Blood pressure (BP) measurement in a clinical setting has been the general recommendation for the diagnosis of hypertension in children and adolescents. This is based on the assumption that the brief but controlled measurement of BP made in the traditional clinical setting predicts the long-term effects of hypertension. Although office BP should be used as a reference, ambulatory BP monitoring is now increasingly recognized as being indispensable to the diagnosis and management of hypertension [1,2]. It has contributed significantly to revealing or ‘unmasking’ BP phenomena that were not readily apparent using office BP. These have included the dipping and nondipping patterns of nocturnal BP, white-coat and masked hypertension.
From the beginning of the use of ambulatory BP, a logical approach was to search for the relationship between ambulatory and office BP values. A positive correlation was observed between ambulatory and office BP for both SBP and DBP in normotensive children, even though ambulatory BP values were generally higher than their office BP counterparts . The higher ambulatory BP in normotensive children was reversed when monitoring hypertensive children. This behaviour seemed to be similar to that observed in the adult population in which the ambulatory BP of normotensive individuals was usually higher than their office BP, in contrast with hypertensive patients . The differences between office and ambulatory BP in normotensive children were attributed, at least in part, to certain factors such as physical activity and the different method of measuring BP. The office BP readings were obtained after a period of rest in a relaxed environment, while in contrast an active lifestyle was permitted during ambulatory BP recordings. In addition, when ambulatory BP was measured with an oscillometric device, an overestimation of BP values, as compared with the auscultatory method, was present in individuals in the lowest office BP range .
In the current issue of the Journal of Hypertension, Salice et al.  compare office and ambulatory BP values in a large cohort of children of different ages and with a wide range of BP values. The conclusions of the study are in agreement with previous observations that differences between office and ambulatory BP largely differ according to office BP values. In the study by Salice et al. , the strongest variable to predict these differences is office BP, even more than sex, age, weight and BMI. Once plotted, the difference between office and ambulatory BP progresses from negative to positive, from low to high office BP values. The difference becomes smaller as the office and ambulatory BP values are closer to the identity line, which is observed at 117 mmHg for 24 h and 123 mmHg for daytime SBP. The authors conclude that the office minus ambulatory BP differences mainly depend on the statistical phenomenon of regression to the mean, based on the differences being more dependent on BP values than on age; and similar discrepancies were observed in adults studied in the PAMELA study. On the basis of the differences observed and according to the data, the clinical conclusion was that diagnosis of white-coat and masked hypertension in children and adolescents is not an easy task. It is worthy to comment on both, the reasons for the office ambulatory differences, and the potential impact on the diagnosis of ambulatory BP-based clinical conditions.
Regarding the office ambulatory discrepancies, a relevant point in the study is the inclusion of normotensive and hypertensive children and adolescents, due to the differences between office BP and ambulatory BP varying largely in the two extremes of BP values. In previous studies, office BP was measured using auscultatory, whereas ambulatory BP was measured with oscillometric devices. As a consequence, the discrepancies were partially attributed to the different methodology. Although auscultatory BP uses the detection of Korotkoff sounds for SBP and DBP, the oscillometric method identifies the largest oscillation, which corresponds to the mean BP, and then an algorithm calculates SBP and DBP. Furthermore, in children and adolescents with an elastic artery and narrow pulse pressure, it may be possible that the results obtained by the algorithm have bias in the extreme values of BP, normotensive and hypertensive. The present study, however, was performed with oscillometric devices to measure office BP (SYNC Master 570 & Dynamap V100) and ambulatory BP (Spacelab 90207; Spacelabs, Redmond, Washington, USA), and the discrepancies were still present. Does this observation rule out the possibility that the algorithm plays some role in the observed discrepancies? Maybe not, as the algorithms used in the office BP monitors are not the same as that used in the ambulatory BP monitor. Therefore, office and ambulatory BP measured using the same oscillometric device can help to further investigate the issue. Our group analysed previous data that had used the same monitor (Spacelabs 90207) to measure office BP immediately prior to initiating 24-h ambulatory BP monitoring in 454 children and adolescents with an age range of 6–18 years. When office and ambulatory BPs were plotted (Fig. 1) (Lurbe et al., unpublished data), the distribution of the points follows a close relationship at either low or high BP values. Then, different algorithms may also contribute to the discrepancies observed between office and ambulatory BPs.
From a diagnostic point of view, how common and important the intraindividual differences are between office and ambulatory BP is the keystone to the use of ambulatory BP monitoring. Reference office BP data have been developed from the BP distribution of thousands of children grouped by age and sex, or age, sex and height. Up to now, although studies have provided information, the data from ambulatory BP are insufficient to create similar reference tables due to the low number of individuals included in each category of age and sex or height and sex. However, preliminary information is available and used in the clinic . The importance of the discrepancies between office and ambulatory BP at the time to diagnose hypertension is meaningless, as the thresholds to define hypertension for office and ambulatory BP are very close and they are at levels in which the differences are minimal.
As for the existence of white-coat or masked hypertension in children, its importance as a clinical entity will depend on whether it carries risk for future cardiovascular outcome. Prevalence of white-coat hypertension differs largely among the studies published ranging from very low values (1%) , to very high as much as 44% , due to the dependence on thresholds and kinds of population included in the studies, as well as the procedure of office BP measurements. Children with white-coat hypertension tend to have a higher left ventricular mass index than confirmed normotensive individuals [9,10]. However, there are currently no data on the long-term follow-up after initial assessment, neither in the reproducibility nor in the risk to develop persistent hypertension.
The opposite phenomenon, masked hypertension, occurred in approximately 10% of children and adolescents [8,10,11]. Key issues such as the persistence and the clinical significance of the phenomenon were analysed in a prospective study . The abnormal elevation of ambulatory BP persisted in nearly 40% of individuals, who had higher left ventricular mass index, and a prevalence of left ventricular hypertrophy of 10%. The long-term prognostic value of masked hypertension to progress to hypertension in youths has been established and that masked hypertension is a precursor of sustained hypertension. Moreover, the risk of developing sustained hypertension is higher in masked hypertensive boys than in masked hypertensive girls.
In conclusion, differences between office and ambulatory BPs have been recognized since the initial years of research in the field and were the underlying reasons for the use of ambulatory BP monitoring. The differences have been attributed to several factors such as absence of alarm reaction, method of measurement and regression to the mean, among others. The contribution of each of these reasons may differ individually in extreme BP values as well as in extreme age groups, in which other factors should be considered. This is the case of the differences among algorithms to calculate SBP and DBP used by the oscillometric methods. Although in a middle range of BPs the algorithms result in close BP values, the calculated BP may differ largely when BP values are very low or very high. Although further studies will provide more light on the issue, the diagnosis of ambulatory BP hypertension using the threshold, age or height and sex-specific 95th percentile is in the range of ‘confident values’ when using ambulatory BP with validated monitors. In children and adolescents, the clinical use of ambulatory BP monitoring is supported by the prognostic value of repeated ambulatory BP monitorings in normotensive type 1 diabetic individuals  and in hypertensive individuals with chronic kidney disease .
This work was supported in part by grant number PI11/00144, Instituto de Salud Carlos III, Spain.
Conflicts of interest
There are no conflicts of interest.
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