Research ArticleAmbulatory Arterial Stiffness Index and circadian blood pressure variability
Introduction
An increase in blood pressure (BP) variability is associated with adverse cardiovascular outcomes.1, 2, 3 However, the physiological mechanisms that underlie such variability remain unclear. Increased arterial stiffness is one potential mechanism because it is more difficult to maintain a steady mean arterial pressure via fluctuations in systemic vascular resistance when arterial stiffness is likely to magnify the pressure effects of small fluctuations in stroke volume. In addition, a stiffer vessel may not “report” the degree of stretch in a baroreceptor like those found in the carotid sinus resulting in less autonomic input to dampen BP.4, 5
The measurement of arterial stiffness has several approaches. These include methods such as a noninvasive estimate of pulse wave velocity6 which reflect stiffness in large vessels like the aorta or methods that analyze the interaction between the diastolic and systolic pressures over a period of observation (typically 24 hours) using regression approaches like the Ambulatory Arterial Stiffness Index (AASI).7 Methods such as the AASI may provide more insight into the nonlinear relationship between distending pressure and BP8 over the course of 24 hours because AASI captures data over the course of a day and a night, as opposed to pulse wave velocity which captures data supine at a single moment.
There are several methods that quantify variability. A commonly used method is the simple calculation of the standard deviation (SD) around the mean BP value. A second method is the calculation of the average real variability (ARV).9 ARV may be more suitable when assessing BP variability more than a 24-hour period because it reflects changes on a measurement-by-measurement basis and is not subject to the limitation of the SD which measures dispersion around a single mean value and is enhanced when patients show a normal “dipping” pattern characterized by daytime readings which are higher than nighttime BP readings.9
In this study, we examined the relationships between AASI and BP variability in an untreated sample of subjects with prehypertension and Stage 1 hypertension. We measured BP variability using the SD and the ARV of 24-hour pulse pressure (PP) measurements in our cohort, and we evaluated the variability measures within the day versus nighttime segments. We examined variability within the PP as our BP metric because PP is felt to be the most relevant standard clinic BP measure related to arterial stiffness.10 Our objective in this study was to examine variability measures of the brachial PP profile over 24 hours, as well as within the daytime and nighttime settings, and to determine if two different ways of measuring variability generate similar results. Furthermore, we sought to examine how well measures of variability within the PP were related to the AASI during the full 24 hours, as well as during the day and night segments.
Section snippets
Participants
Participants included 64 adults between the ages of 22 and 69 years with untreated systolic BP in the range of 130–160 mm Hg and diastolic BP <100 mm Hg. We excluded people with current use of any medications or dietary supplements that could affect BP, body mass index >40 kg/m2, presence of diabetes mellitus, presence of cardiovascular disease, autonomic neuropathy, current tobacco use, renal insufficiency defined as an estimated glomerular filtration rate of less than 60 mL/min/1.73 m2, >10
Results
Table 1 lists demographic information on the cohort. They were 49 years of age on average, with an average body mass index in the overweight–obese range, and of predominantly African American or Caucasian race. Across our cohort, the average PP (±SD) was 51.3 ± 11.0 mm Hg. The average of the within-subject 24 -hour PP SD was 8.3 ± 2.1 mm Hg compared with a value of 24-hour PP ARV of 7.4 ± 1.9 (P < .001). Half of our cohort (32 of 64 subjects) had night-to-day ratio of systolic BP of ≤0.90
Discussion
In our cohort, we observed a positive correlation between AASI and PP variability using both the SD and ARV assessments. Interestingly, the relationships between AASI and the two measures of PP variability showed different strengths of correlation when the 24-hour cycle was divided into day and night time components. We noted that the average AASI value for the 24 hours of monitoring in our cohort was 0.45, similar to the report of Dolan et al7, who observed a value of 0.41 in an untreated
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Mean value of pulse pressure: The key feature in ambulatory arterial stiffness index estimation using regression models
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