Can Simultaneous Ambulatory Blood Pressure and Activity Monitoring Improve the Definition of Blood Pressure?

In this study we measured activity and sleep time, using a wrist actigraph, in a population of 319 patients referred for ambulatory blood pressure monitoring (ABPM). Mean waking and sleeping blood pressure (BP) and the diurnal BP variation derived with this technique were compared to daytime (defined variously as 7 AM to 8 PM, 7 AM to 10 PM, or 7 AM to midnight) and nighttime pressure. To study the relationship of actigraph-derived activity data to BP variability in more detail, 30 patients underwent paired monitoring 1 to 18 (mean 8) days apart. Mean waking BP was then corrected for activity to determine whether this technique could improve reproducibility. Statistically but not clinically significant differences between waking and daytime BP were seen, with larger and potentially clinically relevant differences between sleeping and nighttime BP. The midnight to 7 AM time period gave the best estimate of observed sleeping time and the nocturnal dip of BP. The average correlation between activity and BP was 0.25 for systolic and 0.34 for diastolic BP, but with wide variation in the strength of the relationship both between and within individual patients. On average, activity accounts for 20% of systolic and 26% of diastolic BP variation. Correcting the mean waking BP to a standard activity level reduced within-patient variation by 6%/9%. This made no significant difference to overall reproducibility, perhaps because variability in this sample was low before adjustment, with the standard deviation of the difference equal to 8/4 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)