Visualization of the interplay between arterial properties and 24-hour ambulatory blood pressure during daytime and nighttime, for different age ranges

Blood pressure (BP) is the driving force providing blood flow to the body tissues, via the arterial system, in an attempt to meet the local need for gas and nutrient exchange via the capillaries. These needs vary over a 24-hour period with physiological activities and environmental changes, and are affected by age and other demographic characteristics. The mechanical properties of the arterial system are known determinants of BP. It may be clinically informative to graphically present mean BP of individuals, with domains representing ranges of arterial properties, age and BP control status. For the purpose of the present Comment, systolic BP (SBP) and diastolic BP (DBP) are, respectively, the mean measured values of the maximum and minimum arterial pressure waveforms generated during the heart cycle, in response to the left-ventricular ejection of the stroke volume. The difference between SBP and DBP (SBP-DBP) is the mean pulse pressure (PP) that represents the pulsatile part of the arterial pressure, and the mean arterial pressure (MAP), defined by DBP + PP/3. The two main mechanical properties of the arterial system defined at the site of BP measurement and affecting it are peripheral resistance (‘resistance’) and arterial stiffness (‘stiffness’). Resistance is the ratio between MAP and mean blood flow. Resistance is contributed by the arterial vessels beyond the site of the BP measurement towards the periphery. These include small arteries (lumen diameter 100 to 400 microns) that branch into arterioles (<100 microns). Stiffness, in the present context, is the ratio between pressure change and the corresponding relative change in arterial volume (or the mean cross-sectional lumen). PP, the mean arterial pressure change during the early systole, is a widely accepted correlate of stiffness [1]. While resistance involves blood flow along the arterial axis, stiffness involves radial blood flow, resulting in accumulation of blood in the elastic arteries during the systole that is released during the diastole at a rate depending on the resistance. This so-called ‘Windkessel effect’ buffers the pulsatility of the arterial pressure, which increases the efficiency of the blood gas and nutrient exchange in the tissues through the capillary walls [1, 2]. Distribution of blood flow in response to tissue needs is handled by changes in the arteriolar lumen diameter following variation in the tone of the smooth muscles embedded in the arteriolar wall. This tone is controlled by the neural sympathetic activity (SNA) and metabolites, e.g., partial pressure of oxygen (PO₂), causing vasoconstriction/vasodilation that decreases/increases local blood flow. Resistance reflects mainly the integrated resistance of the arteriolar system downstream beyond the BP measuring point. It should be mentioned that resistance is proportional to blood viscosity, which is affected by the concentration and deformability of the red blood cells. Blood viscosity may greatly increase in slow blood flow and under some conditions, e.g., diabetes mellitus. Looking from the heart side, MAP is equal to cardiac output ([stroke volume]x[heart rate]) multiplied by resistance. Therefore, a persistent elevation of resistance, e.g., due to increased SNA, requires higher MAP to preserve the blood supply, which occurs in arterial hypertension. A recently reported phenomenological model enables the determination of resistance and stiffness from 24-h ambulatory BP (24hABP) measurements [3]. Using this method, relatively higher resistance (Resistance↑) and stiffness (Stiffness↑) were shown to occur for SBP > 135 mmHg and PP > 55 mmHg, respectively [4]. Arterial properties are among the determinants of BP variation with age: resistance at rest increases with age [5]; stiffness, as measured by carotid-femoral pulse wave velocity, increases gradually with age from below 20 to over 70 years old [6]. The well-known association between elevated BP and risk was the basis for defining borders characterizing uncontrolled 24 hABP during daytime and nighttime (see below). Additional expression of the arterial properties in the 24 hABP reflects the fact that stiffness depends on arterial pressure, in such a way that PP can be viewed as a sum of two components: ‘Elastic PP’ (elPP), that represents the value of PP, assuming that stiffness is constant having its DBP value, and ‘Stiffening PP’ (stPP), that expresses the variation of the PP due to the change of stiffness between DBP and SBP. Thus, stPP equals PP-elPP. Both elPP and stPP can be derived from 24 hABP and have been found to have prognostic significant for age ≥ 50 years [7]. Figure 1 shows, probably for the first time, SBP vs. DBP of individual patients at daytime and nighttime for age < 50 and age ≥ 50 years (‘Younger’ and ‘Older’, respectively), with the marked borders enclosing controlled BP during daytime (≤135/85 mmHg) and nighttime (≤120/70 mmHg), and lines defining regions of higher resistance (SBP > 135 mmHg) and higher stiffness (PP > 55 mmHg). These lines divide the SBP-DBP plane into four zones of arterial property phenotypes (using the symbol ↑ for ‘higher’), including (1) Resistance↑, (2) Stiffness↑, (3) both Resistance↑ and Stiffness↑ and (4) neither Resistance↑ nor Stiffness↑. The patient population is a representative sample taken from a previous study [7], and includes 671 patients: 507 age ≥ 50 year (‘Older’) and 104 age < 50 year (‘Younger’). Mean (SD) of 24 hABP was 137 ± 16/78 ± 10 mmHg, age 56 ± 16 years, 55% women and 59% treated with antihypertensive medications. The data were kindly provided by Prof. Michael Bursztyn. Results showed that in patients with uncontrolled BP, the percentage of older and younger patients in some of these zones was very different. For example, for uncontrolled daytime BP, 75% of older vs. 37% of younger patients had both Resistance↑ and Stiffness↑, compared with 35% vs. 7% of patients having uncontrolled nighttime BP. While elPP was rather similar in daytime and nighttime and in controlled and uncontrolled BP for all zones (about 40–60 mmHg), stPP varied largely by zone and was much larger in older than in younger patients. This example suggests that BP control status is likely to be more clinically meaningful for specific age groups and arterial property zones.

Fig. 1.

A plot of the mean 24-hour ambulatory systolic BP (SBP) versus mean diastolic BP (DBP) of 671 individual patients measured during daytime (A) and nighttime (B). Patients younger or older than 50 years of age are distinguished by color and symbol. The white-filled rectangles are the regions of controlled BP < 135/85 mmHg for daytime and <120/70 mmHg for nighttime). The mean pulse pressure (PP) is constant on the diagonal lines (dotted) with marked values (20, 30,…,110 mmHg). The lines to which arrows are attached point to zones of elevated peripheral resistance (green: SBP > 135 mmHg) and elevated arterial stiffness (red: PP > 55 mmHg). These lines define the zones 1, 2, 3, and 4 (see text)

In this issue Li et al. [8] presented results of their study showing the low control rates of 24hABP in hypertensive Chinese patients treated with antihypertensive medication, especially in the night time and morning time windows. Furthermore, the lower ambulatory BP control rates were associated with measures related to arterial stiffness, including ambulatory pulse pressure (PP), its elastic and stiffening components, and the arterial ambulatory stiffness index (AASI). The finding that the BP control rate was much lower than in populations from other countries was attributed to the lower usage of long-acting antihypertensive drugs in China in general, and in this study in particular. An important aspect of this study, which included 4408 patients from 77 hospitals, was the use of a web-based data collection system (Shuoyun) that enabled collection of data from validated ambulatory BP monitors, and generation of a standardized report. The finding that the PP and its components were all significantly associated with the ambulatory BP control status was explained by previous findings and rationales that antihypertensive drugs affect DBP more than SBP, as the former reflects peripheral resistance, while the latter is associated with arterial stiffness. Thus, patients with stiff arteries are less responsive to BP control by reduction of high SBP [9]. The study results are particularly important in light of a very recent study showing that with 24 hABP, SBP, particularly at nighttime, appears to be more informative regarding the risk of all-cause mortality and cardiovascular death than office measurements [10, 11]. The first part of this Comment suggests that deeper insight regarding daytime and nighttime BP control and its association with arterial properties could be gained by expanding the analysis to different ranges of age, arterial properties zones, PP and its components.

Compliance with ethical standards

Conflict of interest

The authors declare no competing interests.