Chapter 36

• The patient has an extensive history of exercise training
• There may be profound sinus bradycardia, sinus arrhythmias, or atrioventricular conduction delays at rest that disappear with exertion.
• The athlete's heart may demonstrate four-chamber enlargement and mild left ventricular hypertrophy, but normal diastolic function and B-natriuretic peptide levels. The diastolic wall-to-volume ratio should be < 0.15 mm/m2/mL by magnetic resonance imaging.
• Morphologic changes reverse with detraining.

The salutary physiologic effects of exercise training on the cardiovascular system have been studied extensively, but intense exercise training can produce cardiac adaptations that mimic pathological conditions. Consequently, clinicians should be aware of the normal cardiac structural and functional responses to exercise training in order to distinguish these changes from clinical disease that could also affect athletes.

### Physiology of Exercise Training

The “athlete's heart” refers to the normal structural and physiologic cardiac adaptations to exercise training. Clinical characteristics of the athlete's heart include resting sinus bradycardia (occasionally profound), sinus arrhythmia, atrioventricular (AV) conduction delays, systolic flow murmurs, four-chamber enlargement, and an increase in cardiac mass, but usually normal or augmented ventricular systolic function.

The magnitude of these cardiac changes depends on a variety of factors including the duration and intensity of the exercise training, the body size of the athlete, the sport, and the physiologic demands of the exercise used to train for that sport. Sports can be roughly classified according to the type and intensity of the exercise performed and the degree of static and dynamic exercise required, but such classifications are innately flawed because they do not consider exercises used in training. Dynamic (isotonic) exercise and sports primarily involve changes in muscle length and joint movement with rhythmic contractions and small intramuscular force. Static (isometric) exercise and sports mainly involve large intramuscular force with little or no change in muscle length or joint movement. Most sports require elements of both.

The acute cardiovascular response to dynamic (aerobic) exercise includes a decrease in peripheral vascular resistance and increases in heart rate, stroke volume, cardiac output, systolic blood pressure, the arteriovenous oxygen difference, and oxygen consumption. Endurance exercise training increases maximal exercise capacity as measured by maximal oxygen uptake. This increase in exercise capacity is produced by increases in the arteriovenous oxygen difference and cardiac output due to increased stroke volume.

Endurance (aerobic) sports, such as long-distance running, and their required training produce the greatest reductions in heart rate and the largest increases in maximum oxygen consumption, cardiac output, stroke volume, and cardiac chamber dimensions. Endurance exercise predominantly produces a volume load on the left and right ventricles. Static or strength sports, such as weight lifting, in contrast, produce only small increases in oxygen consumption and minimal changes in cavity size, but may be associated with mild to modest increases in wall thickness, some of which may be due to increased body mass in such athletes. These changes in wall thickness with little change ...

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