Athletic heart syndrome is a physiological disorder characterized by various structural and functional changes that occur as a response to the athletic activities. Hypertrophy and increased diameter of the left ventricle, increased cardiac output, changes in stroke volume, and several other adaptive mechanisms can manifest as sinus bradycardia, or sometimes even as arrhythmia. The diagnosis is made through an electrocardiography and echocardiography, both of which are able to discriminate athletic heart syndrome from pathological diseases.
Although numerous structural changes occur in athletic heart syndrome, such as hypertrophy of the heart (but mainly of the left ventricle), significant rise in the maximal oxygen uptake, increased stroke volume, and thickening of the myocardium, this phenomenon is considered to be a physiological response that meets the demands of the body to perform the necessary movements and actions    . This syndrome is most commonly encountered among athletes who participate in sports that require endurance - triathlon, long-to-middle distance running, speed skating, and cross-country skiing . Because of the heart's increased size, diameter, and the strength with which it expulses blood, one of the most common signs of athletic heart syndrome is sinus bradycardia . Heart rate can be as low as 30 beats per minute on Holter monitoring, or it can go even less than 30 beats during sleep . Sinus arrhythmias are the second important component of the clinical presentation of athletic heart syndrome . First or second degree atrioventricular (AV) blocks, incomplete right bundle branch block (RBBB), extrasystoles, and early repolarization (presenting as ST elevation) are frequent exercise-related findings that are interpreted as physiological  . Atrial fibrillation is noticed in aged athletes who were involved in endurance training  .
Many studies stress the need for discriminating athletic heart syndrome from pathological conditions that may present in a similar fashion and pose a life-threatening risk -hypertrophic cardiomyopathy, pathologically enlarged left ventricle, Wolff-Parkinson-White syndrome, Brugada syndrome, arrhythmogenic right ventricular dysplasia, and QT syndrome should be included in the differential diagnosis     . The physician should obtain a detailed history during which the patient must be asked about the sport he/she is practicing, for how many years, and on what level (amateur or professional). A thorough physical examination should follow, with an emphasis on cardiac auscultation that will almost universally reveal bradycardia at rest. Electrocardiography (ECG) and Holter monitoring are very useful diagnostic modalities that evaluate the cardiac conduction system and determine if any abnormalities are present   . As mentioned previously, sinus bradycardia, AV blocks, extrasystoles, and early repolarization are hallmarks of athletic heart syndrome  . If, however, ECG reveals left anterior or posterior hemiblocks, complete bundle branch blocks, ventricular preexcitation (an indicator of Wolff-Parkinson-White syndrome), abnormal Q waves, ST depression, or epsilon waves, further evaluation is mandatory  . Cardiac ultrasonography is crucial for examining the morphology of the heart and its characteristics and should be used to confirm that the dimensions of heart are within the criteria for athletic heart syndrome   . The end-diastolic diameter of the left ventricle should not exceed 63 mm in men and 60 mm in women, Myocardial thickness of the left ventricle should not exceed 13 and 12 mm respectively, and the heart mass should not be over 7.5 g/kg for men and 7 g/kg in women  .