Does the relationship of ambulatory blood pressure to position and location vary by age, sex, race/ethnicity, or body mass index?

The effects of position and location on ambulatory blood pressure, and the variability in these effects across individuals was examined. The extent to which the variability among individuals could be predicted by age, sex, race/ethnicity, and body mass was also assessed. The data include 24-hour ambulatory blood pressure recordings of 246 subjects from an ongoing worksite study. A random coefficients model yielded estimates of within-person and between-person effects. The position and location of individuals significantly affect their ambulatory blood pressure. As position changes from reclining to sitting to standing, blood pressure increases, after controlling for location. Likewise, blood pressure is typically higher at work than at home, controlling for position. The effects on blood pressure of both position and location vary considerably across persons. interpersonal variability, measured by the standard deviation, is typically 30-75% as large as the variability in mean levels of blood pressure. There is also an interaction effect of position and location: the difference in blood pressure between standing and sitting down at work is not the same as the corresponding difference at home. Age, sex, race/ethnicity, and body mass index (BMI) predict 17% of the interpersonal variability in average systolic blood pressure while sitting at home, and 6% in diastolic blood pressure. The analysis of whether the effects of position or location vary by age, sex, race/ethnicity or BMI shows that the difference between work and home blood pressures is not significantly related to any of these four factors. However, the difference between awake and sleep diastolic blood pressures can be predicted, in part, by age and perhaps race/ethnicity. As age increases, the drop in diastolic blood pressure during sleep is attenuated. Hispanics also appear to experience a smaller drop in blood pressure during sleep. The drop in blood pressure associated with lying down (while awake) is similarly related to age and race/ethnicity. The increase in blood pressure associated with standing (vs. sitting) while at work is also attenuated in older individuals. Although significant interaction effects were demonstrated, these four major risk factors for hypertension predicted only small percentages of the interpersonal variability in the effects of position and location on blood pressure. (C) 1998 Wiley-Liss, Inc.