How to improve the assessment of 24-h blood pressure variability

An increased 24-h blood pressure variability, expressed as SD of 24-h average ambulatory blood pressure values, is associated with target organ damage and cardiovascular risk in hypertension, while a physiological nocturnal blood pressure fall has been associated with reduced cardiovascular risk. Nocturnal blood pressure fall, however, may contribute markedly to the overall blood pressure variability. The aim of our study was to quantitatively assess the contribution of nocturnal blood pressure fall to 24-h blood pressure variability, and to propose a new method for computing 24-h blood pressure variability correcting for nocturnal blood pressure fall. From a large database of ambulatory blood pressure recordings obtained in two hypertension centres (Milan, Italy and Krakow, Poland), we selected 1995 recordings of a sufficiently high quality (> or =70% valid readings, > or =1 measure/h). We calculated (1) blood pressure variability, as SD of 24-h mean blood pressure, both directly from all 24-h individual readings and as a weighted mean of separately computed daytime and night-time blood pressure SD; and (2) the size of nocturnal blood pressure fall. The weighted mean SD of 24-h blood pressure was significantly lower than the corresponding direct 24-h SD of blood pressure. The size of the difference between direct SD and weighted mean SD was strongly correlated with the absolute size of nocturnal blood pressure fall (SD: r=0.89 and 0.86 for systolic and diastolic blood pressures, respectively, P<0.001 for all). The 24-h SD of blood pressure is markedly influenced by the size of nocturnal blood pressure fall, while the weighted mean SD is not. The inclusion of nocturnal blood pressure fall in the calculation of 24-h blood pressure variability may thus lead to the overestimating of this phenomenon. Given that blood pressure variability and fall at night may have opposite prognostic significance, it may be advisable to calculate 24-h SD as the weighted mean of daytime and night-time values, which excludes the interference of night-time blood pressure fall on overall blood pressure variability and allows a more precise assessment of the clinical value of 24-h blood pressure variability. The actual clinical relevance of this new parameter has to be assessed by longitudinal outcome studies.