Wednesday, August 24, 2011

Exercise & Prevention of HTN in Children & Adolescents

Obesity, Salt, Exercise and Blood Pressure in Children : Exercise & Prevention of HTN in Children & Adolescents Regular physical activity reduces the risk of cardiovascular disease morbidity and mortality, but also lowers BP and prevents the development of HTN. In a population-based prospective cohort study over an 11-year follow-up period, the incidence of HTN was reduced by 28% in men and 35% in women who engaged in high levels of physical activity. An immediate reduction in BP occurs after an aerobic exercise session (postexcercise hypotension). Several studies using ABPM demonstrated that the BP-lowering effects of exercise are most pronounced in people with HTN who engage in endurance exercise, with 24-h daytime BP decreasing by 5–7 mmHg after an isolated exercise session (acute) or following aerobic exercise training (chronic). BP remains lower for the rest of 24-h period after each 30-min period of moderate exercise (50% of maximal O2 uptake) with greater BP reductions for vigorous exercise (75% of maximal O2 uptake). The mechanisms involved in the postexercise hypotension may involve the reduced activity of sympathetic nervous and renin–angiotensin–aldosterone systems. Brownley et al. reported that 65% of the postexercise mean BP response difference could be accounted for by changes in sympathetic factors, with change in norepinephrine and pre-ejection period elongation being the single best predictors. Restoration of balance in functions of the autonomic nervous system was proposed to serve as a possible exercise-dependent mechanism of BP reduction in obese children.Moderate-intensity aerobic exercise has also been shown to augment endothelium-dependent vasodilation in humans through the increased production of nitric oxide. Data from a subset of the 1998–2002 NHANES survey, including 3110 healthy adolescents (aged 12–19 years) and 2205 adults (aged 20–49 years), revealed that cardiorespiratory fitness, estimated by the duration of a maximal treadmill exercise test, was inversely associated with the risk of developing HTN in both adolescents and adults. Low fitness was identified in 33.6% of adolescents and 13.9% of adults. Lobelo et al. also found an excess cluster of cardiovascular risk factors including SBP in both overweight and normal weight adolescents with lowest quintile of cardiovascular fitness distribution. SBP measured during exercise at the age of 9 years predicted resting SBP 6 years later in adolescent in healthy Danish children. Decline in duration and intensity of physical activity was associated with higher SBP in 12-year-old adolescents followed longitudinally for 5 years. A decline of one session of moderate to vigorous exercise session per week each year of age was reported to result in 0.40 and 0.18 mmHg higher SBP in boys and girls, respectively, at the end of the follow-up period. Intervention studies showed that aerobic exercise training at an intensity of 70–80% of maximal fitness, 5 days per week, reduced SBP in hypertensive and obese adolescents. However, a meta-analysis of 12 randomized controlled trials by Kelley et al. reported that exercise led to small, but not statistically significant, reductions in resting BP in children and adolescents. The lack of statistical significance was attributed to the fact that the majority of the subjects were normotensive, as well as to the lack of strictly defined sedentary control group in most of the included studies. More recent data provide increasing evidence for the positive influence of regular aerobic exercise in resting and 24-h SBP in obese and hypertensive children and adolescents.Apart from the positive effects on BP and HTN rate, Meyer et al. demonstrated that regular exercise, three-times per week, 60–90 min per day, over 6 months, improved early vascular changes as measured by flow-mediated dilation and carotid intima-media thickness. In a similar study by Maggio et al. the beneficial effects on BP, BMI and arterial stiffness remained 2 years after the cessation of training in obese children, in a recently published follow-up study.The majority of the subjects maintained physical activity with further improvements of BP and arterial function. Dietary supervision should be combined with regular physical activity, aiming to control BP, reduce extra weight, improve insulin sensitivity and establish a new lifestyle for children prone to developing HTN and cardiovascular disease. Children of parents with relatively high physical activity have been reported to be 5.8-times more likely to be active themselves than children of two inactive parents The main correlates of physical activity in a group of Singaporean adolescents were self-efficacy, enjoyment of physical activity, parental support and participation in sport teams. Current recommendations for exercise in healthy children and adolescents include 30–60 min per day of moderate-to-vigorous physical activity at least three-times per week.Moderate-to-vigorous-intensity exercise equals to 5–8 metabolic equivalents. This intensity should be maintained for a long duration of time to lower BP values in children with mild essential HTN (five-times per week for 30–60 min). Ideal levels of physical activity should be calculated for each individual separately, in connection with the expected cardiovascular benefit from a greater oxygen consumption. Moreover, children should be encouraged to reduce time spent in sedentary activities, such as playing video games or watching television to less than 2 h per day.

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