Among the total of 5637 examinations, 4608 had valid determinations of LVM. Missing values of LVM mainly resulted from the observer being unavailable for echocardiography at the time of examinations (810), participants being authorized to miss this measurement (191), and participants’ refusal (113). Age at data collection ranged from 8.1 to 17.98 years (M 12.06 years). The gender-specific Ms and SDs of LVM and the concurrent measures of age, weight, height, BSA, FFM, fat mass, PBF, BMI, and SSF are shown in . Mean value of LVM was 96.67 g overall, and it was approximately 15 g greater in boys than in girls. Weight, height, and BSA were slightly greater, and FFM was much greater, in boys than in girls. BMI was similar in both genders, and fat mass, PBF, and SSF were much greater in girls than in boys. Race and race-by-age interaction terms were not significant in any steps of the multilevel modeling and therefore were excluded from the LVM models.
Left ventricular mass, and body-size and body-fat measurements by gender, Project HeartBeat!, 1991–1995
The basic LVM–gender–age model is presented in . Gender, age, and gender-by-age interaction terms were significant determinants of LVM. Mean LVM according to the fixed part of the model is estimated as
Estimated model for left ventricular mass on gender and age, Project HeartBeat!, 1991–1995
where Age is the centered age, which equals chronologic age minus 12.0 years. At age 12, LVM was greater in boys than in girls by 11.26 g, whereas at age 9 or 16 years, it was about 10 g or 31 g greater in boys than in girls, respectively. Total variance of LVM was also age-dependent.
Considered individually, each of the nine body-size indicators significantly improved the model compared with the basic LVM–gender–age model. In each case, the p-value associated with the addition of a single body-size indicator was <0.0001 (), showing that each of these variables contained important additional information about the outcome. To assess the importance of including a body-fatness indicator in addition to each body-size indicator, fat mass, PBF, SSF, and BMI were added individually to the nine models, each containing a single body-size indicator. The results for the 36 models incorporating a body-fatness indicator in addition to a body-size indicator are shown in . The added body-fatness indicator is significant except when SSF or BMI are added to the model with BSA, or when PBF, SSF, or BMI are added to the model with BSA1.5.
Significance of each body-size indicator alone and with each of four BFIs
One of the resulting 36 models, containing the combination of FFM and PBF, among other variables, is presented in . Compared with the basic LVM–gender–age model in , the gender-by-age and gender-by-age2 interaction terms were no longer significant. Significant between-subject variance of FFM indicated that the measurement variance of LVM varied with FFM, whereas the between-subject variance of age, observed in the LVM–gender–age model, was no longer significant. Average LVM was 8.959 g lower in girls aged 8–18 years than in boys of the same age. Age remained a significant predictor of LVM. FFM was the strongest determinant of LVM, as shown by the estimated regression coefficient. An increase of 1 kg in FFM was associated with a 2.425-g increase in LVM, equivalent to the change of 27.8 g in LVM with a change of 1 SD in FFM (see ; SD of FFM=11.48 kg). PBF was also positively associated with LVM. A 1% increase in PBF was related to a 0.6759-g increase in LVM, which was a change of ~5.4 g in LVM with every 1-SD change in PBF.
Estimated model for left ventricular mass, Project HeartBeat!, 1991–1995
Similarly, four multilevel LVM models, containing the combinations of FFM, weight, BSA, or height with FM, in addition to gender and age terms, are shown in . The gender effect was consistent across these models, with girls having lower LVM ranging from 8.6 to 12.3 g estimated from different models, when other predictors in each model were held at their M levels. Age continued to be a significant predictor of LVM in all four models after adjustment for gender, body size, and body fatness. In Models B–D, where weight, BSA, or height3 were used instead of FFM as the body-size indicator, gender-by-age interaction terms remained significant, indicating an additional negative relationship between age and LVM in girls compared with boys.
Estimated models for left ventricular mass on gender, age, body-size, and fat mass, Project HeartBeat!, 1991–1995
The estimated effects of body size and body fatness on LVM from Model A in were very similar to those estimated from the model presented in . Compared with the model in , the only different predictor in Model A was fat mass instead of PBF. In this model, every 1-kg change in FFM was associated with a change of 2.09 g in LVM (24.0-g change in LVM per 1-SD change in FFM), and every 1-kg change in fat mass was associated with a change of 1.121 g in LVM (7.2-g change in LVM per 1-SD change in fat mass).
Use of weight, BSA, or height3 as body-size indicators in Models B–D revealed similar strong, positive effects of body size on LVM; however, results differed with respect to the effect of fat mass. A 1-kg change in body weight was estimated to be associated with a change of 2.022 g in LVM (33.3-g change in LVM per 1-SD change in weight); a 1-m2 change in BSA was associated with a 110.0-g change in LVM (31.9-g change in LVM per 1-SD change in BSA); and a 1-m3 change in height3 was associated with a 24.51-g change in LVM (25.2-g change in LVM per 1-SD change in height3). The regression coefficients of fat mass were negative, being −0.8069 g/kg (−5.3 g/SD) and −0.3153 g/m2 (−2.03 g/SD), respectively, in Models B and C, when weight or BSA was used as the body-size indicator, respectively. In model D, where height3 was used, the regression coefficient of fat mass remained positive (1.295 g/m3 [8.3 g/SD]).
Further examination of the regression coefficients of body-fatness indicators among the 36 LVM models revealed a consistent pattern of the effect of body fatness: in models with a non–fat free body-size indicator (weight, BSA, or BSA1.5) for prediction of LVM, effects of body-fatness indicators were all estimated to be negative (significant or not), whereas in those models containing a fat-free body-size indicator (FFM, height3, height2.7, height2.3, height2, or height), effects of body-fatness indicators were all estimated to be significantly positive (data not shown).