(Hypertension. 1997;30:377.)
© 1997 American Heart Association, Inc.
Articles |
Relations of Left Ventricular Geometry and Function to Body Composition in Children With High Casual Blood Pressure
From the Nutrition Unit (G. de S., G.F.M., R.G., A.E.D.P., F.C.), Department of Clinical and Experimental Medicine, Department of Food Science (L.S.), and Department of Pediatrics (A.F.), Federico II University Hospital School of Medicine, Naples, Italy; and the Division of Cardiology (G. de S., R.B.D.), New York Hospital, Cornell Medical Center, New York.
Correspondence to Dr Giovanni de Simone, Dipartimento di Medicina Clinica e Sperimentale, Policlinico dell’Università Federico II, via S. Pansini 5, 80131 Napoli, Italia. E-mail simogi{at}unina.it
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Abstract |
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Abstract To determine whether abnormal casual blood pressure (BP) is associated with left ventricular (LV) abnormalities in children, 190 6- to 11-year-old children (77 girls, 113 boys) were studied at a school site in Naples, Italy, by limited echocardiography and bioelectric impedance to calculate fat-free body mass (FFM). Single-visit BP measurements (defined as casual BP) were high (based on the Italian tables of BP) in 34 children (18%; 9 girls, 25 boys; 133±8/81±10 mm Hg) and obesity was present in 44 (23%; 15 girls, 29 boys). Sex- and age-independent risk of high casual BP value was 2.9-fold (odds ratio) greater in obese than in normal-weight children (95% confidence interval, 1.3 to 6.5; P<.01). LV mass (as both absolute value and normalized for height2.7 or FFM) was higher and relative wall thickness increased in children with high casual BP (all P<.01). Prevalence of LV hypertrophy was 21% among children with high casual BP (P<.004 versus 4.3% in normal group). Risk of LV hypertrophy was 5.5-fold higher in the presence of high casual BP (P<.004), whereas obesity, age, and sex did not have independent effects. Endocardial shortening was slightly higher in children with high casual BP (36.8±8.2%) than in children with normal BP (34.3±4.8%, P<.02), whereas midwall shortening was identical in the two groups (20%). Both endocardial shortening and midwall shortening were negatively related to end-systolic stress (r=-.62, SEE=3.8% and r=-.32, SEE=2.4% in normal children). Shortening as a percentage of predicted from wall stress was increased in children with high casual BP at the endocardial level (P<.001), whereas it was normal at the midwall. Therefore, (1) casual detection of high BP in school children is associated with LV geometric abnormalities similar to those found in adults with sustained hypertension (LV hypertrophy, concentric pattern); (2) similar to in adult hypertension, endocardial chamber function in children is supranormal; and (3) in contrast to findings in adults, midwall shortening is normal in children with high casual BP.
Key Words: children • echocardiography • obesity • hypertrophy • ventricular function, left • body composition
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Introduction |
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Echocardiographic determination of LV mass has facilitated stratification of risk in patients with arterial hypertension and other cardiovascular diseases.1 2 3 4 5 6 By extending concepts initially developed by Gaasch et al,7 8 9 depressed LV performance measured at the level of the midwall has been recently found to improve the prediction of cardiovascular risk in patients with arterial hypertension.10 Therefore, assessment of echocardiographic LV mass and midwall performance may give information that helps to clarify the need for and the intensity of antihypertensive drug treatment.
In many studies,11 12 13 14 children with sustained arterial hypertension have been reported to have increased LV mass, but no data are available concerning LV anatomic and functional adaptation in children who exhibit high BP values in a single clinical examination, most likely affected by emotional factors. Additionally, the effect of body size in the majority of available studies had a major impact on the detection of LV hypertrophy in hypertensive children. Because body growth directly influences organ growth, the influence of body size is especially important during childhood and adolescence in identifying deviations from normal LV mass.15 16 17 18 19 A new approach for the normalization of LV mass for body size that uses height to the power of 2.7 of the allometric relationship identified in large population samples might be a more convenient means of evaluating the separate effects of body size and BP in children6 19 20 21 because such an approach linearizes the relation between LV mass and height across a wide range of ages.
The present study was therefore designed to assess LV geometry and circumferential function in children with occasionally high casual BP using the allometric signal approach and midwall measurement of LV minor axis shortening, respectively, and to elucidate the interaction between body composition and BP in the determination of LV mass.
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Methods |
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Participants, Site, and Procedures
The study was conducted in an elementary school in Naples, Italy. The families of 200 pupils registered at the school (50% random sample of the student population) were asked to participate in the study; 196 (98%) accepted the invitation. Anthropometric and echocardiographic measurements were completed in 190 white children aged 6 to 11 years (8.0±1.5 years; 77 girls, 133 boys). Six children were excluded from the study because of technical inadequacy of the echocardiograms (4) and/or incomplete collection of anthropometric measurements (3). Social class distribution, as indicated by the observation of the different educational levels among the children’s parents, was proportional to that of the general population.22 Informed consent was obtained from the children’s parents or guardians as well as from the principal of the school. An echocardiograph equipped with a 3.5-MHz mechanical transducer (SIM3000, ESAOTE) was moved to a comfortable room in the school together with other medical equipment that included a scale, a mercury sphygmomanometer, and a bioelectric impedance analyzer (Bia101, Akern s.r.l., distribution for RJL-Systems). The staff, consisting of a sonographer, nutritionist, and pediatrician, worked during morning hours (9 AM to 1 PM) to gather information by means of several questionnaires (nutrition, family history, social status, etc) and from the children’s parents. Each child underwent a number of measurements of body proportions, including body weight; height; triceps and subscapular skinfold thicknesses; circumference and length of arms; and circumferences of legs, calves, head, waist (at the umbilical level), and hips (at the iliac crest). After the anthropometric measurements, the children underwent a limited echocardiogram23 in the two orthogonal parasternal planes. After the echocardiograms, BP was then measured by a sonographer previously trained for standardization of BP measurements with the London School of Hygiene tape set.24
BP and Body Size Measurements
BP was first measured at the first and fifth Korotkoff sounds
with the child seated. BP was taken twice during a period of 20 minutes
with a mercury sphygmomanometer equipped with two types of cuff: a cuff
of 17x9 cm for the majority of children and a regular adult-size cuff
for children with arm circumferences exceeding 26 cm. BP also was
measured directly after the echocardiogram with the child in a supine
position, after a 10-minute resting period. Children were classified
according to the tables of the Italian Society of
Pediatrics,25 which were developed from a survey based on
the average of three BP measurements (1-minute interval) in a single
visit of 16 772 children and adolescents (birth to 18 years) in 12
cities in Italy. Therefore, children participating in the present
study were defined as normotensive if the average value of the two
initial measurements was below the 95th percentile of the normal CI of
height- and sex-specific values according to the Italian Society of
Pediatrics Tables. Children were classified as having high casual BP if
the average BP value exceeded the 95th percentile of those tables. BP
values measured after the echocardiogram were used for computation of
wall stress and are presented in this study.
Children were considered overweight to obese if the observed value of body mass index was greater than the sum of age+13 for males and the sum of age+14 for females.26
Bioelectric Impedance
In population-based studies, bioelectric impedance has been
shown to determine accurately the extent of FFM. 27 With
the subject in a supine position, measurements of resistance were taken
from the hand to the foot on both sides of the body by using a
tetrapolar placement of the electrodes28 ; metal objects
were removed from around the limbs and trunk of the body. The arms were
not in contact with the trunk of the body. To ensure good
reproducibility of environmental conditions, all measurements were
performed between January and March from 9 to 11 AM at a
room temperature between 24°C and 26°C. The bioelectric impedance
analyzer used in this study was checked daily with objects of
known resistance. Bioelectric impedance assessment was performed twice
in 15 children on 2 different days. With the stable environmental
conditions, between-day coefficient of variation of whole-body
resistance was less than 2%. Bioelectric impedance obtained by the RJL
System was compared with the impedance measured on the same day with a
different device (Human Hi-Scan, Dieto-System) in 500 children,
including those enrolled for the present study. Differences in
impedance between the two devices did not exceed 5 
(1% of the
measured value).
Measurements of resistance of both sides of the body were
averaged. Based on the principle that the impedance of a conductor
(
) is =L/a, where L is the length and a is cross-sectional area,
impedance index was calculated at 50 MHz. In the human body, L is body
height (h) and can be measured. Cross-sectional area can be transformed
into body volume by the following equation:
h/a=(h/a)·(h/h)=h2/v, where v is volume (ie, area times
height). Therefore, because and h are known, v (impedance index)
can be easily determined as v=h2/.
Because the impedance index is inversely related to the conductivity of electrolytes in body fluids, it is a reliable measure of body water.29 Most body fluids are contained in an FFM,28 and therefore impedance index can be used to approximate the FFM. In children this has been obtained using the following equation: FFM=(v·0.59+BW·0.065+0.041)/(0.769-Age·0.0025-0.019·Sex), where BW is body weight in kilograms and sex is 0 for women and 1 for men.30
Adipose body mass was estimated by subtracting the value of FFM from the body weight.
Echocardiography
Two-dimensionally targeted M-mode echocardiograms were performed
as described previously20 31 with the subjects in a
partial left decubitus position. Tracings were recorded on
strip-chart paper at 50 mm/s, coded, and interpreted blindly by
two investigators at the end of the screening phase. Measurements of
interventricular septal thickness, posterior wall
thickness, and LV diastolic dimension were taken at or just
below the mitral valve tips, according to the American Society of
Echocardiography,32 using a graphic
tablet connected to a PC computer for storing data. A second set of
measurements was performed using Penn convention
criteria33 and was used only for the computation of LV
mass. Relative wall thickness was calculated by dividing the posterior
wall thickness by the LV internal radius.
Endocardial fractional shortening was calculated by using the standard formula.34 Systolic shortening of LV minor axis at the midwall was calculated, taking into account the epicardial migration of midwall during systole.35 Midwall shortening (mFS) was computed as mFS=([Dd+Hd]-[Ds+Hs])/(Dd+Hd)*100, where D is LV chamber diameter, H is 1/2(posterior wall+septum), d is diastole; and s is systole. The value of Hs takes into account the epicardial migration of midwall during systole and was calculated assuming a constant wall volume during cardiac cycle based on a geometric model identical to that used to calculate LV mass.35
Myocardial afterload was represented as circumferential
end-systolic wall stress (cESS), calculated at the
midventricular level using a cylindrical
model7 35 :
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LV volumes were calculated from M-mode LV chamber dimension by the Teichholz formula36 and used to compute stroke volume, cardiac output, and peripheral resistance (as 80 times the ratio of mean BP to cardiac output).
Normalization for Body Size
LV chamber dimension was indexed for height and LV mass for
height2.7 on the basis of their geometric
dimensions.20 LV mass was also normalized for FFM.
Statistical Analysis
Data are expressed as mean±SD. Descriptive statistics are
presented using the 
2 test and frequency
distribution.
One-way ANOVA was used to detect the impact of casual high BP on LV geometry and performance using, when appropriate, covariates forced into the model according to a hierarchical design.37
Least-squares linear regression analysis was used to describe univariate relations between study variables. Stepwise multiple regression analysis was used to study the independent effect of anthropometric, structural, and hemodynamic variables on LV geometry. F to enter and F to remove were set at P<.05 and P<.10, respectively. Sex was treated as a dummy variable by assigning 1 to males and 2 to females. Stability of the estimates of regression coefficients was assessed using collinearity diagnostics.37 Stepwise logistic regression was used to determine the risk of high casual BP values and LV hypertrophy, expressed as odds ratio and 95% CI.
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Results |
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The overall prevalence of obesity was 23%, slightly but not significantly higher in boys (29 of 113 or 26%) than in girls (15 of 77 or 19.5%). Obese and normal-weight children were similar in age.
High casual BP values were found in 34 of 190 children (18%), with a slightly higher prevalence in boys (25 of 113 or 22%) than in girls (9 of 77 or 12%, P=.06). The prevalence was similar among normal-weight boys (13 of 84 or 15%) and girls (7 of 62 or 11%). Among overweight children, 12 of 29 boys (41%) and 2 of 15 girls (13%) exhibited high BP values, but this sex difference did not attain statistical significance because of the small size of the cells. No significant age difference was detected between children with normal (8.1±1.5 years) and high casual BP (7.7±1.3 years).
Table 1 shows the differences in anthropometric measurements between the groups of normotensive and hypertensive children. Children with high casual BP exhibited higher body weight, higher FFM and body mass index, thicker triceps skinfold, and larger arm circumference (all P<.05). All the differences retained or increased their statistical significance after controlling for sex and age, which also identified intergroup differences in subscapular skinfold, umbilical circumference, and adipose body mass (adjusted means for adipose body mass were 7.73 and 9.73 kg for children with normal and high casual BP, respectively; all P<.01).
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Correlates of BP
In the entire study population, systolic BP was positively
related (P<.05) to body mass index (r=.22); body
weight (r=.19); FFM (r=.24); arm
(r=.25), calf (r=.28), leg (r=.19),
hip, and waist (both r=.20) circumferences; and subscapular
(r=.19) and triceps (r=.15) skinfolds. There were
no significant relations of systolic pressure to age, sex,
height, adipose body mass, and waist-to-hip ratio.
Diastolic BP was related to body mass index
(r=.15), arm (r=.16) and calf (r=.20)
circumferences and subscapular skinfold (r=.15). The age-
and sex-adjusted prevalence of high casual BP was almost threefold
greater in overweight children than in normal- weight children (odds
ratio, 2.94 [95% CI, 1.33 to 6.48]; P<.008).
LV Anatomy
Table 2 shows the partial
correlations of LV mass with a variety of anthropometric measurements
in the entire study population, after controlling for sex. LV mass was
more closely related to most measurements of body size than to BP.
However, the effect of BP was clearly apparent when children with high
casual BP were compared with their peers with normal BP.
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After controlling for sex and age, LV chamber size was normal, and LV mass was increased in children with high BP, as both absolute value and indexed for height2.7 or FFM (all P<.01, Table 3). As a consequence, relative wall thickness was also increased in children with high BP (P<.01, Table 3).
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Prevalence of LV hypertrophy, defined as a value of LV mass index >32.22 g/m2.7 (ie, the 95th percentile of the distribution in the 156 normotensive children), was 21% (7 of 34) in children with high casual BP (P<.004 versus controls). The risk of LV hypertrophy was 5.5-fold greater in children with high casual BP than in normal children (1.8 to 17.0, 95% CI of odds ratio, P<.004), a risk that was independent of age, sex, and presence of obesity (logistic regression).
Multiple linear regression analysis revealed that LV mass increased by 1.75 g per kg of FFM and 1.84 g per year of age, and was 6.43 g higher in the presence of high casual BP (.03<P<.0001, multiple r=.68, SEE=12.4 g, P<.0001), without appreciable independent effect for the adipose body mass (partial r=.07).
The relatively minor role of overweight as a direct stimulus for LV hypertrophy compared with the effects of higher FFM and the presence of high casual BP values was also confirmed in the normotensive group (n=156): BP was indeed similar in the 30 overweight children (112/69±9/9 mm Hg) and in the 126 normal-weight normotensive children (110/68±8/8 mm Hg, both P=NS), with no evident abnormalities in LV geometry (LV mass/height2.7 was 24±5 g/m2.7 in overweight and 22±6 g/m2.7 in normal-weight children; LV mass/FFM was 1.9±0.4 and 2.0±0.5 g/kg, respectively; both P=NS). Even in pooled normotensive and hypertensive children, the effect of overweight on LV mass was obscured after controlling for the effect of BP.
Determination of Stress-Shortening Relations
There was a close inverse relation between wall stress and
fractional shortening measured at the level of endocardium in the 156
normal children (r=-.62;
y=100-13·log(x)±3.8%;
P<.0001) and in the 34 children with high casual BP
(r=-.81; P<.0001). The proportion of children
with high casual BP exhibiting endocardial shortening greater than that
predicted by their end-systolic stress was only slightly higher
than in the normotensive group of children (2 of 34 or 6% compared
with 3 of 156 or 2%).
Similar to findings in adult populations, the relation between wall stress and midwall shortening was not as close, but the SEE was lower than in the regression model, with endocardial shortening either in normal children (r=-.32; y=38-35.5·log(x)±2.4%; P<.0001) or in children with high BP (r=-.58; P<.0003). No children with high casual BP exhibited midwall shortening below the normal confidence limit. The normality of midwall shortening, as both raw values or as percentages of the value predicted by end-systolic stress, was also confirmed in the group of children (n=7) with both high BP value and LV hypertrophy.
LV Pump Function
Table 4 shows that after controlling
for sex and age, stroke volume was not significantly different in the
two groups of children, whereas cardiac output was modestly increased
in children with high casual BP (P<.03). After controlling
also for FFM, stroke volume values became identical in the two groups,
and the statistical difference in cardiac output disappeared. Table 4
also shows that peripheral resistance was higher in
children with high BP than in children with normal BP
(P=.05) and that this difference attained statistical
significance after controlling for FFM.
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Discussion |
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This study was performed in a school setting with prepubertal children encompassing a small range of ages (coefficient of variation, 18%) and an even narrower range of body heights (coefficient of variation, 8%). Such small ranges of variation did not permit the association of BP values to age and height to achieve statistical power comparable to previous epidemiological studies.25 38 The prevalence of obesity as assessed by the Himes and Dietz simple method was lower than the mean prevalence estimated using different partition values of body mass index from other countries (France, Holland, United States, UK; estimated prevalence, 36%)39 and similar to the 21% prevalence reported in Milan, Italy.40 Therefore, this population of children is characterized by a high prevalence of obesity in a relatively narrow range of ages and body heights.
In this study, as strongly recommended,38 high BP was identified using height- and age-based classification measured only on a single clinical examination, similar to the procedures used in the survey of the Italian Society of Pediatrics.25 Because of the great fluctuation of BP values in children,41 the presence of occasional high BP values in this study population was not considered indicative of sustained arterial hypertension. However, cardiac anatomy and performance were analyzed to determine whether a single high BP measurement in prepubertal children could result in echocardiographic abnormalities. Clear-cut cardiac modifications were identified in children with high casual BP similar to those reported in sustained arterial hypertension during childhood. Precocious development of LV hypertrophy with a tendency toward the concentric geometric pattern is the characteristic cardiac abnormality in these children.
Body Size, BP, and LV Mass
Obesity is a potent predictor of arterial hypertension
in adults as well as in children or adolescents in population
studies.42 43 In this study, obesity markedly clustered
the detection of abnormal BP values in a single visit. In this study
population, obese children exhibited a threefold higher risk of high
casual BP than their normotensive peers.
Body size was a substantial contributor to variability of LV mass, and similar to findings in US children,13 this contribution was especially due to the magnitude of the FFM. Obesity, however, did not influence LV mass independently of BP values in our children, as opposed to what has been observed during adulthood.44 45 This finding is also in apparent contrast to previous reports showing increased LV mass index in overweight children classified as normotensive.46 Those children, however, exhibited higher average BP, even though within the normal range. Although the association of LV mass with BP was weaker than with measures of body size and composition, categorization of LV mass to separate children with LV hypertrophy highlighted a strong association with high casual BP values and not with obesity. This apparent inconsistency should be evaluated together with the evidence of the close association between occurrence of high casual BP and body size abnormalities. In fact, obesity in children appears to be associated with LV hypertrophy primarily because of its association with higher BP values, therefore suggesting that the body changes occurring with puberty may be required for obesity to assume an independent role as a stimulus to LV hypertrophy.
As expected, the waist-to-hip ratio did not have a significant relation to LV mass values in these prepubertal children. It is well established that sex differences in waist-to-hip ratio begins at puberty,47 when the magnitude of LV mass also begins to diverge between boys and girls.21 In the present prepubertal study population, the waist-to-hip ratio was identical in boys and girls (both 0.94±0.04), although some degree of association with individual pubertal maturation could not be excluded.
LV Systolic Function
In contrast to adult hypertension, the presence of LV
hypertrophy and the tendency to concentric remodeling does
not adversely affect LV performance in children, thus
confirming previous observations on endocardial shortening-stress
relations.48
The hemodynamic pattern observed in our children with high casual BP, after adjusting for potential confounding effects of body size, is more similar to adult sustained hypertension than that commonly associated with borderline or labile hypertension,49 with normal cardiac output and elevated systemic resistance.
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Conclusions |
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The presence of high casual BP is very prevalent in students of an elementary school in southern Italy. This occurrence clusters with the presence of obesity and is characterized by significant prevalence of LV hypertrophy, tendency to LV concentric remodeling, high peripheral resistance, and normal myocardial contractility.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This work was supported in part by grant MUST-185/1993-1994 from the Italian Ministry of University Research and grant 93/00630PF41 CNR/Italy (FATMA Project). The authors thank Dr Giuliana Valerio, AnnaRita Cadara, and Corinna Lanzetta for their expert assistance in the management of the children at the school site and Maurizio Marra for his cooperation in performing and evaluating the bioelectric impedance analyses. We also thank the staff of the elementary school in Naples, III Circolo Didattico, for their enthusiastic cooperation, which made this study possible.
Received January 30, 1997; first decision February 5, 1997; accepted February 26, 1997.
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References |
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