A new approach to define the upper normal limits of ambulatory blood pressure

One of the major problems with ambulatory blood pressure monitoring is the lack of clearly defined normal limits that can be used as reference standards. In the present work normal levels of ambulatory blood pressure were established on the basis of the average disparity between office and daytime blood pressure calculated in a population of 60 normotensives, 109 borderline hypertensives and 353 subjects with established hypertension. Non-invasive 24-h blood pressure monitoring was carried out during a non-working day in freely moving subjects. The office-ambulatory daytime blood pressure difference averaged 14.2/6.7 mmHg and the office-24-h blood pressure difference averaged 18.4/9.6 mmHg in these subjects. The pressure difference was unrelated to the level of ambulatory blood pressure. In fact, when the subjects were subdivided into five classes of increasing ambulatory blood pressure levels the office-ambulatory mean blood pressure disparity was constant throughout the five classes, even though in each class a wide intersubject variability was present. We established the upper normal levels of ambulatory blood pressure by subtracting the average office-ambulatory blood pressure difference from the World Health Organization (WHO) normal limits for office blood pressure. For ambulatory daytime blood pressure the upper normal limits were calculated as 125.8/83.3 mmHg and for 24-h blood pressure as 121.6/80.4 mmHg. In contrast with previous studies, which were conducted exclusively on normotensive subjects, this approach avoids selecting patients on casual blood pressure readings and has the advantage of including the WHO blood pressure limits.