A 24-year-old male rower was referred to the cardiologist office for palpitations. He described palpitations that occurred after exercise and denied any lightheadedness or syncope. Echocardiogram was performed, which showed a dilated left ventricle, dilated left atrium, mild concentric left ventricular hypertrophy, and normal left ventricular function. In endurance athletes, cardiac remodeling may occur; distinguishing exercise-induced cardiac adaptations from pathology is important.

Athletic heart syndrome encompasses a variety of significant physiological and morphological changes that occur in a human heart after repetitive strenuous physical exercise. Physical exercise is dependent on the cardiovascular system’s ability to provide oxygenated blood to the critical organs, removing deoxygenated blood from tissues, as well as the lungs’ ability to clear carbon dioxide (CO₂). With exercise a greater proportion of the oxygen (O₂) is extracted from the blood.

Cardiopulmonary stress testing can be used to assess the athletic performance of an athlete. Gas exchange during rest and exercise can be analyzed. Myocardial oxygen demand is directly related to exercise intensity. The increasing oxygen demands cause an increase in pulmonary oxygen uptake (VO₂). Physiological changes that occur with training include lowered blood pressure and heart rate, enhanced contraction and relaxation of both ventricles, as well as an increase in the VO₂ max. VO₂ max is widely considered to be the gold standard metric for cardiovascular fitness.

Cardiac output is the product of stroke volume and heart rate and may increase 5- to 6-fold during maximal exercise. Heart rate increase accounts for a majority of the augmentation in cardiac output with exercise and may range from 40 bpm at rest to ≥200 bpm in a young maximally exercising athlete.¹ Hemodynamic conditions, specifically changes in cardiac output and peripheral vascular resistance (PVR), vary widely across sporting disciplines.

SPORT-SPECIFIC REMODELING

Structural adaptions of the heart with exercise include increases in heart cavity dimensions, augmentation of cardiac output, and increases in heart muscle mass. Although some overlap exists, exercise activity can be segregated into 2 forms with defining hemodynamic differences.

Isotonic exercise (endurance training) involves sustained elevations in cardiac output with normal or reduced PVR. This hemodynamic effect causes a volume challenge for the heart that affects all 4 chambers. Cardiac adaptations seen with daily sustained exercise include an increase in left ventricular (LV) mass, LV chamber dilation, enhanced LV diastolic function, biatrial enlargement, and right ventricular (RV) dilation with increased systolic and diastolic function. Examples of isotonic sports are long-distance running and swimming.

Isometric exercise (strength training) is characterized by increased PVR and normal or only slightly elevated cardiac output. This increase in PVR causes a pressure load on the ventricle with a transient but potentially marked systolic hypertension and LV afterload. Concentric left ventricular hypertrophy (LVH) and reduction in LV diastolic function occur with repetitive short burst-type power exercises. Football, weightlifting, and discus throwing are some examples of sports involving isometric training. Various sports such as cycling and rowing require both isotonic and isometric forms of training.

Routine athletic training causes alterations in chamber sizes with associated normal systolic and diastolic function in approximately 50% of athletes.² The Morganroth hypothesis is based on a study that compared M-mode echocardiographic LV measurements in wrestlers (strength training), swimmers (endurance training), and sedentary control subjects and found significant LV differences across the groups. Athletes exposed to strength training demonstrated concentric LVH whereas individuals exposed to endurance training demonstrated eccentric LV enlargement. This study led to the concept of sport-specific cardiac remodeling.³ LV ejection fraction is generally normal among athletes.^4,5

The pattern of LVH may help to distinguish between pathology and an athletic heart. The heart of an athlete is almost always symmetrical; whereas, pathology such as hypertrophic cardiomyopathy (HCM) results in asymmetric hypertrophy of the left ventricle. In an athletic heart, LVH and concomitant left ventricular cavity dilatation results in preservation of the LV relative wall thickness or the ratio between the posterior LV wall thickness and the LV end-diastolic diameter. Patients with hypertrophic cardiomyopathy have pathological hypertrophy, which results in a reduced LV cavity size in athletes.

Left atrial remodeling is a physiological adaptation present in highly trained endurance athletes and usually seen accompanying LV cavity enlargement.⁶ Increased transverse left atrial dimensions (≥40 mm) occur in 20% of athletes where larger dimensions (≥45 mm) occur in approximately 2% of athletes.² Although increased in size, the overall left atrium should remain proportional to the left ventricular cavity size. There is an increased risk of atrial fibrillation in endurance athletes.^7,8

Physiological adaptations occur in both male and female athletes; however, the degree of physiological remodeling varies among genders. Women exhibit less physiological remodeling compared to men.^9,10 Race is also a variable in physiological remodeling and black athletes have more LVH than their white counterparts.^11,12

Echocardiography is a useful modality when working to evaluate athletic adaptations from the differential diagnosis of diseases that can cause sudden cardiac death (SCD). This portable and noninvasive modality can analyze the left ventricle for pathologies such as hypertrophy cardiomyopathy, dilated cardiomyopathy, and left ventricle noncompaction, and analyze the right ventricle for arrhythmogenic right ventricular cardiomyopathy.

A commonly encountered ECG-based diagnostic dilemma is that of increased QRS voltage, which occurs in both healthy athletes and pathologic cardiac hypertrophy. A number of echocardiographic studies have documented increases in LV chamber dimensions, wall thickness, and mass among healthy, highly trained individuals.^13-15 The increase in LV wall thickness that can result from exercise training has received particular attention because it can share similarities with that caused by HCM.

Hypertrophic cardiomyopathy (HCM) is a well-recognized cause of exercise-related SCD among athletes. This condition is the most common cardiovascular cause of sudden death among athletes in the United States and has the propensity to affect individuals at all levels of competition.¹⁶ Affected individuals have characteristic myofibrillar disarray on histologic inspection of the myocardium while gross analysis demonstrates LVH with increased wall thickness of either symmetric or asymmetric distribution. Roughly 80% of individuals with HCM have electrocardiographic (ECG) abnormalities including interventricular conduction delay, voltage criteria for LVH, ST-segment depression, T-wave inversion, and anterior/inferior Q waves producing a pseudo-infarct pattern.^17,18 While such findings are typical of HCM, they are encountered commonly among trained athletes with no underlying structural heart disease.

Echocardiography is the preferred method of diagnosis for HCM. Findings seen on echocardiography that are suggestive of HCM include increased LV wall thickness, small or normal LV chamber size (LV end-diastolic diameter <45 mm), altered diastolic filling pattern, and systolic anterior motion (SAM) of the mitral valve with associated mitral regurgitation. When reporting HCM it is important to describe area of maximum hypertrophy along with left ventricular outflow tract gradient at rest and with valsalva.

Research has been done to evaluate the magnitude of LVH in athletes without HCM. Pelliccia et al performed echocardiographic assessment of LV wall thicknesses among 947 elite athletes and found that 1.7% of athletes have LV wall thickness >13 mm and all those patients had concomitant LV cavity dilation, a combination not typically found among individuals with HCM.¹⁹ This has been shown again in other studies suggesting that adaptive remodeling can be differentiated from structural heart disease by consideration of LV cavity dimensions.²⁰ LV wall thickness ≥16 mm is very rare among healthy athletes and this finding must be considered as highly suggestive of HCM. Atrial dilation is less likely to occur in HCM patients compared to patients with exercise-induced LV remodeling. Caselli et al found that LA transverse diameter measurement >40 mm was highly reliable (sensitivity 92% and specificity 71%) in excluding HCM.²¹

Other echocardiographic data can be used to differentiate between HCM and cardiac adaptations of an athlete. Several studies have shown that diastolic tissue velocities are reduced among individuals with HCM.^22-24 In contrast, a number of authors have independently shown that diastolic tissue velocities are normal or elevated among athletic individuals even in the context of LV enlargement.^21,22

Other echocardiographic techniques such as strain can be helpful in differentiating an athletic heart from HCM. Strain measurements define changes in muscle fiber length as they contract and relax during the cardiac cycle while strain rate measures these length changes as a function of time. A study by Serri et al found that those with HCM have lower LV systolic strain values than normal controls.²⁵ In contrast, individuals with adaptive remodeling have strain values that are normal or higher than those found in sedentary healthy controls.^26,27

SCD in an athlete is tragic and often occurs as a first manifestation of disease. Although HCM is the most common etiology of SCD, other LV pathologies, such as left ventricular noncompaction (LVNC), need to be excluded in athletes.

LVNC cardiomyopathy is a rare condition characterized by increased LV trabeculations and intertrabecular recesses (crypts). LVNC cardiomyopathy is thought to occur due to failure of myocardial compaction during embryological development. Coronary circulation is usually normal and is not associated with the intratrabecular recesses. Clinical presentation of LVNC is variable and can present with HF, thromboembolic events, arrhythmia, or SCD; or it can be asymptomatic. Bhatia et al reported a yearly cardiovascular event rate of 4% in 241 adults with lone LVNC diagnosed by echocardiography with a familial inheritance in first-degree family members of 30%.²⁸