24 h non-invasive ambulatory blood pressure monitoring: do the number of recordings per hour and/or ways of analyzing day and night matter?

OBJECTIVE: To analyze whether the number of blood pressure registrations per hour and/or different ways of defining day and night have any influence on the result of ambulatory blood pressure monitoring. DESIGN AND METHODS: The study used blood pressure data from 40 normotensive subjects (20 men) and 40 (20 men) treated hypertensive patients who had undergone 24 h non-invasive ambulatory blood pressure monitoring twice (on a work day and a non-work day), three recordings per hour being made. Calculations were made based on all recordings and with one and two recordings per hour randomly removed. The definition of night was the subject's own (from going to bed until getting up), and an arbitrary designation of 2200 h-0600 h. RESULTS: The mean blood pressure (systolic/diastolic and standard deviation) over 24 h showed no significant difference between calculations based on three, two or one blood pressure registrations per hour in either hypertensives or normotensives, nor was there any significant difference when the same kind of calculation for day and night was made based on the subject's own statement or on arbitrary dividing lines. The between-patient standard deviation increased, albeit marginally, with fewer registrations. Individual comparisons revealed a difference over 24 h of 8.4/6.7 mm Hg between three and one registrations per hour in one hypertensive patient during a non-work day. All the others had lesser and very small differences. CONCLUSION: An analysis of our data revealed no major differences in mean blood pressure if one, two or three recordings per hour were used. Arbitrary dividing lines for day and night or according to the individual's own statement did not have any major effect on the result. This must be considered to be an advantage when data from different studies are compared or pooled. It may, however, be wise to perform recordings no less than every 30 min when evaluating individual patients.