Predictors of interindividual variation in ambulatory blood pressure and their time or activity dependence

Ambulatory blood pressure and coronary artery calcification in middle-aged and younger adults

BACKGROUND

The objective of this study was to investigate relationships between blood pressure (BP) determined by ambulatory monitoring and coronary artery calcification (CAC) determined by electron beam computed tomography (EBCT) in middle-aged and younger adults without symptoms of coronary artery disease.

METHODS

Measures of office and ambulatory BP were analyzed in 298 asymptomatic adults (134 women and 164 men) from the white population of Rochester, MN, who were 20 to 60 years old (mean +/- SD, 40 +/- 9 years). For the ambulatory BP measurements, the active period of the day was defined as the daytime, out-of-bed hours and the inactive period as the nighttime, in-bed hours. Logistic regression was used to assess whether ambulatory measures of BP influenced the probability of having CAC detected by EBCT.

RESULTS

After adjusting for sex, age, and office measures of BP, ambulatory diastolic BP during the active and inactive periods were each statistically significant additional predictors of the probability of having CAC. Similarly, after adjusting for sex, age, and ambulatory systolic BP, ambulatory diastolic BPs during each period were also statistically significant additional predictors of the probability of having CAC. In contrast, measures of ambulatory systolic BP, pulse pressure, and diurnal dipping of BP levels from the active to the inactive period did not make statistically significant additional contributions to the probability of having CAC.

CONCLUSION

These findings emphasize the role that the hemodynamic stress of diastolic BP may play in the early development of atherosclerotic coronary artery disease.

Control of blood pressure in the ambulatory setting

Control of blood pressure still relies on conventional office or clinical blood pressure measurement using mercury sphygmomanometry. However, it has long been known that office measurement, even when repeated, does not fully reflect usual blood pressure. The additional use of ambulatory devices for prolonged periods of blood pressure measurement is now clinically feasible. Previous research has indicated that ambulatory blood pressure measurement is better than office measurement at predicting individual cardiovascular risk. Guidelines for clinical use of ambulatory blood pressure measurement and for quality control of devices are available. Ambulatory measurement has revealed characteristics of circadian rhythm and variability that are promising with regard both to improving our understanding of the aetiology of high blood pressure and to individual risk assessment. Some of the latest developments in research on ambulatory blood pressure measurement are discussed.

Effect of time of day on intraindividual variability in ambulatory blood pressure

The aim of this study was to determine whether intraindividual blood pressure (BP) variability, measured by noninvasive ambulatory monitoring, differs between the active (daytime) and inactive (nighttime) periods of the day. We obtained ambulatory BP recordings in 143 healthy adults (95 men, 48 women) from Rochester, Minnesota. Readings were obtained every 10 min for a 24-h period. We calculated the standard deviation of each individual's BP readings about the means for the active period and for the inactive period as measures of intraindividual BP variability. In men, mean within-individual standard deviations for both systolic (SBP) and diastolic blood pressure (DBP) were significantly greater during the inactive period than during the active period (for SBP: 10.3 +/- 2.1 v 11.9 +/- 2.7, P < .0001; for DBP: 8.8 +/- 2.0 v 9.7 +/- 2.5, P = .0027). In women, the mean within-individual standard deviation for SBP did not differ significantly between the active and inactive periods (9.7 +/- 2.2 v 10.3 +/- 2.4, P = 0.225) but for DBP was significantly greater during the inactive period than during the active period (8.1 +/- 2.0 v 9.2 +/- 2.3, P = .020). Statistically significant predictors of intraindividual BP variability included measures of age and body size, metabolic traits, neuroendocrine traits, erythrocyte cation traits, and renal function traits. This study demonstrates that intraindividual BP variability, as measured by noninvasive ambulatory monitoring, is as great or greater during the inactive period as during the active period of the day.