Comparison of ambulatory blood pressure reference standards in children evaluated for hypertension

Serum uric acid and the risk of hypertension and chronic kidney disease

PURPOSE OF REVIEW

To discuss the evolving data regarding uric acid as a potential cause of hypertension and progressive renal dysfunction and its clinical and research implications.

RECENT FINDINGS

The potential mechanisms by which uric acid could cause vasoconstriction and a progressive ateriolopathy were established in animal models between 1999 and 2004. Since then, there has been a growing interest in the topic and numerous retrospective and prospective observational studies. The preponderance of data support the hypothesis that serum uric acid is a cause or exacerbating factor of hypertension and progressive kidney disease. Over the last couple of years clinical intervention trials, including randomized controlled trials in the young have supported this mechanistic role.

SUMMARY

Current evidence supports the role of uric acid as marker and mediator of risk for both hypertension and progressive decline in renal function. Data on the impact of xanthine oxidase inhibitors or uricosurics on clinical hypertension and chronic kidney disease are suggestive but inconclusive. Considerably, more data will be required to determine if uric acid lowering therapy will become a mainstay of management in diseases other than gout and tumor lysis syndrome.

Hyperuricemia and hypertension

Over the past century, uric acid has been considered a possible risk factor for hypertension and cardiovascular disease. However, only in the past decade, animal models and clinical trials have supported a more mechanistic link. Results from animal models suggest a 2-phase mechanism for the development of hyperuricemic hypertension in which uric acid induces acute vasoconstriction by activation of renin-angiotensin system, followed by uric acid uptake into vascular smooth muscle cells leading to cellular proliferation and secondary arteriolosclerosis that impairs pressure natriuresis. This acute hypertension remains uric acid dependent and sodium independent, whereas the chronic hypertension becomes uric acid independent and sodium dependent. Small clinical trials, performed in adolescents with newly diagnosed essential hypertension, demonstrate that reduction of serum uric acid can reduce blood pressure. Although more research is clearly necessary, the available data suggest that uric acid is likely causative in some cases of early onset hypertension.

The role of uric acid in the pathogenesis of hypertension in the young

Uric acid has been suspected to be a risk factor for hypertension since the 1870s. Numerous epidemiological studies demonstrate an association between uric acid and both incident and prevalent hypertension in diverse populations. Studies in elderly patients have had more variable results, raising the possibility that uric acid may be more significant to hypertension in the young. Animal models support a two-phase mechanism for the development of hyperuricemic hypertension. Initially, uric acid induces vasoconstriction by activation of the renin-angiotensin system and reduction of circulating nitric oxide, which can be reversed by lowering uric acid. Over time, uric acid uptake into vascular smooth muscle cells causes cellular proliferation and secondary arteriolosclerosis that impairs pressure natriuresis, causing sodium-sensitive hypertension. Consistent with the animal model data, small clinical trials performed in adolescents with newly diagnosed essential hypertension demonstrate that at least in certain young patients, reduction of serum uric acid can mitigate blood pressure elevation. While more research is clearly necessary, the available data suggest that uric acid is likely causative in some cases of early-onset hypertension.

Pediatric ambulatory blood pressure monitoring: indications and interpretations

The prevalence of hypertension in children and adolescents is increasing, especially in obese and ethnic children. The adverse long-term effects of hypertension beginning in youth are known; therefore, it is important to identify young patients who need intervention. Unfortunately, measuring blood pressure (BP) is difficult due to the variety of techniques available and innate biologic variation in BP levels. Ambulatory BP monitoring may overcome some of the challenges clinicians face when attempting to categorize a young patient's BP levels. In this article, the authors review the use of ambulatory BP monitoring in pediatrics, discuss interpretation of ambulatory BP monitoring, and discuss gaps in knowledge in usage of this technique in the management of pediatric hypertension.

Ambulatory blood pressure status in children: comparing alternate limit sources

The American Heart Association has included alternate ambulatory blood pressure (ABP) limits for children published by Wühl in 2002. These updated limits employ the same pediatric cohort data as the previous ABP limits published by Soergel in 1997 but differ in analysis technique. The implications of changing ABP limit source on the diagnosis of hypertension has yet to be examined in a large pediatric cohort. We reviewed 741 ABP monitorings performed in children referred to our hypertension clinic between 1991-2007. Hypertension was defined as 24-h mean blood pressure ≥ 95 th percentile or 24-h blood pressure load ≥ 25%, by Soergel and Wühl limits separately. Six hundred seventy-three (91%) children were classified the same by both limit sources. Wühl limits were more likely than Soergel to classify a child as hypertensive (443 vs. 409, respectively). There was an increased classification of prehypertension and decreased white-coat hypertension by the Wühl method, whereas ambulatory and severe hypertension counts remained relatively the same by both limits sources. The use of either limit source will not significantly affect most clinical outcomes but should remain consistent over long-term research projects. Collection of new normative data from a larger, multiethnic population is needed for better measurement of ABP in children.