Genetic and congenital cardiac conditions are the primary cause of exercise-related cardiac events in young subjects (see Table 13.1) (2). Acquired conditions such as myocarditis, idiopathic dilated cardiomyopathy, infiltrative myocardial disease, and atherosclerotic disease are responsible for a small proportion of these events in young subjects (3). The causes of exertion-related deaths in young subjects may vary by geographical location. In the United States, hypertrophic cardiomyopathy (HCM) accounts for 36% to 50% of sudden death in young athletes (3) and arrhythmogenic right ventricular cardiomyopathy (ARVC) is an unusual cause accounting for only 1% of such events (3). In contrast, ARVC is the dominant cause of exercise-related deaths in young Italian athletes accounting for 20% of events in the Veneto region of that country (4). It is unclear whether these differences are due to varying prevalence of disease or differences in screening procedures and diagnostic criteria. Other common causes of exercise-related cardiac deaths in young subjects include anomalous origin and course of the coronary arteries (13%), valvular and subvalvular aortic stenosis (8%), and aortic dissection in association with inherited connective tissue disorders such as Marfan’s syndrome (2%) (2,3).

The frequency of exercise-related sudden death in high school and college athletes has been estimated as one death/year for every 133,000 men and 796,000 women (2). Deaths were included if they occurred within 1 hour of practice or competition. These numbers can only be regarded as estimates, and each death is a tragedy for the individual and the family, but these estimates do provide reassurance that the absolute incidence of sudden death during athletic participation is low. These results also place the value of screening in question. The Cardiovascular Disease in Adolescents (CARDIA) Study demonstrates that the incidence of unexplained echocardiographically documented left ventricular hypertrophy (LVH), consistent with HCM, is one to two cases per 1,000 individuals in the general population (5). Because HCM is the most frequent cause of exercise-related cardiac deaths in American athletes, it is surprising that the death rate is not higher. Indeed, if all cases of unexplained LVH and HCM resulted in exercise-related events, the incidence of death could be as high as 200 cases per 100,000 male athletes. Such observations suggest that most athletes with unexplained LVH and possible HCM tolerate the condition and athletic activity without being affected by sudden death (6). Such observations also question the wisdom of routinely screening all athletes and disqualifying all with possible HCM.

Atherosclerotic vascular disease is the primary cause of almost all exercise-related deaths in adults. Ragosta examined the cause of death in 75 men and 1 woman older than 29 years who died during or immediately after recreational exercise (7). All deaths were related to atherosclerotic vascular disease including atherosclerotic coronary artery disease (CAD) in 71, hypertensive heart disease in 2, dissecting aortic aneurysm in 1, and cerebrovascular accident in 1 victim. In addition to being the primary cause of exercise-related death, atherosclerotic CAD causes virtually all exercise-related myocardial infarctions (MIs). Sudden cardiac death and acute MI in previously healthy adults in the general population are generally produced by atherosclerotic plaque rupture with acute coronary thrombosis (8,9) although erosion, rather than rupture, of the atherosclerotic plaque has been noted by some observers in up to 32% of exertion-related deaths (10).

The absolute incidence of cardiac events during exertion is low, but increased relative to the incidence during nonvigorous activities. From a collection of deaths during exertion, we estimated the annual incidence of death during jogging as only one death per 15,640 previously healthy adults, but the hourly frequency of death during exercise was seven times that at rest (11). Others determined an absolute annual incidence of exercise-related cardiac arrest as only 1 per 18,000 previously healthy men, but again the incidence during exercise was greater than at rest (12). Interestingly, for the habitually least active men, the risk of a cardiac arrest was 56-fold greater during exercise than at rest whereas the relative risk of exercise was only 5-fold greater in the habitually most active men. We observed a similar pattern for exercise-related MI (9). Physically inactive men were 30 times more likely to suffer an MI during vigorous exertion whereas the relative risk was only 20% higher (relative risk of 1.2) and not significantly increased in the most active subjects. These results demonstrate that exercise increases the risk of exercise events, but such events in adults are largely restricted to physically inactive individuals, primarily men, performing unaccustomed physical activity.

All of these studies on the incidence of exercise-related cardiac events suffer from small sample sizes because the absolute incidence of events is low. Results have been reported but only in abstract form for almost 3 million members of a national chain of fitness facilities who engaged in 182 million exercise sessions (13). There were only 71 fatal events occurring in 61 men and 10 women. Almost 55% of those suffering cardiac events exercised on average less than once weekly and 10 had participated in less than three sessions. The incidence of events was one death per 25,000 subjects per year. These results are important because they confirm with more events in a larger population the figures reported in smaller studies (12,13) and also support the conclusion that exercise-related cardiac events are more frequent in infrequent exercisers (9,13,14).

This study also has the potential to provide important data on the incidence of cardiac events in adult women who have not been studied previously because of the rarity of exercise events in this group.

The mechanism by which exercise increases acute plaque rupture is not clear. Thompson in 2000 postulated that the increased “twisting and bending” of coronary arteries during vigorous exertion increased the frequency of plaque rupture (1). This motion is exacerbated by the increases in heart rate and contractility produced by exercise. Other possibilities also exist and probably contribute. Exercise dilates normal coronary arteries, but can produce vasoconstriction in atherosclerotic segments (15). Such spasm over a thickened, noncompliant atherosclerotic plaque could itself induce plaque rupture. Exercise also acutely increases platelet aggregability, which would increase thrombosis; platelet to leukocyte aggregation, which could increase leukocyte adherence to an injured arterial wall; and elastase levels, which can attack elastic fibers in the extracellular matrix and facilitate plaque disruption (16).

It should be emphasized that plaque rupture and thrombosis as a cause of exercise-related cardiac death
applies primarily to previously asymptomatic subjects. Patients with known coronary heart disease who die during exertion may, but often do not, demonstrate evidence of an acute coronary lesion or recent myocardial injury, but often show evidence of a previous infarction (17). This suggests that these patients died of ventricular fibrillation (VF), which originated from the infarction site.

Physicians performing cardiac evaluations of highly trained competitive athletes should be aware that the clinical findings characteristic of the athletic heart syndrome can mimic disease and increase the complexity of the evaluation. Clinical distinctions between physiologic athlete’s heart and pathological conditions have critical implications for athletes, because cardiovascular abnormalities can trigger disqualification from competitive sports and a misdiagnosis can result in unnecessary restrictions, depriving athletes of the psychological, social, and economic benefits of sports (18,19,20,21,22,23,24). The athletic heart syndrome was first described by Henschen in 1889 (25) and is characterized by electrocardiographic (ECG) changes and by changes in cardiac conduction and architecture. There are three salient points regarding the athletic heart syndrome. First, cardiovascular findings in the athletic heart syndrome rarely exceed the normal range. Second, cardiac abnormalities are generally found only in endurance-trained athletes. Third, findings that do exceed the normal range are generally restricted to well-trained athletes engaged in endurance sports requiring the use of a large muscle mass, such as rowing and Nordic skiing. The corollary of these principles is that findings exceeding normal parameters in nonendurance or noncompetitive athletes should prompt a search for other etiologies.

The ECG findings of the athletic heart syndrome include ST segment changes of early repolarization, abnormal T waves, and evidence of chamber enlargement. The ST-T segment elevations of early repolarization and T inversions are attributed to the augmented parasympathetic tone characteristic of exercise training.

Habitual endurance exercise produces a global cardiac enlargement, which may affect both the right and left atria and ventricles in the presence of normal systolic and diastolic function (20). The most consistent enlargement is seen in the left ventricular chamber. Mild enlargement of both atria and the right ventricle can occur, but marked enlargement of these structures is suggestive of a disease process, and is not observed in the athletic heart syndrome. The magnitude of physiologic hypertrophy may also vary according to the particular type of sports training (26).

The ECG in well-trained athletes may show mildly increased P wave amplitude suggesting right atrial enlargement, P wave notching suggesting left atrial enlargement, incomplete right bundle branch block (RBBB), and voltage criteria for right and LVH (27). Among endurance athletes, voltage criteria for right ventricular hypertrophy are noted in 18% to 69% of subjects (28). Incomplete RBBB is also common. ECG voltage evidence for LVH is common in endurance athletes and the voltage may be quite large in some athletes. Frequent/complex ventricular tachyarrhythmias, increased R or S wave voltages, Q waves and repolarization abnormalities may also be seen in the ECG, mimicking certain cardiac diseases (21,22).

The resting bradycardia, sinus arrhythmia, and atrioventricular conduction delay of the athletic heart syndrome are due to enhanced parasympathetic and reduced sympathetic tone. These abnormalities in sinus rate, A-V conduction, and early repolarization should disappear with exertion because exercise decreases vagal tone and increases sympathetic activity. A heart rate less than 60 beats/minute is found in up to 91% of endurance athletes (28) and rates as low as 25 beats/minute have been reported (29). Sinus bradycardia and sinus pauses of more than 2 seconds have been documented during sleep in endurance athletes (28). First degree A-V block, defined as a PR interval greater than 0.20 seconds is reported in 10% to 33% of endurance athletes (29). Second degree A-V block of the Mobitz I or Wenchebach pattern or progressive prolongation of the PR interval before a nonconducted P wave is also seen more commonly in the athletic heart syndrome (29) (see Table 13.2). Second degree A-V block with Mobitz II appearance, which appears as a nonconducted P wave without preceding PR prolongation, is not typical
of the athletic heart syndrome. A-V block with Mobitz II appearance may occur in well-trained athletes, but is rare (28) and should be attributed to the athletic training only if the athlete is asymptomatic and no other abnormalities are detected. The prolongation of the A-V interval and decrease in A-V conduction velocity described earlier may also unmask ventricular pre-excitation seen in the Wolff-Parkinson-White (WPW) syndrome (28).

ST elevation of the “early repolarization pattern” is common in endurance-trained athletes whereas ST depression is extremely unusual. Peaked, biphasic, and inverted T waves in the precordial leads may also be seen in endurance athletes. Deeply inverted T waves can also be normal in athletes, but are rare and should be evaluated by echocardiography because they may be a sign of HCM.

Echocardiographic studies have documented global chamber enlargement in the athletes. At least 59 studies have consistently documented increased left ventricular dimensions. Thirteen studies noted that right ventricular dimensions averaged 24% greater in athletes than controls (30). Fourteen studies examined left atrial dimensions and demonstrated that the transverse dimension was 16% larger in the athletes. At least one study has documented a larger right atrial size in the athletes (18).

Pelliccia et al. examined left ventricular wall thickness in 783 nationally ranked Italian male athletes and 209 women (18). Only 16 athletes or 1.7% had a left ventricular wall thickness of more than 12 mm, the upper limit of normal. Fifteen of these athletes were rowers or canoeists, sports that require both isotonic and isometric effort and involve a large muscle mass. All athletes with increased wall dimensions were internationally ranked. The largest wall thickness in any athlete was 16 mm. All of the female athletes had wall thickness values below 11 mm.

These same investigators examined left ventricular cavity dimensions in 1,300 elite athletes participating in 38 different sports (19). left ventricular end diastolic dimension (LVEDD) was greater in male (55 mm) than in female (48 mm) athletes. LVEDD was greater than 55 mm, the upper limits of normal, in 45% of the athletes and exceeded 60 mm in 14%. The largest LVEDD was 66 mm in a woman and 70 mm in a man. The sports that were most associated with an LVEDD equal or greater than 60 mm were those that required a large endurance component or a combination of moderate endurance training and increased body size such as cycling (49% of cycling athletes), ice hockey (42%), basketball (40%), rugby (39%), canoeing (39%), and rowing (34%). Despite enlarged intrachamber dimensions only 14 of the athletes or 1.1% had a septal thickness of more than 12 mm and only 4 athletes (0.3%) exceed this posterior wall thickness.

These results are useful in differentiating the athletic heart syndrome from pathological conditions. Left ventricular wall thickness more than 12 mm affected less than 2% of elite athletes and should not be seen in healthy recreational athletes. No athlete had a left ventricular wall thickness more than 16 mm so that values above this range should raise the possibility of HCM. The largest cardiac dimensions occurred in elite athletes performing sports that require a large amount of endurance training and increased body size. Also, despite enlarged end diastolic ventricular volumes in the athletes, systolic and diastolic function was normal. However, there are occasional ambiguous situations. The two most common are (a) diagnosing HCM in an athlete with mildly increased left ventricular wall thickness, normal ventricular function, and no systolic anterior motion of the mitral valve and (b) differentiating an early dilated cardiomyopathy from athletic heart syndrome in an athlete with normal left ventricular cavity dimensions and a low-normal left ventricular ejection fraction (50% to 55%). Such cases may be clarified by observing the response of cardiac mass to deconditioning, assessment of diastolic filling parameters (23), genotyping and serial observation and echocardiographic studies of the athlete over time (26).

Functional murmurs are also part of the athletic heart syndrome and are produced by the cardiac adaptations to exercise training. Endurance exercise training reduces resting heart rate, increases resting stroke volume, and enhances cardiac performance. These adaptations produce a larger stroke volume. Much of the larger stroke volume is delivered more vigorously in early systole increasing blood velocity and producing systolic “flow murmurs.” Flow murmurs in young athletes are due to flow across the pulmonic valve and often vary with respiration. Athletes aged 50 years and above may have mild sclerosis of the aortic valve leaflets and their flow murmurs are often due to both aortic valve sclerosis and the increased blood velocity. These murmurs may be less “innocent” because they can progress over time to become important aortic stenosis of adults or what has been called “aortic stenosis of the elderly

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