Patterns of evolution of QRS-T relationship were investigated by determining statistical distributions of QRS and STT integral vectors and the ventricular gradient vector in 1492 normal children divided into 12 age groups from birth to the age of 16 years. From birth to the age of 4 days, the ventricular gradient vector shifts posteriorly and to the left due to posterior shift of the STT integral vector and an increase in the spatial angle between QRS and STT integral vectors to a mean value of 1030. These early neonatal changes in QRS-T relationship probably reflect the sudden reduction of hemodynamic load and the subsequent postnatal atrophy of the right ventricle while the left ventricular load slowly increases. The magnitude of the ventricular gradient vector increases from age 3 weeks until about 7 years. This increase appears to be related to a gradual increase in the magnitude of the QRS and STT integral vectors and a drastic decrease in the spatial angle between them. The spatial angle between QRS and STT integral vectors reaches its minimum (22°) in the age group 1.5-4.5 years, suggesting that at that age the average direction of ventricular excitation and repolarization wavefronts are nearly opposite to each other.
In addition to the shifting balance between the left and right ventricular hemodynamic load, other factors, such as the maturation of the sympathetic nervous system, may be important in determining spatial gradients in the duration of action potentials, thus influencing the relationship between ventricular excitation and repolarization.