T wave alternans threshold in normal children

Sustained microvolt-level T wave alternans (TWA) during exercise is a predictor of ventricular arrhythmia propensity in adult populations. TWA occurs in normal adults, but it is rare at < 70% of predicted maximum heart rate. An onset heart rate < or = 110 is believed to be significant. The aim of this study was to examine the feasibility of performing the test in children and to determine the normal heart rate threshold for sustained TWA in children.

Alternans was evaluated during bicycle exercise in 100 normal volunteers aged 8 to 17 years. Adequate resting data were obtained in 76 of 100 children and was negative in all. Exercise data from 16 of 100 was excluded due to excessive noise. Median maximum heart rate was 192 (range 140 to 214). Sustained alternans was absent in 75 (89%) of 84. In the nine children with sustained alternans, median onset heart rate was 138 (range 120 to 158), and 7 of 9 had an onset heart rate > or = 135. Median heart rate threshold as a percentage of predicted maximum heart rate (220 - age) was 67% (range 58% to 76%). Only 1 subject (1.2%) had an onset heart rate < 60% of predicted maximum. There was no significant difference between age, gender, endurance, maximum heart rate, QRS duration, QT interval, or QTc in those with and those without sustained TWA.

Noninvasive assessment of TWA is feasible at > or = 8 years of age. Sustained TWA was present in 11% of normal children, but was absent at heart rates below 120 and rare (1.2%) below 60% of predicted maximum heart rate.