ABPM constitutes a valuable tool in the diagnosis of RH. The identification of white coat RH and masked hypertension (which may fulfill or not the definition of RH) is of great importance in the clinical management of such patients. Moreover, the various ABPM components such as average BP values, circadian BP variability patterns, and ambulatory BP-derived indices, such as ambulatory arterial stiffness index (AASI), add significantly to the risk stratification of RH. Lastly, ABPM may indicate the need for implementation of specific therapeutic strategies, such as chronotherapy, that is, administration-time dependent therapy, and the evaluation of their efficacy. 1. Introduction Ambulatory blood pressure monitoring (ABPM) is the method of obtaining automated brachial blood pressure (BP) measurements at fixed time intervals, during a 24-hour period away from a medical environment. This represents a more “realistic” approach to BP assessment since it involves BP measurement during the usual daily activities and sleep. In this sense, the overall haemodynamic load and BP variability is more accurately estimated. Numerous studies have shown that ambulatory BP compared to office BP is more reproducible and superior in predicting target organ damage and incidence of cardiovascular events in both the general hypertensive population and in subjects with chronic kidney disease [1, 2]. All these advantages of ambulatory BP in comparison to office BP, along with the ability to identify the white coat phenomenon, that is, the combination of increased office BP with normal ambulatory BP, and masked hypertension, that is, the combination of normal office BP with increased ambulatory BP, resulted in the transition of ABPM from a research tool to a clinical modality. The indications for ABPM in the clinical management of hypertensives include among others the resistance to treatment [3]. This has been defined as BP above goal despite the use of three agents of different classes in optimal doses, ideally including a diuretic. More recently hypertension controlled with four or more agents has been proposed to be included in the spectrum of resistant hypertension [4]. Although the prognosis of resistant hypertension (RH) is inadequately substantiated in the literature due to lack of sufficiently powered studies, there is plenty of evidence relating target organ damage and cardiovascular outcomes to BP levels. Uncontrolled BP along with a clustering of other risk factors is a harbinger of poor outcome in RH. Consequently, ABPM has implications in both the diagnosis and
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