Table 1. Candidate Predictor Definitions.

Candidate Predictor Variable	Definition	Coding
NYHA/Ross functional class	NYHA functional classification20/modified Ross heart failure classification for children13 at baseline evaluation	Binary (NYHA/Ross 1 = 0, NYHA/Ross ≥2 = 1)
Unexplained syncope	Defined as a transient loss of consciousness with no identifiable cause at or before first evaluation1,8,14,15	Binary (no = 0, yes = 1)
Nonsustained ventricular tachycardia	≥3 Consecutive ventricular beats at a rate of ≥120 beats/min lasting <30 s on ambulatory ECG monitoring (minimum duration 24 h) at or before first evaluation16,21	Binary (no = 0, yes = 1)
Maximal wall thickness z score	Defined as the number of SDs from the population mean22; the 2-D measurement of maximal wall thickness (millimeters) is at baseline evaluationa	Continuous (z score)
Left atrial diameter z score	Defined as the number of SDs from the population mean23 the 2-D measurement of maximal left-atrial diameter (millimeters) is at baseline evaluationb	Continuous (z score)
Maximal LV outflow tract gradient	The maximum LV outflow tract gradient at rest or with Valsalva provocation using continuous wave Doppler from the apical 3- or 5-chamber views17c	Continuous (mm Hg)

Abbreviations: 2-D, 2 dimensional; ECG, electrocardiogram; LV, left ventricular; NYHA, New York Heart Association.

^aMaximal wall thickness is the greatest thickness as measured by 2-D echocardiography in the parasternal short-axis views of the left ventricle in 4 places at the level of the mitral valve and papillary muscles (anterior and posterior septum, lateral and posterior wall) and in 2 places at the apical level (anterior and posterior septum).⁵

^bLeft atrial diameter is determined by M mode or 2-D echocardiography in the parasternal long-axis plane.

^cPeak outflow tract gradient is determined using the modified Bernoulli equation: gradient = 4V², where V is the peak aortic outflow velocity.