Cardiac adaptation to intensive training in prepubertal swimmers

Background Despite the increasing involvement of child athletes in intensive training regimens, little is known about the influence of such training on autonomic regulation and cardiac structure and function. Patients and methods Twenty-five highly trained (12 - 14 h weekly for at least 4 years) swimmers (aged 11.9 +/- 1.6 years; 15 males, 10 females) and 20 non-training normal children who served as controls (aged 11.3 +/- 0.6 years; 14 males, 6 females) were studied. Heart rate variability analysis in the time and frequency domains was performed on 15 min resting heart rate acquisitions. Left ventricular morphology and systolic function was studied with two-dimensional guided M-mode echocardiography. The transmitral flow velocity profile was assessed with pulsed Doppler. Parameters measured included the peak early (E) and peak late (A) transmittal flow velocity and their ratio (E/A). Left atrial (LA) volumes were determined at mitral valve (MV) opening (maximal, Vmax), at onset of atrial systole (P wave of the ECG, Vp), and at MV closure (minimal, Vmin) from the apical 2- and 4-chamber views, using the biplane area-length method. LA systolic function was assessed with the LA active emptying volume (ACTEV) = Vp-Vmin and the LA active emptying fraction (ACTEF) = ACTEV/Vp. Results Average NN (967.1 141.8 vs. 768.4 +/- 85.6 ms, P < 0.0001), logSDNN (1.89 +/- 0.14 vs. 1.80 +/- 0.17 ms, P < 0.05), logPNN 50% (1.66 +/- 0.23 vs. 1.46 +/- 0.35, P < 0.05), and logHF power (3.13 +/- 0.32 vs. 2.95 +/- 0.26 ms(2), p < 0.05) were greater in swimmers than in controls. Left ventricular end-diastolic diameter was greater (32.3 +/- 3.3 vs. 29.5 +/- 3.3 mm m(-2), P < 0.02) in swimmers than in controls, whereas the left ventricular septal (5.9 I vs. 5.6 +/- 0.8 mm m(-2), P = NS) and posterior wall thickness (5.7 +/- 0.9 vs. 5.4 +/- 0.8 mm m(-2), P = NS) were similar in the two groups. The E/A ratio was greater (2.2 +/- 0.49 vs. 1.78 +/- 0.36, P < 0.003) whereas the A velocity was lower (0.41 +/- 0.09 vs. 0.50 +/- 0.13 m s(-1), P < 0.02) in swimmers than in controls. Vmax was greater (18.6 +/- 4.8 vs. 14.9 +/- 5.3 cm m-2, P < 0.03), whereas ACTEF was lower (36 +/- 12% vs. 44.2 12%, P < 0.04) in swimmers than in controls. Conclusion Cardiac adaptation to intensive training in prepubertal swimmers includes vagal predominance, a mild increase in left ventricular dimensions without significant changes in septal or posterior wall thickness, and increased LA size associated with depressed LA systolic function. Evaluation of LA size and systolic function may contribute to a better understanding of the characteristics of the 'athlete's heart' in children and to the differential diagnosis between left ventricular adaptive and pathologic changes.