Response to Interstudy Variability of Ambulatory Arterial Stiffness Index
We thank Dechering et al1 for their comments on our study, in which we revealed a previously unobserved strong negative association between the degree of nocturnal blood pressure (BP) reduction and ambulatory arterial stiffness index (AASI).2 Dechering et al1 observe that the night:day ratio of BP measurements is able to explain part of the interstudy variability in AASI: the lower the number of nocturnal as compared with diurnal readings, the higher the AASI (see Table 1 in Reference 1).
The dependency of AASI on the number of diurnal and nocturnal readings is a phenomenon related to our demonstration2 that day-night BP reduction is a major determinant of AASI. Indeed, we showed that part of the regression slope (B) of diastolic on systolic BP over the 24 hours is because of the fact that, in dipping subjects, the physiological nocturnal BP fall generates a considerable number of nocturnal systolic and diastolic BP values, which are both much lower than the corresponding daytime values. If the number of nocturnal readings is artificially reduced by an uneven collection of data during daytime and nighttime hours, the contribution of the lower nocturnal BP values to the overall regression slope is reduced, and AASI, or 1−B, is artificially increased. As a further demonstration of this relationship, we found that, in our cohort, AASI calculated on the basis of daytime readings, thus excluding the nocturnal values, was much higher than 24-hour AASI (0.48±0.26 versus 0.31±0.17).3
More generally, the present discussion might contribute to the ongoing debate on what the optimal number of ambulatory BP measurements during the 24 hours should be. It could be argued that programming a lower number of readings during the night might minimize the interference of automated readings with sleep and, thus, improve the reliability of ambulatory BP monitoring in recording the “true” circadian BP profile. In a previous randomized study, however, we demonstrated that a shorter nocturnal interval between BP readings (15 minutes) provided comparable results to a longer interval (60 minutes) in terms of sleep BP and percentage of BP reduction from wake to sleep.4 Moreover, in another previous study of ours, based on performance of 48-hour intra-arterial ambulatory BP monitoring combined with noninvasive automated BP monitoring during 1 of the two 24-hour subperiods, performance of automated BP readings did not modify the intra-arterial day-night BP profile as compared with the invasive recording performed without the noninvasive monitor.5
Thus, a more frequent nocturnal sampling does not seem to significantly interfere with the physiological nocturnal BP reduction. On the contrary, a higher number of nocturnal measurements allows a more accurate estimate of nocturnal short-term BP variability.6 Accordingly, the recently issued European guidelines for the management of hypertension7 recommend an interval of ≤30 minutes at night to obtain an adequate description of the 24-hour BP profile by ambulatory BP monitoring.
Taken together, the comments on AASI calculation made by Dechering et al1 and by ourselves3 add another argument against the use of a disproportionately small number of readings during the night by showing that a uniform frequency of measurements during the 24 hours avoids the pitfalls related to an uneven collection of data during daytime and nighttime hours.
Dechering DG, Adiyaman A, van der Steen M, Thien T. Interstudy variability in the ambulatory arterial stiffness index. Hypertension. 2007; 50: e65.
Schillaci G, Parati G, Pirro M, Pucci G, Mannarino MR, Sperandini L, Mannarino E. Ambulatory arterial stiffness index is not a specific marker of reduced arterial compliance. Hypertension. 2007; 49: 986–991.
Schillaci G, Parati G, Pirro M, Pucci G, Mannarino MR, Sperandini L, Mannarino E. Response to digging deeper into the ambulatory arterial stiffness index. Hypertension. 2007; 50: e61–e62.
Villani A, Parati G, Groppelli A, Omboni S, Di Rienzo M, Mancia G. Noninvasive automatic blood pressure monitoring does not attenuate nighttime hypotension. Am J Hypertens. 1992; 5: 744–747.
Di Rienzo M, Grassi G, Pedotti A, Mancia G. Continuous vs intermittent blood pressure measurement in estimating 24-hour average blood pressure in hypertension. Hypertension. 1983; 5: 264–269.