The cardiovascular responses to exercise in children across the blood pressure distribution. The Muscatine study

The Cardiovascular Stress Response as Early Life Marker of Cardiovascular Health: Applications in Population-Based Pediatric Studies-A Narrative Review

Stress inducement by physical exercise requires major cardiovascular adaptations in both adults and children to maintain an adequate perfusion of the body. As physical exercise causes a stress situation for the cardiovascular system, cardiovascular exercise stress tests are widely used in clinical practice to reveal subtle cardiovascular pathology in adult and childhood populations with cardiac and cardiovascular diseases. Recently, evidence from small studies suggests that the cardiovascular stress response can also be used within research settings to provide novel insights on subtle differences in cardiovascular health in non-diseased adults and children, as even among healthy populations an abnormal response to physical exercise is associated with an increased risk of cardiovascular diseases. This narrative review is specifically focused on the possibilities of using the cardiovascular stress response to exercise combined with advanced imaging techniques in pediatric population-based studies focused on the early origins of cardiovascular diseases. We discuss the physiology of the cardiovascular stress response to exercise, the type of physical exercise used to induce the cardiovascular stress response in combination with advanced imaging techniques, the obtained measurements with advanced imaging techniques during the cardiovascular exercise stress test and their associations with cardiovascular health outcomes. Finally, we discuss the potential for cardiovascular exercise stress tests to use in pediatric population-based studies focused on the early origins of cardiovascular diseases.

Health Fitness Training Responses of Normotensive and Elevated Normotensive Children

The purpose of this study was to compare health fitness profiles of normotensive (blood pressure < 108/76) and elevated normotensive (blood pressure ≥ 108/76) children and to evaluate the health fitness training responses of children with higher and lower blood pressures to a regular program of exercise over an eight month period. The subjects were 386 fourth grade students (lower blood pressures = 305; higher blood pressures = 81). They were measured for height and weight and evaluated before and after an exercise intervention program for flexibility, muscular endurance, cardiovascular endurance, and body fat levels. The results show that children with higher blood pressures were fatter and had lower cardiovascular fitness levels before and after the intervention. They had health fitness profiles similar to hypertensive adults. Their rate of health fitness improvement, with training, was similar to children with lower blood pressures. Therefore, elevated normotensive children have an increased risk of cardiovascular disease but can change their risk profile with regular exercise.

Flow-mediated dilation and exercise blood pressure in healthy adolescents

OBJECTIVES

Exercise blood pressure is a robust predictor of cardiovascular disease risk. Endothelial dysfunction occurs early in development of cardiovascular disease and is associated with greater exercise blood pressure in adults. However, it is not yet clear whether endothelial function is associated with exercise blood pressure in youth. The purpose of this study was to examine the relationship between endothelial function, indexed by brachial artery flow-mediated dilation, and submaximal exercise blood pressure in healthy adolescents.

DESIGN

Cross-sectional study.

METHODS

Adolescents (N=45) completed a graded submaximal treadmill test. Blood pressure was measured during rest and each exercise stage. Ultrasound measurement of brachial artery flow-mediated dilation was completed on a separate visit. Pearson correlations and multiple regression were used to assess the unadjusted and multivariate adjusted associations between flow-mediated dilation and exercise blood pressure, respectively.

RESULTS

Lower flow-mediated dilation was associated with lower diastolic blood pressure (r=0.37, p=0.01) and greater pulse pressure (r=-0.38, p=0.01) during exercise. The significance did not change when adjusting for age, gender, fitness, or resting blood pressure. Exploratory analyses suggest that flow-mediated dilation was associated with exercise diastolic blood pressure primarily among adolescents with low resting diastolic blood pressure.

CONCLUSIONS

Studies in youth are important to understand the early pathogenesis of cardiovascular disease. Findings from this study suggest that endothelial function may play a role in regulating blood pressure responses during submaximal exercise in healthy adolescents.

Changes in Metabolic Syndrome Variables Since Childhood in Prehypertensive and Hypertensive Subjects

That essential hypertension is associated with metabolic syndrome is known. However, information is scant regarding the course of development of adverse levels of blood pressure and other risk variables of metabolic syndrome in youth at risk for developing hypertension. This aspect was studied, retrospectively, in a community-based cohort of normotensive (n=2206), prehypertensive (n=721), and hypertensive (n=328) subjects examined serially during childhood (4 to 11 years), adolescence (12 to 18 years), and adulthood (19 to 42 years). Prehypertensive subjects versus normotensive subjects had significantly higher body mass index and subscapular skinfold, systolic (SBP) and diastolic (DBP) blood pressures, and triglycerides beginning in childhood; higher glucose in adolescence; and higher low-density lipoprotein cholesterol, fasting insulin, and insulin resistance index in adulthood. Hypertensive subjects versus normotensive subjects had higher adiposity measures, SBP and DBP, glucose, and triglycerides beginning in childhood; higher insulin and insulin resistant index in childhood and adulthood; and lower high-density lipoprotein, cholesterol in adulthood. Most of these variables progressed adversely at an increased rate in prehypertensive and hypertensive subjects. In a multivariate analysis, adverse changes in adiposity, SBP, and DBP were independently associated with prehypertensive status; and adverse changes in adiposity, SBP and DBP, insulin resistant index, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides with hypertension status. As young adults, prehypertensive and hypertensive subjects showed significantly higher prevalence of obesity, hyperinsulinemia, hyperglycemia, and dyslipidemias. Thus, excess adiposity and blood pressure beginning in childhood and accelerated adverse longitudinal changes in risk variables of metabolic syndrome through young adulthood characterize the early natural history of hypertension.

Leptin is favourably associated with vascular function in obese Caucasians, but not in obese Africans

The comparison of the associations between chronically elevated leptin levels and cardiovascular function in obese Africans and Caucasians has not yet been determined. Therefore, the aim of this study was to compare leptin's associations with cardiovascular function in obese African and obese Caucasian women to determine whether leptin's associations differ between these two groups. This study consisted of two case-case control studies. The first study included a sample of 102 apparently healthy African women and the second, 115 apparently healthy Caucasian women. All lean and obese subjects were selected from each study. The Finometer apparatus was used to obtain a more elaborate cardiovascular profile. Serum leptin levels, insulin levels and the lipid profile were determined. Stroke volume (SV) and cardiac output (CO) were significantly (P< or =0.01) elevated in both obese African and Caucasian groups compared to their lean controls. Total peripheral resistance (TPR) was significantly decreased and arterial compliance (C(W)) significantly increased in both obese African and Caucasian groups. In the obese Caucasian group, diastolic blood pressure (DBP) was significantly (P< or =0.01) lower, SV and C(W) significantly higher (P< or =0.01) and TPR significantly lower compared to the age, body mass index (BMI), and leptin-matched obese African group. After adjusting for age and BMI, leptin correlated negatively with DBP (P< or =0.05; r=-0.33) and TPR (P< or =0.05; r=-0.36) in the obese Caucasian group, but not in the obese African group. Even though leptin levels were similar in obese African and Caucasian women, leptin is favourably associated with vascular function in obese Caucasians, but not in obese Africans.

Cardiovascular responsivity to stress in adolescents with and without persistently elevated blood pressure

OBJECTIVES

The goal of this study was to compare the cardiovascular responses to behavioural stressors of three groups of adolescents who differed in blood pressure status across assessments.

DESIGN

Casual blood pressure of adolescents who were identified as having elevated blood pressure during a school screen was re-evaluated in the laboratory. The adolescents were classified into two groups: (i) those with consistently elevated blood pressure across school and laboratory assessments and (i) those with labile blood pressure whose blood pressure in the laboratory was below 130/80 mmHg. A comparison group of adolescents with consistently normal blood pressure was also included.

METHODS

Cardiovascular parameters were assessed during rest and during two behavioural stressors, the evaluated speaking task and the mirror tracing task.

RESULTS

Adolescents with elevated blood pressure were more vascularly responsive across stressors than adolescents with labile blood pressure, who, in turn, were more reactive than adolescents with normal blood pressure.

CONCLUSIONS

These results suggest that vascular reactivity to behavioural stressors may be useful in predicting risk of hypertension because of its sensitivity in distinguishing adolescents with consistently elevated blood pressure from those with labile blood pressure and those with normal blood pressure.

Obesity-hypertension: the effects on cardiovascular and renal systems

Longitudinal and cross-sectional studies suggest that a large number of obese patients have a high prevalence of hypertension. This association causes the following changes: insulin and leptin resistance with a suppressed biologic activity of natriuretic peptide, which contributes to sodium retention with concomitant expanded cardiopulmonary volume and increased cardiac output. The cellular metabolism of cations may be altered in obesity and may lead to changes in vascular responsiveness and increased vascular resistance. These changes lead to structural adaptations in the heart characterized by concentric-eccentric left ventricular hypertrophy. The hypertrophic condition provides the basis for the development of congestive heart failure and cardiac arrhythmias that may explain the higher rates of cardiac sudden death in those patients. In the kidneys, obesity hypertension may initiate a derangement of renal function. The increased deposit of interstitial cells and of extracellular matrix between the tubules induces higher interstitial hydrostatic pressure and tubular sodium reabsorption. The consequent increase in renal flow and glomerular filtration enhances albuminuria excretion and the susceptibility to the development of renal damage. In summary, the hemodynamic and structural adaptations related to obesity hypertension is the cause of greater risk for adverse cardiovascular and renal events.

Genetic regulation of hemodynamic variables during dynamic exercise. The MCV twin study

BACKGROUND

Both resting and exercise levels of blood pressure in individuals have been used as predictors of adult hypertension. One possible mechanism underlying the relation between childhood resting and exercise blood pressure and future blood pressure is a set of genes expressed in childhood that persists to regulate adult blood pressure.

METHODS AND RESULTS

To investigate the genetic relation of blood pressure and heart rate during both rest and exercise, we asked: (1) Are the genes that regulate resting hemodynamic variables the same genes that regulate these variables during exercise? (2) How much of the variance in exercise hemodynamic variables is genetic and how much is environmental? (3) Do the genetic and environmental influences on hemodynamic responses change with increasing levels of exercise? To determine how genetic and environmental effects expressed at rest influenced responses during dynamic exercise, a genetic analysis was conducted by fitting a series of models to the covariance matrices with the use of the LISREL VII program.

CONCLUSIONS

We found that all the genetic effects expressed at the later stages of exercise can be explained by genetic effects expressed at rest and at the first stage of exercise. The environmental effects appear to be workload specific and include errors of measurement.

Exercise-loaded blood pressure and Li-Na countertransport system in the erythrocyte membrane as predictors of mild essential hypertension prognosis

1. The blood pressure response to exercise loading, Na concentration in the erythrocyte and Li-Na countertransport (Li-Na CT) system in the erythrocyte membrane in 40 male volunteers and 98 patients with mild essential hypertension were investigated. Subjects were divided into a juvenile group (less than 36 years) and a middle-aged (greater than 36 years and less than 65 years) group. Exercise-loaded blood pressure in patients with mild essential hypertension was followed up for more than 5 years. 2. Systolic blood pressure (SBP) at 6 min during exercise loading was significantly higher in the mild hypertension group compared with the control group. 3. Patients with high SBP at exercise loading in the mild hypertension group transferred more often to the moderate hypertension group within 5 years. 4. No difference in the haemodynamic parameter at rest was noted in the transferred group for moderate hypertension and the non-transferred group. 5. A higher blood pressure response to noradrenaline was noted in the transferred group compared with the non-transferred group of the juvenile group. 6. High Na concentration in the erythrocyte and acceleration of the Li-Na CT system in the erythrocyte membrane were noted in the mild hypertension group compared with the volunteer group. 7. A correlation between SBP at exercise loading and Li-Na CT system in the erythrocyte membrane was noted. This correlation was strongly noted in subjects less than 50 years old. 8. From these results it was concluded that SBP at exercise loading and Li-Na CT system in the erythrocyte membrane are useful as indicators for the prognosis of mild essential hypertension.

Combined low-dose medication and primary intervention over a 30-month period for sustained high blood pressure in childhood

Studies of the pathobiologic consequences of high blood pressure in childhood, as well as those following blood pressure levels into young adulthood, indicate that early intervention in the natural history of essential hypertension is warranted. In an exploratory study of the concept, 95 children out of 1604 (aged 8 to 18 years), who persistently scored higher than the 90th percentile for blood pressure over a 4-month period, considering the race, sex, and height of the children, were studied. Five series of replicate measurements with 30 total observations were obtained. Children with evidence of secondary hypertension were excluded. The study children were randomly divided into treatment (n = 48) and high-comparison (n = 47) groups. Treatment consisted of low-dose combined drug therapy (propranolol and chlorthalidone) with an educational program directed towards hypertension and dietary and exercise modification. Monthly follow-up was continued for 30 months. Significant systolic (-3.59 mm Hg) and diastolic (-1.73 mm Hg) changes were noted up to 30 months (p less than 0.05) with minimal side effects. Furthermore, analyses suggested that the blood pressure change, at least in the first month, was mostly attributable to drug therapy. Moreover, the mechanism of blood pressure change appeared to be race-specific, with whites having pulse rate changes and blacks having significant weight changes, which were associated with blood pressure change. This trial shows further research is warranted to determine optimum approaches for early treatment of essential hypertension to prevent future hypertensive disease.

Echocardiographic functions and blood pressure levels in children and young adults from a biracial population: the Bogalusa Heart Study

M-mode echocardiograms were obtained on 651 healthy subjects, 7-22 years of age, whose diastolic blood pressure levels remained in the same height-, race-, and sex-specific decile during two biannual examinations. Echocardiographic measures of heart size and dynamics were compared across the total blood pressure distribution. Left ventricular stroke volume, cardiac output and ejection fraction, minor axis shortening, velocity of circumferential fiber shortening, and peripheral vascular resistance were correlated with blood pressure levels. There were positive correlations (p less than .001) of cardiac output and stroke volume with both systolic and diastolic blood pressure levels. Left ventricular output and stroke volume were associated with measures of body size, especially height, weight, ponderal index, and body surface area (p less than .001). The left ventricular output and stroke volume increased with age and with systolic blood pressure quintiles in the four race-sex groups. With adjustment for systolic blood pressure and measures of body size, white males had greater cardiac output (1.25 l/minute for ages 18-22 years, p = .01) and stroke volume than black males. Black males had higher peripheral resistance (4.5 mm Hg/(l/minute), p = .01) than whites. These results suggest that different hemodynamic mechanisms operate in the early phase of hypertension in blacks vs. whites in this population.

Blood pressure reactivity in children

This paper reviews the relevant pediatric literature about blood pressure (BP) hyperreactivity to stress as a possible precursor of hypertension and coronary heart disease. Two prospective studies of children indicate that BP hyperreactivity predicts later development of hypertension. Several studies have identified correlates of BP reactivity. Race, obesity, Type A and family history of hypertension appear to be associated with BP reactivity in children. There appear to be both genetic and environmental influences, but relatively few psychosocial variables have been studied. There are significant methodological concerns in terms of defining the characteristics of the stressors and the stability and generalizability of responses to laboratory stressors. BP reactivity in children is a potentially important area of inquiry that has been understudied.

The association between cardiovascular response tasks and future blood pressure levels in children: Bogalusa Heart Study

Children were selected from a total community survey and categorized into three blood pressure (BP) groups (low less than 10 percentile, midrange 10 to 90 percentile, high greater than 90 percentile) based on resting diastolic pressure (DBP). To examine the relationship of cardiovascular reactivity with BP levels 4 years later, three cardiovascular response tasks (orthostatic, handgrip, and cold pressor) were administered. Partial correlations, adjusted for initial resting BP, showed that peak BP responses to the three tasks were related to future resting systolic and diastolic BP. Further analyses showed that DBP reactivity was related to future BP tracking in children with high DBP levels. In the high DBP group, while resting DBP levels did not differ for trackers vs nontrackers, peak DBP responses to each of the three tasks did (orthostatic, p less than 0.001; handgrip, p less than 0.05; cold pressor, p less than 0.01). The combination of resting and peak BP levels may provide more powerful predictions of future BP levels in children.

Measurement of left ventricular wall mass in pediatric populations

This paper reviews the techniques for obtaining technically adequate echocardiograms for epidemiologic studies. When these techniques were applied to studies of pediatric populations the following objectives were achieved: reproducible echocardiograms, observations about the relationship of echocardiographic variables to cardiovascular variables, and information about the relationship of echocardiographic and electrocardiographic variables to each other and to blood pressure. We documented the precision of M-mode left ventricular chambers and dimensions, interobserver and intraobserver variability, and the day-to-day variability of these measures. Left ventricular wall mass was significantly larger than expected for age and body size in children with persistently elevated blood pressure. The relationship between the echocardiographic and electrocardiographic variables was poor. Moreover, the electrocardiographic measures of ventricular hypertrophy did not correlate with blood pressure.

Hemodynamics in childhood hypertension

Children with elevated systolic blood pressure have a wide range of cardiac output. We investigated the mechanisms regulating resting and exercise blood pressure in 264 children who were selected from the low, middle, and upper quintile of the distribution of blood pressure of an entire school population. We identified patterns of response to exercise that correlated both with resting cardiac output and resting blood pressure. During isometric exercise, systolic blood pressure adjusted for age and body size increased in all groups. The low group mean pressure remained significantly lower than the high group's pressures throughout the entire exercise period. Body size adjusted group systolic and diastolic blood pressure level differences existed during dynamic exercise. The product of the systolic blood pressure times the heart rate in the high blood pressure group was significantly higher throughout dynamic exercise than in the other two groups. Elevated resting resistance was correlated with elevated resistance during isometric exercise and elevated diastolic blood pressure during dynamic exercise. Cardiac index had a significant negative correlation to age (r = -0.58) at all levels of blood pressure. This observation, in children, lends some support to the concept of evolution from a hyperkinetic circulation in early childhood to a circulation with lower cardiac output and more elevated systemic vascular resistance at an older age.