses

Cardiovascular problems represent the third cause of poor performance in sport horses, after musculoskeletal and respiratory disorders. Among these, arrhythmias are of particular interest although only the role of atrial fibrillation on athletic performance was established. The significance of other arrhythmias, such as supraventricular (SVPDs) and ventricular premature depolarizations (VPDs) is still not completely defined. It has been suggested that premature depolarizations (PDs) have to be considered clinically important if more than two isolated PDs are detected during maximal exercise or if multiple PDs (>5) or pairs or paroxysms of PDs are detected during peak exercise or immediately after exercise [1]. During the last ten years, different researchers investigated the possible influence of PDs on equine athletic performance. Particularly, studies focused on racehorses [2–6] in which even slight variations of cardiac rhythm may play a significant role [7]. The prevalence of PDs (supraventricular and/or ventricular) recorded in poorly performing Thoroughbred racehorses is about 63%, with the main incidence during and immediately after maximal exercise [2]. A significant prevalence of PDs has been reported also in Standardbred racehorses (about 46-50% SVPDs and 4-19% VPDs during and immediately after racing). In these horses the absence of significant association between arrhythmias and echocardiographic abnormalities or increased myocardial damage indicators (troponin I) suggests a physiological origin of the rhythm disturbances [3–5]. Recently, isolated SVPDs and VPDs, with a prevalence of about 70% and 12% respectively, were reported in normal performing showjumping horses [6]. Moreover, isolated SVPDs (28%) and VPDs (5%) were recorded during exercise in normal dressage horses [7]. According to the current knowledge, the importance of PDs is under discussion, and different researchers agree that these arrhythmias could be observed in a significant percentage of clinically normal well performing sport horses [2,6–8]. Nowadays, it might be better referring to ACVIM/ECEIM consensus statement for the management of athletic horses with cardiovascular abnormalities [9]. Some recommendations for equine athletes are quite similar to those reported for human competitive athletes with PDs [10]. In particular, supraventricular premature depolarizations, in absence of structural heart disease and symptoms, are considered not dangerous for the athletes, both human and equine. Instead, when ventricular premature depolarizations or other ventricular arrhythmias occurs, more in depth evaluation is suggested [9,10]. The exact influence of PDs on athletic performance, particularly when they are infrequent, isolated and/or detected in periods different from peak exercise, needs to be further investigated. In addition, different authors state that their role as risk factor for development of sudden death, immediately after intense exercise in athletic horses, require further investigation [3,4,9,11].