Systolic time intervals in adolescents. Normal standards for clinical use and comparison with children and adults.

Systolic time intervals were measured in 147 healthy 13-19-year-old adolescents to derive regression equations for clinical use in this age group and to determine to what extent they differ from those of children and adults. Stepwise regression analysis showed that heart rate was the only variable consistently and significantly related to electromechanical systole (QS2), left ventricular ejection time (LVET) and preejection period (PEP). None of the other variables tested (age, height, weight, body surface area, blood pressure, hemoglobin, hematocrit and serum electrolytes), when used alone, were relevant variables in relation to QS2, LVET or PEP. The PEP/LVET ratio in adolescents was essentially independent of heart rate, as in children and adults. The regression lines for QS2, LVET and PEP in adolescents fall between those for children and adults, indicating that there is a distinct tendency toward lengthening of all the systolic time intervals with age, independent of changes in heart rate. The PEP lengthens proportionately more than the LVET, resulting in a progressive increase in the mean PEP/LVET ratio from childhood (0.30) to adolescence (0.32) and to adulthood (0.345). The precise hemodynamic factors underlying these changes with increasing age remain to be determined.